Evaluation of Ranking Procedures
for Invasive Plants:
Application to British Columbia
Brian Wikeem, P. Ag.
Solterra Resource Inc.
Prepared for:
Val Miller
Range Branch
B.C. Ministry or Forests and Range
March 2007
Evaluation of Ranking Procedures for Invasive Plants
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Contents
EXECUTIVE SUMMARY .............................................................................................. IV
INTRODUCTION .............................................................................................................. 1
WEED RANKING OBJECTIVES AND CRITERIA........................................................ 2
REVIEW AND ANALYSIS OF CURRENT SYSTEMS.................................................. 3
Weed Ranking Systems .................................................................................................. 3
Exotic Plant Ranking System...................................................................................... 6
Northern Prairie Alien Plants Ranking System .......................................................... 6
Virginia Ranking Invasive Exotic Plant Species ........................................................ 8
Australian Weed Assessment Guide........................................................................... 9
NatureServe Criteria for Ranking Invasive Plants.................................................... 10
Alaska Weed Ranking Project .................................................................................. 11
Weed Risk Assessments ............................................................................................... 12
Application of Weed Risk Assessments ................................................................... 14
United Kingdom Non-native Risk Assessment Scheme........................................... 14
Invasive Alien Plant Program (IAPP) Species Prioritization Algorithm.................. 16
Alberta Invasive Species Risk Assessment Tool ...................................................... 17
EVALUATING WEED RISK ASSESSMENT SYSTEMS ............................................ 18
Variable Objectives....................................................................................................... 19
Questions and Content .................................................................................................. 19
Scientific Knowledge.................................................................................................... 19
Biological Characteristics ............................................................................................. 20
Ecological Impact ......................................................................................................... 21
Subjectivity ................................................................................................................... 22
Scoring Procedures ....................................................................................................... 22
Abiotic and Autecological Features.............................................................................. 23
Strengths of Weed Risk Assessments ........................................................................... 25
Summary and Conclusions ........................................................................................... 26
POTENTIAL RANKING PROCEDURES ...................................................................... 28
Unique Features Affecting Weed Populations in BC ................................................... 28
Review Objectives ........................................................................................................ 29
Potential Weed Rating System...................................................................................... 30
Environmental Tolerance.......................................................................................... 32
Abundance and Distribution ..................................................................................... 33
Management Feasibility............................................................................................ 34
Rationale for Provincial and Regional Ranking ....................................................... 35
CONCLUSIONS AND RECOMMENDATIONS ........................................................... 36
LITERATURE CITED ..................................................................................................... 38
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APPENDIX
Appendix 1. Glossary of terms. .................................................................................... 43
Appendix 2. Exotic plant ranking system questions..................................................... 46
Appendix 3. Northern Prairie alien plant ranking system questions. ........................... 47
Appendix 4. Virginia ranking invasive exotic plant species questions and scoring..... 48
Appendix 5. Australian weed assessment guide. .......................................................... 49
Appendix 6. NatureServe questions and scoring. ......................................................... 50
Appendix 7. Alaska weed ranking project questions and scoring. ............................... 51
Appendix 8. IAPP species prioritization algorithm. ..................................................... 52
Appendix 9. Alberta invasive species risk assessment tool questions and scoring. ..... 55
Tables
Table 1. Number of species listed by provincial government legislation and other
sources in British Columbia...................................................................................1
Table 2. Criteria and characteristics of highly ranked species.............................................2
Table 3. Selected primary and secondary criteria used to rank invasive plants. .................4
Table 4. General characteristics of invasive plants..............................................................5
Table 5. Comparison among protocols of criteria and total number of questions
asked. .....................................................................................................................7
Table 6. Desirable information for conducting weed risk assessments. ............................15
Table 7. Examples of questions and responses..................................................................20
Table 8. Potential criteria for ranking invasive plants in British Columbia. .....................31
Table 9. General guidelines for selecting weed ranking criteria. ......................................32
Figures
Figure 1. Potential priority matrix to rank invasive plants in British Columbia............... 35
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EXECUTIVE SUMMARY
This report reviewed and evaluated selected invasive plant ranking (weed ranking
systems) and weed risk assessment procedures from North America, Australia, and the
United Kingdom to assess their relative application in British Columbia at both a
provincial and regional scale. Eight procedures were specifically evaluated, but other
literature and protocols were also reviewed to put the process of evaluating invasive
plants into an historical and current context. The review revealed that numerous
overlapping protocols are presently available and many are currently under development.
The main findings and recommendations from the report are summarized below.
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Weed Ranking Systems were developed in the mid-1980s to screen innocuous
invasive plants from those that could threaten national park resource values in the
United States. Over the next decade, the original protocols were modified and
applied in many similar settings throughout the United States.
Weed ranking systems were initially designed to evaluate small areas. More
recently, some protocols are applied to states (provinces) and countries.
Most weed ranking systems were developed for conservations lands, and exclude
agricultural lands and native plant communities outside conservation areas.
Weed ranking methods combine professional opinion with “scientific”
documentation to predict relative invasiveness among non-native plants based on
a set of predetermined criteria.
Generally, people with good taxonomic and ecological skills can reduce the
number of weed species of potential management concern when the criteria for
weed ranking are rigorously applied. Training will be required to ensure
consistent application of these tools.
Weed Risk Assessments were introduced in late-1990s, and blend weed ranking
objectives with other legal, political, and policy issues involved with the
prevention, introduction, spread, and control of exotic organisms into countries.
Weed risk assessments are strictly technical evaluations that take no account of
the potential benefits from new organisms being introduce into novel
environments, although the process has been moving in that direction.
Numerous guidelines have been developed that provide general direction on the
content and methods for conducting weed risk assessments, but specific protocols
vary considerably in scope and application.
Weed risk assessments have become more comprehensive over time, and
international risk analysis standards have been established to evaluate the effects
of weeds and plant pests on the environment and biological diversity.
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The transition to more comprehensive analyses has not necessarily improved the
weed risk assessment process, and in some cases has made relatively simple
procedures cumbersome, confusing, and appearing imprecise.
Weed risk assessments have been criticized as procedures that are narrowly
focused, subjective, often arbitrary and unquantifiable, and subject to political
interference. There is also widespread concern that they are severely limited in
their ability to identify potential of invasive plants in new geographic
environments because they lack of broad scientific principles or reliable
procedures.
Most weed risk assessments rely heavily on biological traits to determine
“invasiveness” and to project impacts. The inability of “invasive” traits to predict
actual invasions, however, has created skepticism, and some weed managers
doubt they will ever be capable of predicting which species are invasive and
where.
Ecological impacts of weeds on plant community composition, structure, and
ecological processes are the most pervasive criteria used in weed ranking and risk
assessments.
Although ordering invasive species is important for prioritizing management
efforts, little is known about which species cause the greatest ecological impacts,
or the relative effects of the same species from one ecological setting to another.
Consequently, questions regarding impacts are of limited value for objective
analysis until more information is available on the actual effects of weeds on
ecological structure and processes.
Risk assessments have used several approaches to scoring weed threats and
impacts. Measures of uncertainty, and the option for assessors to record that
answers are “unknown,” are common procedures used to compensate for the
subjectivity introduced in these procedures.
Until recently, most weed risk assessments have concentrated on native plant
communities on conservation lands where “disturbances” may be minimized
compared to other lands.
Climate and habitat factors have not been used extensively in weed risk
assessments, although “climate-matching” is gaining interest and application in
some of these procedures. Similarly, environmental factors have only begun to
infiltrate weed risk protocols recently.
Weed risk assessments that are carefully documented and explain the logic of
their rating systems are more likely to result in repeatable conclusions by different
assessors, and allow public scrutiny and independent evaluation.
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Weed risk assessments can provide a consistent and logical decision-making
process for ranking exotic plants. Such an analytical framework encourages
biologist and managers to consider the full range of factors and consequences of
their decisions.
Although all of the methods reviewed provide valuable information for
conducting weed ranking elsewhere, the existing protocols ranged from highly
specific to very comprehensive in geographical scope, organisms addressed, and
the sophistication of questions and summary analyses. None of the applications
reviewed appear satisfactory in their present format for application in British
Columbia.
The following conclusions and recommendations are submitted:
o No weed ranking system presently exists in BC that is widely endorsed
and systematically used.
o A pragmatic and scientifically-based process for ranking invasive plants,
and evaluating the relative susceptibility of habitats, would be a valuable
management tool for the province.
o None of the weed assessment tools reviewed is adequate in its present
format to meet the needs of British Columbia.
o A weed ranking system for British Columbia should account for the
unique features of the province including its geographic size, and its
diversity in climate, soils, and vegetation, and the requirements of invasive
plants relative to ecological conditions in the province.
o BC needs to be divided into “management zones” based on geographic
separation, such as Ministry Forests and Range districts, or regional
district electoral areas. Divisions should be based on anticipated
management authority so that geographic areas and management
responsibilities are linked.
o Criteria for land uses with different management priorities, legal
responsibilities, and government policies need to be considered in setting
priorities.
o Weed ranking needs to be linked to inventory and mapping so that weed
lists can be updated at regular intervals based on actual data collected in
BC.
o Generally, invasive plant lists should be regarded as guidelines rather than
a definitive list of ranked species. Weed lists should be considered “living
lists” with the expectation that they will modified as more knowledge is
gained, and as provincial, regional, and local priorities change over time.
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INTRODUCTION
Although a host of non-native1 plants have been introduced world-wide, most species
have not become problems in their new setting (Hiebert and Stubbendieck 1993; Nelson
1994; Williamson 1996; Radosevich 2005). Those that do become pests, however, can
be highly competitive, and affect the integrity and productivity of natural and agricultural
ecosystems. Management of these species can be difficult, expensive, and often requires
a long-term commitment. Therefore, land managers must direct financial and human
resources to those species that are most likely to interfere with management goals and
objectives.
In 1994, Douglas et al. (1994) listed 553 non-native vascular plants as part of the British
Columbia flora, and numerous introductions have occurred since (Rankin and Associates
2004). Presently, 82 of these species are covered in overlapping lists under three
provincial statutes (Table 1), while management of the remaining species is at the
discretion of those agencies or people responsible for Crown and private lands.
Table 1. Number of species listed by provincial government
legislation and other sources in British Columbia.
Invasive Plant Designation Source
Weed Control Act
49
Forest and Range Practices Act
42
Community Charters Act
50
Ministry of Forests and Range Southern Interior
Forest Region Invasive Plant Categories
Ministry of Forests and Range Coast Forest
Region Invasive Plant Categories
Ministry of Forests and Range North West BC
Invasive Plant Categories
Ministry of Forests and Range North East BC
Invasive Plant Categories
Ministry of Forests and Range Invasive Alien
Plant Code Index
1
Number
Listed
43
49
68
68
>120
See Appendix 1 for a glossary of technical terms.
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Many other invasive plant lists have also emerged in the province over the last decade
that complement, or supplement, those covered under provincial legislation (Table 1), but
no clear and consistent rationale for these lists has been documented. Each of these lists
pertains to large geographic areas and a wide range of ecological conditions. While they
are intended to provide general direction for classifying the possible risks and ecological
threats of invasive species, there exists some debate on their veracity even under local
conditions. In addition, no comprehensive system is presently available for ranking
invasive plants at a provincial and regional level that is broadly accepted.
Many protocols have been developed over the last two decades to assist in separating
innocuous introduced species and those that may become harmful or damaging (Hiebert
and Klick 1988; Hiebert and Stubbendieck 1993; APCC 2002; Randall et al. 2003; Morse
et al. 2004; DEFRA 2005; Pheloung 2007). The objective of this report is to review
selected invasive plant ranking and risk assessment procedures used in North America,
Australia, and the United Kingdom (UK) to evaluate their applicability for use in British
Columbia at both a provincial and regional scale.
WEED RANKING OBJECTIVES AND CRITERIA
Initially, weed ranking procedures were developed to provide resource managers with a
pragmatic tool to classify the relative threat of non-native species to indigenous natural
resources. These models sought to separate harmless exotic species from those deemed
“disruptive” according to their “innate ability to become a pest” and the “level of impact”
they might cause (Table 2). In addition, the process aimed to collect consistent and
accurate information for establishing priorities and making management decisions
(Heibert and Stubbendieck 1993).
Table 2. Criteria and characteristics of highly ranked species.
Primary Criteria
Innate Ability as a Pest
Significance of Impact
Feasibility of Control
Selected Characteristics for High
Priority Species
Highly fecundity, specialized dispersal
abilities, ability to germinate in a wide range
of environmental conditions.
Occur in high quality natural areas, have large
populations that invade and replace natural
communities.
Not widely distributed, small seed banks, and
easily controlled with cultural, mechanical or
chemical methods.
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Since their introduction, exotic plant ranking systems have undergone numerous
modifications to adapt the process for use in other geographic regions and ecological
conditions. Additionally, interest in listing and ranking invasive plants has gained
international interest, and these protocols have been incorporated into other risk
assessments with broader objectives (Heffernan et al. 1999; Hiebert et al. 1999; Morse et
al. 2004; ANHO 2005).
For example, the United States (US), Australia, New Zealand, and the United Kingdom
have developed risk assessment procedures that are primarily directed towards
controlling the importation, introduction, and spread of non-indigenous species into their
countries (NRC 2002; DEFRA 2005; Pheloung et al. 1999; NWRAS Review Group.
2006). Weed risk components of these systems have been adapted to address the legal,
political and regulatory issues surrounding transportation and importation of exotic
species (Pheloung et al 1999; DEFRA 2005; NWRAS Review Group. 2006).
Consequently, both the objectives for making these assessments, and the criteria for
evaluation, have undergone significant changes in recent years (Table 3).
REVIEW AND ANALYSIS OF CURRENT SYSTEMS
Weed Ranking Systems
Weed ranking systems appear to have evolved from early work by Hiebert and Klick
(1988) who constructed a method to classify invasive plants based on a model developed
at Point Reyes National Seashore (Self 1986). Between 1989 and 1991 this procedure
was applied in numerous national parks across the United States, and revised based on
the experiences and recommendations of users and expert reviewers (Hiebert and
Stubbendieck 1993).
Most weed ranking methods combine professional opinion with “scientific”
documentation to predict relative invasiveness among species based on a set of
predetermined criteria (Hiebert and Klick 1988; Dunster 1990; Hiebert and Stubbendieck
1993; Hiebert 2001). Early experience indicated that applying these systems could
greatly reduce the number of weed species of potential management concern when the
criteria were diligently applied by a person with good taxonomic and ecological skills
(Hiebert and Stubbendieck 1993).
Plant characteristics related to “weediness” (Table 4) were considered the principal
attributes that enabled weeds to invade habitats and form persistent populations,
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Table 3. Selected primary and secondary criteria used to rank invasive plants.
Primary Criteria
Secondary Criteria
Seed production, viable seed, average seed mass, interval between seed crops, long flowering and
fruiting period, evidence of reproductive failure in native habitat. Asexual reproduction, small genome
size, biennial or annual. Relative growth rate; produces spines, thorns, burrs, parasitic, pathogenic.
Innate Ability (Invasive Traits)
Allelopathic, unpalatable, causes allergies, toxic to animals. Geophytic, hybridization, self-fertilization,
minimum generative time. Tolerates mutilation, cultivation, fire. Other species in genus occur in area of
interest. Tolerates range of conditions or stressful conditions.
Affects ecosystem processes, community structure, and community composition. Competes with native
plants and animals. Genetic impacts, creates fire hazard. Grows on other vegetation; forms dense
Impact (Environment, Economic, Social)
thickets. Impact agriculture infrastructure, affect agriculture operations, potential risk to forestry
operations. Obstructs vision or movement, diminishes aesthetic appeal.
Feasibility of Management and Control Well controlled by herbicides; biocontrol agents available; cultural/mechanical control possible.
Abiotic Factors
Distribution and Abundance
Climatic suitability, soil requirements, light requirements.
Current global distribution. Current local distribution, proximity distribution in adjacent jurisdictions,
trend in distribution, naturalized beyond native range, garden disturbance weed, weed of agriculture or
forestry, environmental weed.
Disturbance
Extent and kind of disturbance. Requires anthropogenic disturbance to establish.
Ecological Types
Aquatic, riparian, grassland, shrub land, forest land, other.
External Introduction and Spread
Introduction and Spread
Land Use Practices
Capable of surviving in transit, likelihood of detecting at entry point, likelihood of introduction into
country, re-introduction, or dispersal from human pathways. Species listed elsewhere (province/state,
country).
Propagules dispersed intentionally, dispersed unintentionally, persistent seed bank. Capable of longdistant dispersal, wind-dispersed, buoyant and water-dispersed, animal-dispersed. No known predators
(native or non-native); capable of surviving in transit.
Conservation areas, native ecosystems outside conservation areas, agricultural, forestry, parkland,
pasture, rangeland (non-arable grazing), roads and trails, utility corridors, other.
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Table 4. General characteristics of invasive plants.1
• Capable of germinating in many environments.
• Continuous seed production throughout the growing season.
• High seed production and long viability of seeds in the soil seed bank.
• Ability to reproduce both sexually and vegetatively.
• Ability to self-fertilize.
• Ability to cross-pollinate easily by a variety of vectors such as insects
and wind.
• Adaptations for short- and long-distance dispersal.
• Rapid growth from the vegetative phase to flowering.
• If a perennial, vigorous vegetative reproduction or regeneration from
fragments.
• If a perennial, resistance to being broken from ground easily.
• Ability to compete vigorously with other plant species by special means
such as rosettes, rapid growth, or chemical properties (allelochemicals).
1
Modified from Baker (1974).
but potential impacts, and feasibility to control populations, were also regarded as
important factors for making management decisions.
Those species considered to be the greatest threat, and most easily controlled, were given
the highest priority for management; while benign species that were virtually impossible
to control were regarded as a low priority. This information was used to formulate
control and management decisions for the areas surveyed (Hiebert and Klick 1988;
Dunster (1990, cited in White et al. 1993); Hiebert and Stubbendieck 1993; Hiebert
2001).
Initially, weed ranking systems were designed for making evaluations on relative small
areas such as specific sites, or individual national parks. More recently, the scope of these
procedures has broadened significantly, and some applications are now used to survey
entire states (provinces) and countries (Hiebert and Stubbendieck 1993; Morse et al.
2004).
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Exotic Plant Ranking System
This system was among the first published and widely circulated weed ranking protocols
developed (Hiebert and Stubbendieck 1993). Designed primarily for US National Parks
and small areas dominated by native vegetation within parks, the system has been
adopted and revised for use primarily in conservation areas throughout the United States.
The system was developed as an analytical approach for decision-making, and with the
hope of reducing subjective decisions for weed control, which were often based on
incomplete knowledge (Hiebert and Stubbendieck 1993).
The procedure contains 24 questions for each weed species organized into two main
sections: 1) Significance of Impact, and 2) Feasibility of Control or Management. The
significance of impact section is divided further into the “Current Level of Impact” and
“Innate Ability of Species to Become a Pest” (Table 5; Appendix 2).
Each question has a variable number of multiple choice answers which are independently
scored on a relative scale ranging from 0 to 15 points. Each of the two main sections is
based on a scale of 100 points. Species that score more than 50 points for Significance of
Impact are regarded as “seriously disruptive” and require management intervention,
while species that score high in the “Feasibility of Control” section will be easier to
manage than those with low scores. A final question relating to urgency of action ranks
plants as high, medium or low.
Northern Prairie Alien Plants Ranking System
The Alien Plants Ranking (APR) system was cooperatively developed by the US
National Parks Service, University of Minnesota, and the US Geological Survey. Based
on earlier prototypes prepared by the National Parks Services (Hiebert and Klick 1988;
Hiebert and Stubbendieck 1993), this computer-based system was modified for
application in grassland parks in the central United States (Hiebert et al. 1999).
Like the previous protocol, it is designed to help identify invasive species that are
currently affecting grassland sites, and other invasive species that may be a high risk in
the future. The system also evaluates the feasibility of control of each species, which
helps managers compare the cost of control against potential impact.
The APR system contains 23 questions for each weed species known to occur at, or near,
the area of interest (Table 5). The questions are organized into three sections: 1) Current
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5
Impact (Environment, Economic, Social)
6
7
5
6
Feasibility of Management and Control
9
6
1
Alberta 2007
3
BC IAPP 2006
Australia 2002
10
United
Kingdom 2005
Virginia 2001
9
Alaska 2005
Northern
Prairie 1999
Invasive Attributes (Biological Traits)
Criteria
9
20
18
13
5
4
16
6
12
4
3
7
2
n/a
n/a
NatureServe
2004
US National
Park Service
1993
Table 5. Comparison among protocols of criteria and total number of questions asked.
Abiotic Factors (climate, soil, light)
2
Current Distribution and Abundance
3
1
4
5
n/a
Yes
n/a
n/a
Disturbance and Land Use
Yes
n/a
2
Introduction and Spread
35
Trend in Distribution and Abundance
Total Questions
Uncertainty
1
n/a
6
7
24
No
23
No
14
No
12
No
20
No
21
No
> 601
No
31
No
33
Yes
Exact number of questions difficult to determine because of the number of modules in the scheme and sub-questions.
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level of Impacts, 2) Potential of the Species to be Invasive, and 3) Feasibility of Control
Including Costs (Appendix 3). Multiple-choice answers are provided for each question
with the associated scores variably ranging from 0 to 10 points depending on the
weighting given to the question. “Unknown” is provided as a response for all questions
and does not enter into the final scoring for a species.
Scores are tallied within the computer program, and an on-screen summary report can be
produced or a hard copy printed. The program provides electronic fact sheets for each
weed species. In 2000, this system was expanded to 25 questions and the web-based
system was linked to GIS and quantitative sampling systems (Hiebert 2001).
Virginia Ranking Invasive Exotic Plant Species
Beginning in 1991, the Virginia Department of Conservation and Recreation (VDCR)
began compiling a list of invasive plants species to create awareness among land
managers and the public (Heffernan et al. 2001). By 2003, the list had undergone several
revisions and invasive species were classified based on three main criteria: 1) Invasive
Rank, 2) Physiographic Rank, and 3) Habitat Requirements (VDCR 2003). Invasiveness
Rank classified species as highly invasive, moderately invasive, and occasionally
invasive based on cumulative impact on natural areas, effects on other species, potential
to disperse and invade native habitats, distribution and abundance, and difficulty to
manage. Physiographic Rank was determined by dividing the state into three
physiographic regions and assigning each plant to regions where they occurred. Habitat
Requirements focused on light and moisture requirements to provide a broad indication
of their adaptation to different environments. Adaptation to light was partitioned into full
sun, partial sun or shade; and moisture categories included hydric, mesic and xeric.
Although not regulatory, the list provides managers with a relative ranking of invasive
species, a regional assignment of where they may be problematic, and basic habitat
information regarding species adaptation to abiotic habitat features (Heffernan et al.
2001; VDCR 2003). This appears to be the first time habitat conditions were introduced
into an invasive plant ranking procedure.
In 1999, the VDCR List of Invasive Species was combined with a modification of the
Association for Biodiversity Information (ABI) ranking criteria (Randall et al. 2001) to
study 11 non-native species in Virginia (Heffernan et al. 2001). Four components were
used to rank species invasiveness representing aspects of the species biology, ecology,
and potential for management: 1) Impacts on Native Species, Habitats, and Ecosystems,
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2) Biological Characteristics and Dispersal Ability, and 3) Distribution and Abundance in
Virginia and the U.S., and 4) Management Potential (Appendix 4).
Although the protocol contains a series of 14 multiple-choice questions among the four
components (Table 3), some questions are comprised of an additional series of queries.
For example, the first question (Biological Characteristics) under Section 2 of the
protocol (Appendix 4) contains a checklist of 14 biological traits that cumulatively
contribute to the species’ overall “aggressiveness.” Species with three or more of these
traits are classified as “Extremely Aggressive.”
In general, scoring for this procedure is convoluted and complicated. Each question is
answered as insignificant, low, medium, or high; and each category is further assigned a
weighted score. The answers to each question are converted to weighted scores to tally
sub-ranks for each area, and these in turn are adjusted to new weighted scores to provide
an overall rank for the species. A greater weighting is placed on those criteria that most
strongly demonstrate impact on native plant and animal habitat, and biological
characteristics; whereas less emphasis is focused on distribution, abundance, and
difficulty of control. Species invasiveness is ranked as insignificant, low, medium, or
high.
Comments and supporting references are provided for each question. References include
published research articles, internet sources, databases, and personal communications
from field biologists or land managers.
Australian Weed Assessment Guide
This guide was developed as a tool to help make standard, informed decisions on weed
control priorities in South Australia. The protocol consists of 12 questions to compare
the relative importance of various weeds under different land use conditions in
recognition that the same weed species may respond differently to various practices
(Table 3; Appendix 5). The questions are divided into three categories: 1) Invasiveness,
2) Impacts, and 3) Potential Distribution (Appendix 5). Invasiveness focuses on the
relative rate of spread among species, with weeds that disperse faster having a higher
priority for control. Impacts are related to the economic, environmental and social effects
resulting from weed invasion, and Potential Distribution predicts the total area the weed
could eventually cover.
A sequence of multiple choice responses is presented for each of the 12 questions.
Scores are attached to each of the alternatives, which are ranked from high to low. A
fifth category, “Don’t Know,” is also included for those questions where a subjective
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judgment cannot be made. A score, ranging from 0 to 10, is assigned to each of the
criteria, and the product of the three scores determines the “Weed Importance Score” out
of 1000.
Weeds can be assessed separately for various land uses, so that the most important weeds
can be identified in different management situations. These include aquatic, crop/pasture
rotation, forestry, irrigated crops and pastures, native vegetation, non-arable grazing,
perennial horticulture, and urban.
NatureServe Criteria for Ranking Invasive Plants
Two drafts of this procedure were reviewed (Randall et al. 2003; Morse et al. 2004). This
protocol was developed for biologists to evaluate the effects of individual invasive plants
on native plant communities within a specified geographical region (country, state
(province), ecological region, and site). The second draft states that it is “designed to
make the process of assessing and listing invasive plants objective and systematic, and to
incorporate scientific documentation of the information used to determine each species’
rank” (Morse et al. 2004).
The method focuses on native plant communities in conservation areas. It is not intended
for evaluating weed species that grow in cultivated areas, or on disturbed sites outside
cultivation. Two questions are asked before a survey is conducted to determine whether
these criteria are met. If an invasive plant grows outside cultivation, but not in native
plant communities, it is ranked as insignificant (Appendix 6).
This protocol integrates previous weed ranking criteria developed by the US National
Park Service (Hiebert and Klick 1988; Hiebert and Stubbendieck 1993; Hiebert 1997)
with a synthetic approach to explaining relationships among invasive plants and
ecological responses based on models (Parker et al. 1999). The procedure consists of 20
multiple-choice questions organized into four categories: 1) Ecological Impact on
Ecosystem Processes, Communities, and Native Species; 2) Current Distribution and
Abundance; 3) Trend In Distribution and Abundance; and 4) Management Difficulty
(Appendix 6).
Five answers are possible for each of the 20 questions; four pertaining to the physical,
biological, ecological, or economic response to the questions, and the fifth is “Unknown”
(Table 3). “Unknown” is recorded when the evaluator is unable to respond to the question
because of “lack of information.” Evaluators are asked to record comments and the sources
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of information used to arrive at conclusions for each answer in a citation format. This may
include publications, internet sources, expert opinion, and other sources.
The ranking for a species is determined in a three-step process. First, scores are assigned
to each question within sub-rank categories and then tallied to produce a sub-rank score
within each of the four categories. Each of the four sub-ranks, however, is weighted
differently depending on the perceived importance of the category. For example, the
highest weight is credited to Ecological Impact and the lowest weighting is given to
Management Difficulty. The final score is produced by summing the weighted sub-rank
scores to provide the overall invasive rank (I-Rank) for the species. Each species is then
placed in one of the four I-Rank categories: high, medium, low, or insignificant.
Although the protocol was originally designed to assess invasive plants in large,
contiguous, and ecologically diverse regions, NatureServe states it can be adapted to
discrete regions (such as states, provinces, or smaller geographic areas). NatureServe is
now using this protocol to assess the effects on biodiversity of nearly 3,500 exotic plant
species established outside cultivation in the United States (Morse et al. 2004).
In 2004, the NatureServe method was tested in British Columbia by ranking 36 nonnative plants inhabiting coastal communities (Rankin and Associates 2004). Based on
this review, the authors concluded that the method “forced careful and relatively
objective” consideration of a wide range of factors involved in assessing invasive plants”
but further refinement of the protocol would likely be necessary to apply the process on a
regional basis. The following observations and conclusions were drawn that are pertinent
to this report:
• The scope of the process is comprehensive with respect to the organisms
involved; international, national and regional scope; and the potential policy and
political issues addressed.
• The ranking process is subjective.
• A sound “risk assessment procedure” with clear priorities is a necessary first step
in managing invasive plants.
• Actions should be based on a clear direction, and the best scientific and socioeconomic information available.
• Changes would be necessary before the process could be adopted by BC.
Alaska Weed Ranking Project
This protocol is a cooperative project among the Alaska Natural Heritage Program,
several US federal agencies including the Forest Service, National Park Service,
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Agricultural Research Service, Cooperative Extension Service, and the Geological
Survey. The protocol is a web-based system that appears to have elements of both the
National Park Weed Ranking System and the NatureServe model although no references
are provided.
The procedure is divided into two sections. The first section (Climatic Comparison)
correlates the potential climatic suitability of each species to one of three eco-geographic
regions in the state (Appendix 7). The second section (Invasiveness Ranking) contains
21 questions assembled into the same (but reworded) four sections as the NatureServe
protocol: 1) Ecological Impact, 2) Current Distribution and Abundance, 3) Distribution,
and 4) Feasibility of Control (Table 3). Two to four scaled answers follow each question,
and “Unknown” is allowed when the assessor cannot select an answer among the choices
provided. No score (including “0”) is applied for Unknown.
The total score for each of the four sections varies from 40 points for Ecological Impact
to 10 points for Feasibility to Control (Appendix 7). The total score for a species is the
sum of the total points for all four sections out of 100 possible points. This value is
converted to a decimal fraction by dividing the score attained by 100. Information for all
species is displayed in a tabular format listing the score for each species and the climatic
similarity of the species relative to each of the three eco-geographic regions listed for
Alaska. Climate similarity for each species is expressed only as Yes or No for the ecogeographic region.
Weed Risk Assessments
Weed risk assessments were introduced into the invasive plant forum in the mid- to late1990s, and probably owe their beginning to the multi-stage process developed in the US
to assess human health (NRC 1983). Risk assessment was defined as a general process
for linking science to decision-making (NRC 1983).
In 1992, the US Environmental Protection Agency (EPA) described a basic framework
for evaluating scientific information relating to undesirable effects of “stressors” on the
environment. The term “stressor” was defined as “any physical, chemical, or biological
entity that can induce an adverse effect.” “Adverse ecological effects” included a variety
of disturbances ranging from mortality of an individual organism to a loss in ecosystem
function (EPA 1992).
Weed risk assessment has amassed considerable momentum over the past decade to the
extent that the process has been characterized as a “new and developing discipline”
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(Groves et al. 2001; Rahman et al. 2003). The first international workshop for weed risk
assessments was held in Adelaide, South Australia in 1999 with presentations delivered
from at least five countries including Australia, Ecuador (Galapagos Islands), New
Zealand, United Kingdom, and the United States. Since then, weed risk assessment
procedures have been introduced or applied in New Zealand (Pheloung et al. 1999);
Australia (Pheloung et al. 1999; Pheloung 2007), American states (Thomas 1999;
Westbrooks and Madsen 2006), United Kingdom (DEFRA 2005) and Canada (Alberta)
(EMEC 2007a).
Although weed risk assessments can be conducted at any stage of a species’ entry or
spread into a country or specific geographic area, the current focus appears directed
towards preventing the ingress and dispersal of species that will likely become invasive
in a country (IPPC 2004). These procedures are concerned primarily with assessing the
risk of a plant species becoming of sufficient importance to be classified as a pest.
Generally, weed risk assessments are strictly technical evaluations that ignore the
potential benefits that may accrue from the species being introduced or from not
controlling it. Moreover, no mechanism presently exists to balance the risks and the
potential benefits from political processes even though advocates have been trying to
move in that direction (IPPC 2004).
Growing international interest in quarantine legislation concerning weeds (and other nonindigenous organisms) added further impetus to develop weed risk assessment systems to
help prevent the introduction of weed species, and to prioritize existing species for
control (IPPC 2004). Much of the current experience originates in Australia and New
Zealand, where quarantine protocols for preventing the introduction and spread of
unwanted plant species are in place. The success of weed-risk assessment systems in
these countries results from a regulatory environment that enables plant protection
organizations to restrict the movement of plants across the borders and within the
country. Without such legislation, weed-risk assessment by itself cannot prevent the
entry and spread of weeds (IPPC 2004).
More recently, risk assessments have become even more comprehensive and address
some of the legal and policy issues related to international trade (Powell 2002). In
addition, international risk analysis standards have been established to evaluate the
effects of vascular plants and plant pests to the environment and biological diversity,
including those risks affecting uncultivated/unmanaged plants, wild flora, habitats and
ecosystems (IPPC 2004).
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Application of Weed Risk Assessments
Numerous guidelines have been developed over the last decade (IPPC 1996; APHIS
2002; Williams 2003; Lehtonen 2004; IPPC 2004), which provide general direction on
the content and methods for conducting weed risk assessments (Table 6). Although these
assessments often aim to be quantitative, they are mostly qualitative procedures (Powell
2004). In the generic risk assessment process (EPA 1992), “experts” first estimate
probabilities (as low, medium, or high) that a species will disperse, establish, and spread;
and then they forecast the environmental and economic consequences of the species’
introduction (Simberloff and Alexander 1998).
The total risk of establishment is “quantified” as the product of the independent
probabilities of dispersal, survival, initial establishment, and spread. The “Total Effect of
Establishment” is defined as the product of the perceived economic and environmental
consequences, which are arbitrarily, but consistently, scored according predetermined
tabular values (Simberloff and Alexander 1998).
Categories of uncertainty often accompany the estimated probabilities for each
component of the assessment. These are usually expressed as “very certain, reasonably
certain, or moderately certain,” or as “reasonably or very uncertain.” These estimates do
not contribute to the calculation for “total organism risk potential” but they can serve as a
guide for decision makers in evaluating the strength of the assessment (Simberloff 2005).
United Kingdom Non-native Risk Assessment Scheme
Impetus for this protocol emerged from the Review of Non-native Species Policy: Report
of the Working Group analysis conducted in 2004, which recommended that “a scheme
for assessing the risks posed by any non-native organisms to species, habitats or
ecosystems in all or part of the UK” be undertaken (DEFRA 2005). The project was
conducted by a consortium from six universities and institutes in the UK using a draft
pest risk assessment scheme developed by the European and Mediterranean Plant
Protection Organization, and the procedures developed for plants invading Western
Australia (Pheloung et al. 1999).
The procedure is divided into two parts. The first section contains 14 questions that
determine whether a detailed risk assessment is warranted. The second part includes 51
detailed questions in three categories: 1) Entry and Establishment; 2) Capacity for
Spread; and 3) the Extent to Which Significant Economic, Environmental or Social
Impacts may Occur (Table 3). Each question is accompanied by five levels of responses
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Table 6. Desirable information for conducting weed risk assessments.1
Indicator
Taxonomic Information
Pathway or Source of Entry
Species History and Closely
Related Taxa
Characteristics
Taxonomic description and valid scientific name including
synonyms of accepted scientific name and common name(s).
Possible pathways include:
• Plant purposefully imported for urban horticulture, commercial
horticulture, or forestry.
• Contaminants of imported commodities such as seeds for sowing
or as a food source for humans or livestock.
• Accidental introduction of plants not included in either of the
above pathways e.g., attached to clothing.
• Has the species become a weed elsewhere?
• Does the species have weedy relatives in the same genus or
family in other countries or the receiving country?
• What effects has the species had where it has become a weed?
Environmental Tolerances
• Characteristics of the climate and soils in both its home range
and its extended weedy range.
• Do similar climates and soils exist in the present country? Note
however, that many species tolerate a wider range of
environmental condition when released from the constraints
present in their home range.
Biological Characteristics
Undesirable biological traits that defined it as weed elsewhere or
characteristics that may contribute to it becoming invasive in the
new location.
Introduction and Spread
Biological traits that promote the spread and persistence of the
species in native or agricultural ecosystems.
Potential Impacts
Possible adverse effects if the species is permitted entry and not
controlled including:
• Damage to human health.
• Loss of production in agricultural/horticultural systems.
• Reduction in population levels of valued native plants and
animals.
• Disruption to natural ecosystems resulting in reduced ecosystem
services e.g., lower water supply, reduced tourist attraction.
Ability to Control
1
Cost, effort, efficacy and possible outcomes of control options.
Source: IPPC 2004.
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(very low, low, medium, high, very high) and three levels of uncertainty (low, medium,
high).
Written comments are required for each question justifying the response, and a literature
reference (or personal communication) is recommended where possible. The manual
provides guidance on the protocol, and for actions that should be taken when information
is lacking or uncertain (DEFRA 2005).
Six additional modules provide methods for identifying invasive attributes, evaluating
pathways of introduction, determining the vulnerability of receptors, quantifying
economic impacts, summarizing risks and uncertainties, and selecting risk management
options. Three of the modules are designed to determine whether the species has
invasive attributes (Module 1), to quantify economic effects (Module 4) and to
summarize risk and uncertainty (Module 5). Two other modules allow assessment of the
relative importance of introduction pathways (Module 2) and the vulnerability of
receptors (Module 3) to invasion. Module 6 provides a decision-making framework for
selecting risk management options should the assessor believe they are warranted. Draft
Excel templates are available to conduct analyses, but these are considered rudimentary
at this time and require further development (DEFRA 2005).
Invasive Alien Plant Program (IAPP) Species Prioritization Algorithm
This draft protocol was developed to provide a means for ranking invasive plants in
coastal British Columbia. The procedure is an Excel-based system that has integrated
components of existing ranking procedures into a product applicable to temperate,
maritime conditions.
The procedure contains 30 questions distributed among four sections: 1) Biological
Criteria; 2) Impact Criteria; 3) Controllability Criteria, and 4) Containability Criteria
(Table 3; Appendix 8). In the first section, the assessor answers each of 17 questions
independently and if the answer is “Yes” for the question, the appropriate points (ranging
from 2 to 5 points) are entered into the “Score” column of the spread sheet. If the answer
is no, the question scores zero. No “Unknown” choice is available for any question.
Sections 2 to 4 contain five, four and three alternatives to select among, respectively. In
these sections, the assessor chooses the most applicable impact, or scenario, and then
records the point value for that question in the “Score” column beside the choice and at
the bottom of the section in the “Subtotal” cell (Appendix 8).
Sub-total scores for each of the sections are automatically calculated in the Excel version
or hand-calculated on paper. These values are transferred to a summary table and
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weighted with a “prorated factor” as follows: Biological (1.3), Impact (2.0),
Controllability (1.5) and Containability (1.5). The total score for each species is
calculated by summing the four prorated scores from each section and the dividing by
three to produce a final score with a maximum value of 10 (Appendix 8). Although
many of the questions found in this protocol are similar to those found in weed risk
assessments, the objectives of this procedure are not stated, and weed ranking may be the
intent of the procedure.
Alberta Invasive Species Risk Assessment Tool
This risk assessment tool is a computer software application that allows a “predictive,
quantitative assessment of the likelihood of adverse impacts from potential invasive alien
species in Alberta” (EMEC 2007a). The tool is intended to predict invasive species
impacts by providing information for management decisions, prioritizing management
strategies, and by assisting in enhancing management communications in the province.
Similar to the UK scheme, this protocol has adopted the “classic risk assessment
paradigm” developed in the US (NRC 1983; EPA 1999), and is broader is scope than
invasive vascular plants alone. Hence, the protocol addresses all potentially invasive
organisms from pathogens and parasites to vascular plants and animals. It also
accommodates organisms from a variety of environments including aquatic, terrestrial,
agricultural, natural areas; and native, disturbed habitats. It does not directly deal with
issues concerning national or international policy.
The procedure contains 32 questions distributed among six “Indicators:” 1) Biological
Characteristics, 2) Introduction, 3) Environmental Impact, 4) Economic Impact 5), Social
Impact, and 6) Control Management (Table 3; Appendix 9). All but one question is
accompanied by four levels of responses (ranked 0, 1, 2, 3, 4) based on an ordinal scale
from “least” to “most.” Zero in the scale means “low” or “none.” The procedure also
provides the assessor with the opportunity to answer “unknown” for each question.
When this occurs, or when no answer is provided, the question does not contribute to the
final score. The protocol also accommodates uncertainty when the assessor is not sure of
the strength of a subjective answer. These are ranked as low, medium or high and
averaged over all six indicators and 32 questions.
The risk assessment for a species is “quantified” by averaging the response scores for
each of the six “Indicators” and converting the value to a percentage. Each indicator is
adjusted with an “indicator-specific weight” and these values are then summed to
produce an overall score “reflecting the adverse impacts from the species.” No rationale
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is provided for the weighting, but in the “default” mode, each indicator is assigned equal
emphasis. Modifications to the weighting scheme provide a mechanism to “represent
scenarios in which certain aspects of the impacts or potential “invasiveness” might be
emphasized.” An “Overall Risk Score” and “Overall Confidence Score” is presented in
the final summary table for each species (EMEC 2007a). A manual accompanies the
protocol providing rationale and guidance for each question, and for actions that should
be taken when information is lacking or uncertain (EMEC 2007b).
EVALUATING WEED RISK ASSESSMENT SYSTEMS
The ability to predict with confidence that non-indigenous plants will become invasive in
a new environment could yield enormous ecological and environmental benefits. Such
predictions, however assume that information about the biology of new species, their
capacity to survive and spread; and their actual impacts on organism, ecosystems, and
economies are known, or can be reasonably estimated. These assumptions have plagued
weed risk assessment protocols since their inception; and as the process escalates in
biological complexity, technological innovation, and theoretical and statistical
development, there is little evidence that verifiable predictions are at hand.
For example, Simberloff (2005) offers “risk assessment procedures are narrowly focused,
subjective, often arbitrary and unquantifiable, and subject to political interference.”
In addition, there is widespread conviction that the “lack of broad scientific principles or
reliable procedures” severely limits their ability to recognize the potential of invasive
plants in new geographic environments (NRC 2002; Powell 2004; Simberloff 2005).
Powell (2004) speculates, however, that the demand for these systems to contribute to
international legal, policy, and political processes will require greater scientific certainty
in the future. This will require a “balance between the demand for accuracy and
precision,” and “the constraints of limited information, time and other resources” (Powell
2004).
The National Research Council report, Predicting Invasions of Non-indigenous Plants
and Plant Pests (NRC 2002), concluded that a scientifically based system to predict
invasiveness must satisfy three general criteria:
•
It must be transparent, be open to review, and have been evaluated by peers.
•
It must have a logical framework that includes independent factors that are
identified through critical observation, experimentation, or both; and that are
important in the invasion process.
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•
Use of the framework must be repeatable and lead to the same outcome,
regardless of who makes the predictions.
Some of the specific limitations of weed risk assessments are summarized below.
Variable Objectives
Beginning as a simple process to provide objective lists of weeds to assist managers in
making informed decisions (Hiebert and Stubbendieck 1993), risk assessments have
evolved over the last decade into a much larger process biologically, geographically,
legally, and politically (NRC 2002; Powell 2004; Simberloff 2005). Legal, political and
policy issues are now challenging the weed risk assessment process to extend beyond its
traditional scope of identifying hazards, pathways, and the biological consequences of
plant invasions (Rahman et al. 2003; Powell 2004; Simberloff 2005). The transition to
more comprehensive analyses has not necessarily improved the process, and in some
cases has made relatively simple procedures cumbersome, confusing, and appearing
imprecise.
Questions and Content
The number, and the content, of questions contained in weed risk assessments continue to
escalate. The modest number of questions (ranging from 20-24) found in early protocols
has now surpassed 60 (Table 3; Table 5). While some commonality remains, there is
considerable dissonance among protocols as counties, states (provinces), and regions
attempt to modify existing procedures to include new objectives and address local
conditions.
The construction and clarity of questions varies considerably among protocols. Some
protocols present clear, concise questions containing a single concept. Others submit
questions that are more complex, contain multiple concepts and unnecessary “ecological”
or “biological” jargon, and the linkage between the question and the prospective
responses is not clear (Table 7). Such lack of clarity can lead to faulty conclusions, and
reduce the accuracy and precision of the protocol.
Scientific Knowledge
In an evaluation of the risk assessment process in the public health sector, the US
National Research Council stated “the basic problem with risk assessment is the
sparseness and uncertainty of the scientific knowledge of health hazards addressed.
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Table 7. Examples of questions and responses.
Question
Possible Responses
Number of seeds per plant.
1) Few (0-10); 2) Moderate
(11-1000); 3) Many-seeded
(> 1000).
Is the production of offspring
prolific and consistent?
Guidance: Does the organism
produce viable offspring?
Does the organism have
higher fecundity than other
organisms its size?
1) Very few or none; 2) Few;
3) Moderate; 4) Many.
Comment
Question and responses
clear and concise.
Response easily connected
to question.
Question vague, guidance
introduced additional
concepts into the question
(e.g. “viability” and “other
organisms its size.” Not
clear which part of the
question and guidance the
responses refers.
Re-organization of the risk assessment function will not create the data and underlying
knowledge that assessors need to make risk assessments more precise” (NRC 1983).
While these conclusions were directed to health, they apply equally well to risk
assessment in the biological sciences. For example, Parker et al. (1999) concluded
“Despite the considerable attention invasive plant species receive, our lamentable paucity
of data on impacts leaves us largely ignorant about the ecological changes they have
brought about.” Five years later, Powell (2004) reported “there are currently no known
broad scientific principles or reliable procedures for identifying the invasive potential of
plants… in new geographic ranges.”
Biological Characteristics
The recurrence of some invasive species in widely separated parts of the world elevates
speculation that weeds possess traits that enhance their ability to flourish in new
environments. Although the importance of traits as a main factor determining invasive
potential holds considerable appeal in weed risk assessments, there are few data available
that convincingly demonstrate their predictive capability (Reichard and Hamilton 1997;
Parker et al. 1999).
While Baker (1974) listed 11 botanical characteristics of the “ideal weed” (Table 3), no
individual plant species has acquired all these traits (Radosevich and Holt 1984).
Moreover, many species that possess these traits are not invasive (NRC 2002), while
other non-native species, which are invasive, have few or none of these characteristics
(Schierenbeck et al. 1994; Rejmanek and Richardson 1996).
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Nonetheless, biological characteristics have been extensively used in weed risk
assessments, and extrapolated to a wide range of habitats even though species with the
same genetic makeup often demonstrate different phenotypic responses in new
environments (Marsden-Jones and Turrill 1945; Radosevich 2005). Consequently, it is
difficulty to predict the response of an introduced species based on biological
characteristics without considering the biotic and abiotic conditions in a new
environment. Unfortunately, environmental factors have only begun to make their way
into weed risk protocols recently (Westbrooks and Madsen 2006).
In many protocols, biological characteristics are evaluated as if the effects of perceived
negative characteristics are cumulative, and progressively define one plant as more
invasive than another. Williams (2003), however, suggests there may be no “suit of
endogenous plant characteristics …that reliably predicts potential weediness.”
Accordingly, the inability of “invasive” traits to predict actual invasions has created
skepticism, and some doubt it will ever be possible to predict successfully which species
will become invasive (Crawley 1987; Williamson and Fitter 1996; NRC 2002).
Ecological Impact
Ecological impact is the most pervasive criterion motivating weed ranking and weed risk
assessments, and the need to manage invasive plants. Some typical criteria used to assess
weed impacts among many (Table 3) include:
•
Plant Community composition and structure;
•
Ecosystem Processes including fire occurrence, erosion, sedimentation rates,
hydrological regimes, nutrient cycling, among others;
•
Higher Trophic Levels such as extirpation or endangerment of native plants and
animals; and
•
Genetic Integrity including production of fertile or sterile hybrids that can out
compete or lower the reproductive potential of native species.
Although ordering invasive species is usually the primary objective of virtually all
procedures, surprisingly little is known theoretically or operationally about which
invasive species cause the greatest ecological impacts. Moreover, less is known about
the relative impacts of an individual species from one ecological setting to another
(Parker et al. 1999).
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Such fundamental knowledge gaps make assessments of complex processes at best highly
spurious, and at worst virtually impossible. Consequently, questions regarding impacts
are of limited value in an objective analysis until more information is available on the
actual effects of weeds on ecological structure and processes.
Subjectivity
Subjectivity is perhaps the most consistent criticism levied against protocols for weed
ranking and weed risk assessments (Simberloff and Alexander 1998; NRC 2002; Powell
2004). These systems rely principally on observational data, or data that originate in
different geographic locations and climates compared to the present distribution of the
weed.
Currently, there is no known way to collect, or introduce, the actual data that are required
to conduct unbiased weed assessments using existing protocols (NRC 2002). “Measures
of uncertainty,” and the option for the assessor to record that the answer is “unknown,”
are common procedures used to offset the subjectivity introduced in these protocols
(Hiebert et al. 1999; NRC.2002; Morse et al. 2004; DEFRA 2005). Unfortunately, these
measures of uncertainty do not compensate for inadequate or incorrect information.
“Expert” opinion is usually the other main source of information used to determine the
invasive characteristics of weed species, and their anticipated environmental and
economic impact in new environments (Hiebert and Stubbendieck 1993; NRC 2002;
Powell 2004). Opinions of a weed’s performance in a new environment, however, often
vary among professionals even when they have the same information (Simberloff and
Alexander 1998; Parker et al. 1999). Although disagreement among experts is not
unexpected (Parker et al. 1999), the prospect that different results and interpretations can
be produced using the same protocol detracts from the aspiration of veracity and
repeatability generally sought in assessment protocols (NRC 2002).
Scoring Procedures
Risk assessments have used several approaches to scoring weed threats and impacts. One
technique simply sums the scores for each of the “risk elements” evaluated in the
assessment that reflect impact. Relative weed risk potential is assigned in descending
order of the highest score (Hiebert and Stubbendieck 1993; Hiebert et al. 1999; ANHP
2005).
A second approach allocates weights to individual questions, or to each of the risk
elements, to reflect their relative contribution to potential risk. The specific procedures
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for weighting and for calculating a final score vary among protocols (Heffernan 2001;
Morse et al. 2004). Other protocols use more complicated algorithms that involve
summing scores, weighting risk elements, converting scores to probabilities, and
combinations of these mathematical procedures (APCC 2002; DEFRA 2005; EMEC
2007a). In addition, uncertainty may be involved in some of the calculations but not
others (DEFRA 2005; EMEC 2007a).
Many risk assessments use slightly different characteristics and methods for determining
probabilities or likelihoods, depending on the specific conditions where they are applied
(DEFRA 2005; EMEC 2007a). These changes can be valid, though, if the ecological
characteristics of the plant, the environmental features of the location, or both, justify
modifying the procedure to accurately account for local conditions.
Although the simple enumeration of scores for each question has been considered a
weakness in some protocols, no analysis has been conducted to validate any algorithms
used to classify invasive plants and their impacts (NRC 2002). Despite this, weed risk
assessments are likely to continue in their present form until a less subjective and more
quantitative system can be developed. In the interim, weed risk assessments must have a
clear rationale for scoring if protocols are to gain credibility (NRC 2002). Risk
assessments that are carefully documented and explain the logic of their rating systems
are more likely to result in repeatable conclusions by different assessors, and allow public
scrutiny and independent evaluation (NRC 2002).
Abiotic and Autecological Features
All plants, including weeds, are simultaneously affected by the amount of heat, light,
moisture, and nutrients available to them (Daubenmire 1974). Consequently, their
relative distribution on the landscape reflects the degree to which a particular
environment can provide an optimum mix of these factors for individual plants to
complete their life cycle, and for populations to grow and spread.
From a predictive standpoint, the specific components driving weed populations cannot
be evaluated individually. Consequently, indices that integrate these factors are sought to
help predict where suitable habitats for weeds exist, and where other habitats limit
invasive weed growth. So far, four inter-related components have been considered in risk
assessment procedures: climate, habitat type, light, and disturbance.
Climate. Although some non-native plants can adapt well to a variety of habitats, they
usually are most successful within the climatic variation of their native range first
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(Panetta and Mitchell 1991; Radosevich 2005). In the past, climate has not entered into
weed risk assessments except in the most rudimentary manner (Heffernan et al. 2001;
APCC 2002; ANHP 2005; EMEC 2007; Pheloung 2007). More recently, however,
“climate matching” appears to be gaining interest and application in some of these
assessments (Busby 1986; Panetta and Dodd 1987; Panetta and Mitchell 1991; Cramer
and Solomon 1993; Lehtonen 2004; Randall and Stuart 2000; Williams 2003; Powell
2004; Westbrooks and Madsen 2006).
Ideally, climate matching should compare the climatic conditions from the species’ native
and extended weedy range to the climate in the present area where it grows or could
infest. One of the limitations of climate matching, however, is the assumption that
climate alone determines the distribution of invasive plants in new environments, even
though other abiotic and biotic factors (including random dispersal) may be important
(Sutherst and Maywald 1985; Mack 1995; Davis et al. 1998).
Habitat Type. Several analyses have concluded that the most useful criterion for
predicting whether an invasive plant will thrive in a new environment is the history of
where it has succeeded elsewhere (Reichard and Hamilton 1997; Daehler and Carino
2000; Daehler et al. 2004). Such comparisons are usually expressed in terms of
“ecosystem types” or “habitat types,” which are described in relatively board terms such
as aquatic, riparian, grassland, open forest, closed forest, etc.
Even though habitat matching was addressed in some of the procedures reviewed
(Heffernan et al. 2001; NatureServe 2004; DEFRA 2005), it has generally received a low
weighting for scoring. While climate and habitat matching may have limitations and be
considered a first evaluation for screening new species (Kriticos and Randall 2001), there
is growing agreement that both deserve greater emphasis in profiling invasive plants than
most current weed risk assessment protocols assign now (NRC 2002). Recent protocols
have given habitat matching a higher profile (Warner et al. 2003; Duncan 2005), and the
Nature Conservancy suggests that all species proposed for introduction into the US
should be screened based on invasiveness outside their native range (Randal and Gordon
2007).
Requirement for Light. Shade can either promote or restrict the potential for an
invasive plant to establish and flourish in new environments. Knowledge of the light
requirements of weed species can be a powerful tool for predicting weed distribution in
novel environments. So far, shade tolerance has not been given much consideration in
most weed risk assessment protocols (Heffernan et al. 2001; DEFRA 2005; IAPP 2006;
Pheloung 2007).
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Disturbance. The importance of natural and anthropogenic disturbance in promoting the
establishment and spread of weeds is generally well accepted. For example, Radosevich
(2005) considers disturbance as a primary factor favoring plant invasions, while PrieurRichand and Lavorel (2000), state that disturbance is “the only clear factor lending to a
community’s invasibility.”
Most weed risk protocols have concentrated on native plant communities in conservation
lands and largely ignored disturbance as a factor promoting weed populations (Hiebert
and Stubbendieck 1993; Heffernan, et al. 2001; Morse et al. 2004; ANHP 2005). Even
on conservation lands, natural events such as fire, floods, and storms disturb soils, modify
plant communities, and create temporary openings in forested canopies. These
disturbances allow entry of invasive plants when light, moisture, heat, and competition
are modified. Outside conservations lands (protected lands), forest harvesting, road
construction, livestock and wildlife grazing, and recreation also create conditions that
foster invasive plant dispersal into new areas (APPC 2002; Radosevich 2005).
Strengths of Weed Risk Assessments
Currently, the limitations of weed ranking and risk assessments appear to exceed the
strengths and benefits. Most limiting is the extent to which they both depend on
subjective decisions to make “scientific” predictions (Hiebert and Stubbendieck 1993;
NRC 2002; Morse et al. 2004; Simberloff 2005). Nevertheless, weed risk assessments
have value, provided that conclusions drawn from them are supported with relevant data,
scientific literature, and they are peer-reviewed (NRC 2002).
One of the most important benefits of these systems is that they provide a consistent and
logical decision-making process for ranking exotic plants. Such an analytical framework
encourages biologist and managers to consider the full range of factors and consequences
of their decisions. If the procedure documents the methods used, rationale for decisions,
and the assumptions made in conducting an assessment, then assessors can conduct
evaluations that should be consistent and defendable within the bounds of the knowledge
available and the assumptions made (Hiebert and Stubbendieck 1993; NRC 2002).
Consistently using an analytical tool helps ensure that ecological knowledge contributes
to the process, and can reduce the subjectivity often associated with decisions based on
personal judgment and precedent alone. Consequently, the credibility of management
decisions is enhanced when they are based on acceptable biological information (Hiebert
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and Stubbendieck 1993). Additionally, the process of conducting risk assessments may
be as valuable as the specific results they produce because they provide a medium for
gathering and analyzing relevant information and knowledge about invasive plants, and
their management (NRC 2002).
Experience with weed ranking and weed risk assessments suggests that the number of
weed species of potential concern is often reduced when assessments are conducted by
experienced ecologists who have a sound understanding of weed ecology (Hiebert and
Stubbendieck 1993). However, the results of these analyses should be considered as
guidelines and not interpreted as definitive.
Summary and Conclusions
Weed Ranking Systems and Weed Risk Assessments are terms that are now
commonplace in the invasive plant and weed management vernacular. Although these
tools have similar origins and overlap extensively in objectives, methods, and scope of
application, there are differences. The following summary identifies the most pertinent
features of both tools (hereafter collectively called “weed assessment tools”) based on the
survey of procedures listed above and additional readings.
•
•
•
•
•
•
Weed Ranking Systems were developed in the mid-1980s as tools to
discriminate among innocuous non-native plants and those that could threaten
natural resource values primarily in US national parks.
Weed Risk Assessments emerged in late-1990s, and blended weed ranking
objectives with other legal, political, and policy issues involved with the
prevention, introduction, spread, and control of exotic organisms at a national and
international level.
Numerous guidelines provide general direction on the content and methods for
conducting weed risk assessments, but specific protocols vary considerably in
scope and application.
The focus of most weed assessment tools has concentrated on native plant
communities and especially on conservation lands.
Almost all weed assessment tools exclude agricultural lands and native plant
communities outside conservation areas, where disturbance usually plays a
significant role in promoting the introduction, establishment, and spread of
invasive plants.
Weed assessment tools combine professional (expert) opinion with “scientific”
documentation to predict relative invasiveness among non-native plants based on
a set of predetermined criteria.
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•
•
•
•
•
•
•
•
Weed assessment tools have been criticized as narrowly focused, subjective, often
arbitrary and unquantifiable, and subject to political interference.
There is concern that the ability of weed assessment tools to identify potential
invasive plants in new environments is severely limited because they lack broad
scientific principles and reliable procedures to make such determinations.
Most weed assessment tools rely heavily on biological traits to determine
“invasiveness” and to project “impacts.” The inability of biological traits to
predict actual invasions has created skepticism, and some doubt whether they will
ever successfully predict which species are invasive and where.
Ecological impacts of weeds on plant community composition, structure, and
ecological processes are the most common criteria used in weed ranking and risk
assessments.
Although ordering invasive species is essential for prioritizing management
efforts, little is known about which species cause the greatest ecological impacts
or the relative effects of the same species from one ecological setting to another.
Consequently, questions regarding impacts are of limited value for objective
analysis until more information is available on the actual effects of weeds on
ecological structure and processes.
Measures of uncertainty, and the option for assessors to record that answers are
“unknown,” are common procedures used to compensate for the subjectivity
introduced in these assessments.
Climate and habitat factors have not been used extensively as predictors in weed
assessments, although interest in “climate matching” is growing and some
procedures contain questions regarding climate. Similarly, other habitat factors,
such as shade tolerance and disturbance, have only begun to infiltrate weed risk
protocols recently.
Generally, people with good taxonomic and ecological skills can reduce the
number of weed species of potential management concern when the criteria for
weed ranking are rigorously applied. Training will be required to ensure
consistent application of these tools.
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POTENTIAL RANKING PROCEDURES
No weed ranking system presently exists in British Columbia that has widespread
endorsement. Invasive plant management in BC, however, could benefit from a simple,
objective, and scientifically-based process that evaluates the potential threat of individual
weed species, and the susceptibility of ecologically diverse habitats. Such a tool should
be provincial and regional in scope, but flexible enough to address local needs.
A comprehensive approach to listing invasive species should recognize that weeds have
variable potential in different environments, but effective management requires
agreement among stakeholders in assembling provincial, regional, local and management
mandated lists. Accord among participants fosters communication, cooperation and joint
understanding of invasive plant issues and solutions.
Unique Features Affecting Weed Populations in BC
A weed ranking system for British Columbia must take into account the unique features
of the province. British Columbia covers nearly 1 million square kilometers, extends
over nearly 110 of latitude, and spans an elevational range from sea level to 5950 m on
Mount Logan. This encompasses a region nearly equivalent to the combined area of
Washington, Idaho, Oregon, and California; and a latitudinal variation similar to that
from San Francisco to the BC border. As a result, the province has a diverse climate
ranging from dry desert-like conditions in the southern interior, to Mediterranean and
temperate forests on the coast, and to alpine and arctic climates at the highest elevations
and far north.
Plant communities reflect the diversity in climate, soils, and topography in British
Columbia. Fourteen biogeoclimatic zones and over 75 subzones (Meidinger and Pojar
1991) have been classified in the province. Each of these can be further divided into
smaller units and contain many seral vegetation types. This range in climate and
vegetation types provides enormous opportunity to both accommodate invasive species in
some biogeoclimatic units, and to restrict them in others. It should be recognized that
ecological conditions in British Columbia will likely meet or exceed the ecological limits
of many species of Mediterranean or Central Asia origin somewhere in the province.
Although numerous biological factors contribute to the introduction and spread of
invasive plants globally, forest canopy is one of the most pervasive influences controlling
their distribution throughout most of British Columbia. The presence of climax forest
stands, or seral communities dominated by deciduous trees and shrubs, restricts weed
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introduction and growth for most species that are already in BC, or those that immigrate
north from grassland communities in the United States. Conversely, the removal of
canopies at any seral stage can provide favorable conditions for invasion and further
spread, especially when soils have been disturbed when the canopy is removed.
About 3% of the land base in British Columbia is agricultural lands (MOAL 2006), while
14% resides in provincial parks and protected areas (MOE 2006). Regional parks and
conservations areas also contain other “natural areas.” The larger part of the provincial
land base (>80%), however, remains forest land, rangeland, alpine, and urban areas;
which fall under federal, provincial, corporate, and private jurisdictions. These areas are
primarily managed as natural ecosystems used for a wide range of purposes such as
forestry, mining, livestock and wildlife grazing, recreation, and other activities.
Jurisdictions and land use practices often overlap, and are not equally distributed
throughout the province. Therefore, each jurisdiction and land use represents a different
opportunity or threat for weed invasion and control.
To be effective, a weed ranking system must address species within all jurisdictions and
disturbance regimes throughout the province. Although the potential for weed invasion
may vary significantly between highly disturbed agricultural lands and minimally
disturbed natural areas, a weed ranking system must take into account effects on all
provincial lands including agricultural, natural areas, and integrated-use lands.
Review Objectives
In addition to the weed ranking and risk assessment systems reviewed, many more
protocols are available world-wide that could be drawn upon in planning a weed ranking
procedure for British Columbia (Thomas 1999; Tye 1999; Duncan 2005; Westbrooks and
Madsen 2006; Pheloung 2007). It may be tempting to modify an existing system for
immediate application, however, none of the protocols reviewed appear satisfactory for
use in the province in their present format.
Before adopting a particular approach, the objectives for a weed ranking system in BC
requires considered contemplation. Such critical review is imperative given the wide
range of products and approaches currently available, and the large variation in
objectives, requirements, and criteria used in these tools. In particular, the objectives for
a weed ranking system in BC should consider the following questions:
•
What will the tool be used for? Is the tool intended for ranking weeds within an
individual Ministry, for provincial scope weed ranking through the Invasive Plant
Council, or is it intended as part of a risk assessment process for all invasive
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•
•
organisms as suggested in the Invasive Alien Species Framework for BC (Rankin
et al. 2004)?
Is there a need for a comprehensive tool that embraces the ecological,
physiographic and geographic variability in the province? Will it focus on
protected and conservation lands or consider the broader perspective of all land
susceptible to weed infestation in the province?
Will the tool apply to the wide range of jurisdictions that must by considered in
developing a comprehensive system that will achieve broad acceptance?
Potential Weed Rating System
The selection of criteria for weed ranking and risk assessments has gone through
considerable vacillation over the last two decades as researchers and managers attempt to
determine the best mix of biological, physical, and management factors to use in these
assessments. Most previous protocols have emphasized criteria related to “invasiveness”
and “impact,” which are usually unknown or highly speculative. In contrast, relatively
less importance has been assigned to climatic and habitat factors, which can be more
objectively defined, although these factors are becoming more prominent in new
protocols.
Three primary factors are suggested for a weed ranking system in BC: 1) Environmental
Tolerance, 2) Abundance and Distribution, and 3) Management Feasibility (Table 8).
Each primary criterion is broad in scope and implies a “potential effect” that is not
measurable. Secondary criteria are indicators more directly related to weed biology,
ecological adaptation, current and potential distribution, and likelihood of control.
The veracity of any weed ranking system depends upon the quality of information
available to assess the primary and secondary factors for each species. Therefore, the
secondary criteria proposed have emphasized questions where actual data can be acquired
or reasonable estimates can be made. Some of these criteria may require more precise
definition and amplification if they are selected, and other criteria can be considered
depending on the specific objectives and scope of the weed ranking system. General
guidelines for selecting primary and secondary criteria for weed ranking are found in
Table 9.
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Table 8. Potential criteria for ranking invasive plants in British Columbia.
Environmental Tolerance
1
Adaptation of species to BC climates.
Range in climatic conditions the plant species is adapted to in its native range compared to local
climates in British Columbia.
2
Adaptation of introduced species in novel environments to BC climates.
Range in climatic regimes within the introduced range of non-native species outside BC compared
to climatic profiles for actual or potential habitats occupied by the species within the province.
3
Proportion of province, region, local area with similar climate.
The geographic area (hectares) of the province with a similar climate as the native or exotic range
of the plant defines its potential distribution provincially, regionally or a locally.
4
Habitat suitable for supporting populations.
This analysis entails matching ecological types in British Columbia to analogous types outside
BC, and throughout regions of the province. Ecological matching is primarily based on
biogeoclimatic or ecoregion classification. This analysis will require comparisons with other
vegetation classification systems to make “relative” comparisons. Ecological types include forest,
grassland, riparian, aquatic, alpine, etc.
Abundance and Distribution
5
6
7
Biological characteristics that promote introduction and establishment.
The most conspicuous trait that will limit weed infestation in BC on most forest range is shade
tolerance. Other factors, however, that promotes dispersal and establishment to sites where light
conditions are adequate may also be relevant.
Does the species occupy similar habitats in adjoining US states, Alberta or Yukon.
The relative proximity of species already in the province, or species that inhabit similar habitats to
those in BC outside the province, provide an estimate of potential for dispersal.
Current estimated area covered provincially, regionally.
This question attempts to quantify in general terms the area presently covered by individual
invasive plants (high, medium, low). Documentation of the area cover combined with the habitats
used provides an index of plant success and relative threat to successfully establish in similar
habitats in the province.
8
Recent trend in total area infested within the province.
Estimated rate of spread provincially, regionally, locally based on observations (increasing,
stable, declining).
9
Disturbance regime matching.
Requirement for natural or anthropogenic disturbance for the species to establish and form
persistent populations. Generally these relate to land use practices that result in various kinds,
degrees, timing and frequency of disturbance.
Management Feasibility
10
Likelihood of control.
Relative ability to control invasive plants using cultural/mechanical, chemical or biological means
within the context of integrated pest management principles. The effects of management on nontarget organisms are also important, especially when weeds occur in natural environments.
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Table 9. General guidelines for selecting weed ranking criteria.
1.
All criteria should relate to clearly defined objectives for ranking weeds in British
Columbia.
2.
Criteria must be relevant to British Columbia conditions and management priorities
(ecologically, economically, and socially).
3.
Each criterion can be evaluated in an objective and unbiased manner (accuracy)?
4.
Each criterion should be easy to measure or estimate; or factual information can be
acquired to provide an answer.
5.
Repeated assessments (same person, other people) will produce the same result
within reasonable limits (precision).
6.
Criteria should relate to legal requirements and government policy where appropriate
and necessary.
7.
Each criterion should contribute (additive) to making management decisions.
8.
All statements for each criterion should be clearly stated in simple, but accurate, text
with minimal extraneous jargon.
9.
Where possible, each criterion should be presented unaccompanied by other criteria
that could result in contradictory or ambiguous answers for the same question.
10. The criteria should be flexible so they can be adapted to meet local or regional needs
as required.
Environmental Tolerance
Environmental tolerance endeavors to predict the response of new non-native plants into
British Columbia by matching existing climates and habitats in the province with habitats
where the species has succeeded in its native or introduced range elsewhere (Table 8).
Such analyses can provide meaningful information on the potential range and
distribution of invasive species within the province although they do not imply measures
of relative “invasiveness” or “aggressiveness.”
Climate matching could be achieved using computerized tools such as CLIMEX (Vickery
1974) or BIOCLIM (Bioclim 2007), or by using climate atlases such as Hardiness Zone
Maps (USDA 1990; AAFC 2000). Other products may be available such as climate
maps and long-term climate data from Environment Canada.
Habitat suitability is the interaction between the abiotic components of the habitat and the
biological characteristics of the plant. Habitat-matching can be accomplished by
surveying literature to determine the native distribution of individual species (range in
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latitude, longitude, and elevation), their non-native global distribution outside North
America, and their current North American distribution. This information can be
compared to the species’ current distribution in British Columbia or projected by
comparing analogous habitats where the plant is found elsewhere with biogeoclimatic
units in BC. These comparisons can provide a “suitability index” that indicates a
“likelihood of invasiveness.” Actual impacts on habitats, native plants and animals; and
rare and endangered species are unknown. These can only evaluated only by
unquantifiable inference.
Abundance and Distribution
Abundance and distribution depicts the diversity of conditions under which a species can
subsist (Table 8). Abundance generally refers to the size of a population and relates to
ecological factors such as cover, density, and frequency. Distribution may be
geographically oriented, or it may refer to the extent of infestation within various
ecological types.
Many biological characteristics are used in weed risk assessments to predict invasive
potential. Shade intolerance , however, is likely the most important biological trait
affecting the introduction, establish and spread of invasive plants in BC, where forest
canopies play such an important ecological role in plant communities. Most invasive
species presently in BC are best adapted to open conditions and are mostly shade
intolerant. Therefore, those biogeoclimatic units with no canopy, or with an open forest
canopy, are at greater risk of invasion than those that will attain a closed canopy.
Consequently, weed adaptation to shade (shade tolerance) is an important biological
factor that determines the ultimate range of a species.
Disturbances such as logging, fire, and beetle kill can create artificial opening in
biogeoclimatic units that would otherwise be forested. Under these conditions, the
distribution and abundance of species, which were previously restricted by shade, can
change dramatically as previously marginal habitats become suitable. Local experience
provides ample information to predict shade tolerance of many non-native species that
presently occur in the province. Other information can be obtained through literature and
experience elsewhere can help predict response of new invaders.
Confirmation of the presence of an invasive species and its estimated area provincially,
and regionally, are the first steps in establishing its persistence and distribution in the
province. As more inventory data become available, the distribution and the frequency of
infestations can be determined on a local, regional and provincial basis. With GIS
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analyses, these data can be superimposed on biogeoclimatic maps and climate maps (if
they exist) to provide information on the geographic distribution of species, and the
habitat and climatic regimes where the species occurs. Results from these analyses can
help develop projections of possible infestations elsewhere.
Disturbance is a primary factor favoring plant invasions in virtually all ecological
systems (Radosevich and Holt 1984; Prieur-Richand and Lavorel 2000). Land use
practices such as forest harvesting, road construction, livestock and wildlife grazing,
agriculture, urban, and recreation remove forest or herbaceous cover and disturb soils,
which permits invasive plants to disperse, establish, and form persistent populations in
new areas.
Land use and disturbance can be inferred from forest cover maps and possibly overlaid
onto biogeoclimatic maps to determine cover and distribution of various land use types
by biogeoclimatic unit. Plant response to disturbance and abundance under various land
uses can potentially be mapped from inventory data and specific responses of plants
determined from literature review. Not every weed will necessarily need to be cross
referenced against each land use, but important land use activities that promote weed
invasion can be identified for the most important species that are presently known.
Management Feasibility
Management feasibility evaluates the ability to prevent the introduction, establishment,
and spread of invasive plants (Table 8). It also assesses the secondary impacts of
treatments on non-target species and the physical environment, as well as the financial
costs of associated with management of the species.
Species that are difficult to manage are expected to have a greater potential to adversely
affect other species, environments, and economies than those that can be easily
controlled. Predictions on expected management feasibility can be determined from
current experience locally, or for the species in similar habitats elsewhere. Literature
already exists in British Columbia to provide as background on numerous species (OLA
2002), but information on other species can be gathered through literature review. The
implications of management practices on non-target organisms such as native plants and
red- and blue-listed species are also important to consider. Predictions and costs of the
expected effects of treatment applications can be gathered from local experience,
professional advice from neighboring states or provinces, or literature searches where
relevant information is available.
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Rationale for Provincial and Regional Ranking
A classification of invasive plants as of provincial, regional or local significance should
follow a hierarchical order based primarily on climate, biogeoclimatic distribution,
geographic distribution, and land use. Although the three categories are described
separately, they are linked and overlap. Figure 1 provides a simple priority matrix to
assist decision-making.
Provincial Priority. A plant of provincial priority is one that is adapted to a broad range
of climate and habitat conditions, and has the potential to establish and limit land use
management objectives within the majority of the regional units designated. Provincial
and regional invasive plant lists should be prepared that are consistent with provincial
level government policies and responsibilities in various ministries.
Invasive Potential
High
Low
Medium Priority
High Priority
Adapted to BC climates and wide range
of biogeoclimatic units. Presence
confirmed and dispersal potential high.
Containment unlikely; biocontrol
possible.
Adapted to BC climates and wide range
of biogeoclimatic units. Presence
confirmed and dispersal potential high.
Eradication and containment possible;
biocontrol likely.
Low Priority
Medium Priority
Poorly adapted to BC climates and few
biogeoclimatic units. Presence
confirmed and dispersal potential low.
Containment and biocontrol low or not
possible.
Marginally adapted to BC climates and
few biogeoclimatic units. Presence
confirmed and dispersal potential low to
moderate. Containment/biocontrol likely.
Manageability
High
Figure 1. Potential priority matrix to rank invasive plants in British Columbia.1, 2
1.
Invasive Potential. Ecological suitability to local climates and suitability to habitat conditions
in British Columbia. Habitat conditions are best described as biogeoclimatic units, likely at the
subzone level.
2.
Manageability. Expected responses to management based on local experience in British
Columbia or experience from abroad in similar habitats and climates.
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Regional Priority. A plant of regional priority is one that is adapted to a moderate range
of climate and habitat conditions, and has the potential to establish and limit land use
management objectives within most of the local sub-units designated within the region.
These may be based on geographic designations (MOFR Districts, Electoral Areas, etc.),
biogeoclimatic units within the region, or both.
Local Priorities. A plant of local priority is one that is adapted to a narrow range of
climate and habitat conditions that has the potential to establish and limit land use
management priorities within particular biogeoclimatic units, plant community types
(aquatic, riparian, grasslands), land use practices (range use, forestry, utility corridors,
parks and protected land), and/or sensitive areas (sites for red- and blue-listed species).
Lists at this level they should focus more on specific jurisdictional concerns and
mandates within local ecological areas.
CONCLUSIONS AND RECOMMENDATIONS
No weed ranking system presently exists in British Columbia that is widely endorsed and
systematically used. A simple, objective, and scientifically-based process for ranking
invasive plants, and evaluating the relative susceptibility of ecologically diverse habitats,
would be a valuable tool for management. Such a tool should be provincial and regional
in scope, but flexible enough to address local needs. None of the weed ranking or risk
assessment procedures reviewed, however, is adequate in its present format to meet the
needs of British Columbia. The following recommendations are made for consideration
in developing a weed ranking system for the province.
•
A weed ranking system for British Columbia should account for the unique
features of the province including its geographic size, and its diversity in climate,
soils, and vegetation.
•
A weed ranking system for BC should be cognizant that many species of
Mediterranean or Central Asia origin will be at their northern limits of ecological
tolerance somewhere in the province. Therefore, focus should be directed to
discerning the relative susceptibility of habitats (biogeoclimatic units) to the
introduction and spread of invasive species. Attention to biological traits should
center on those characteristics that clearly promote or limit spread of weed
species, such as shade tolerance.
•
BC needs to be divided into “management zones” based on geographic separation.
Ministry Forests and Range districts, or regional district electoral areas, are two
possibilities of existing, recognized boundary designations in the province. These
divisions provide a hierarchy of geographic areas of diminishing size from
province, to regions and local areas. Divisions should be based on anticipated
management authority so that geographic areas and management responsibilities
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are linked. Whatever choices are made, there should be consistent application
across the province.
•
Criteria for land uses with different management priorities, legal responsibilities,
and government policies need to be considered in setting priorities. Local
concerns, and the mandated responsibilities of particular agencies, may over-ride
geographically based priorities occasionally, and the flexibility to accommodate
such possibilities needs to be provided for.
•
Weed ranking needs to be linked to inventory and mapping so that weed lists can
be updated at regular intervals based on actual data collected in BC.
•
Generally, invasive plant lists should be regarded as guidelines rather than a
definitive list of ranked species. Weed lists should be considered “living lists”
with the expectation that they will modified as more knowledge is gained, and as
provincial, regional, and local priorities change over time.
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Simberloff, D. 2005. The politics of assessing risk for biological invasions: the USA as a
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control. Canadian Wildlife Service, Ottawa, Canada. 121 p. http://www.cwsscf.ec.gc.ca/publications/inv/index_e.cfm
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Appendix 1. Glossary of terms.
Annual (plant). A plant species that lives for only one year or growing season.
Biodiversity (Biological diversity). The diversity of plants, animals, and other living
organisms in all their forms and levels of organization, including genes, species,
ecosystems, and the evolutionary and functional processes that link them.
Biogeoclimatic zone. A geographic area having similar patterns of energy flow,
vegetation, and soil as a result of a broad, regional climate.
Biological control. The use of living organisms, such as predators, parasitoids, and
pathogens, to control invasive plants.
Chemical control. The application of herbicides to control or eradicate plant species.
Climate. The average weather conditions of a place over many years.
Community. Any group of organisms interacting among themselves.
Containment. An invasive plant practice that aims to geographically isolate infestations
and prevent them from increasing beyond the edge of their current infestations.
Control. An invasive plant practice that aims to prevent seed production and recruitment
of new plants within the target patch, and eventually reduce the area and density of
the target plant over time. Control measures acknowledge that a low level of the
invasive plant will likely persist after treatment.
Crown land. Land that is owned by the government of Canada or British Columbia.
Cultural control. An invasive plant management practice that manipulates plant
populations by cultivation, pulling, cutting, or other hand-applied techniques.
Dispersal. The scattering of seeds or spores of a plant to a new habitat.
Ecosystem. Organisms together with their physical environment, forming an interacting
system, inhabiting an identifiable space.
Endangered species: Any indigenous species, or sub-species, threatened with imminent
extinction throughout all, or most of its range.
Environment. The sum of all external conditions that affect an organism or community
and influence its development or existence.
Eradication. Elimination of every individual plant of an invasive plant population,
including all viable seeds, and vegetative propagules.
Establishment. The process of a plant species entering and reproducing in a new habitat at
a sufficient level to ensure continued survival of subsequent generations without
immigration of new plants from outside the habitat.
Habitat. The natural dwelling of a plant or animal, including all biotic, climatic, and
edaphic factors affecting life.
Herbicide. A chemical that kills or regulates growth of plant species or groups of species.
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Hybrid. A plant (or animal) that is the product of a cross between two genetically
different plants.
Integrated pest management. An approach to invasive plant management that uses
more than one control option including prevention, cultural, mechanical, chemical,
and biological controls in an integrated program.
Invasion. The arrival of an organism in an area where it was not formerly represented.
Invasive plant. A plant that is non-native to the ecosystem under consideration and
whose introduction causes, or is likely to cause, economic or environmental harm or
harm to human health.
Landscape. The fundamental characteristics of a specific geographic area, including its
biological composition and physical environment.
Mechanical control. Control of invasive plants by physical and mechanical means such
as mowing, cultivation, chain sawing, and weed-whacking.
Native plant. A plant species that are part of the original flora of an area.
Non-native. A species that is not native to the region in which it is found.
Non-target. Any plant that a management practice is not aimed at, but may accidentally
be injured by the application.
Noxious weed. Any plant species so designated by the Weed Control Act of British
Columbia.
Perennial. A plant species that lives for more than two years.
Plant community. An association of plant species growing together in different areas
with similar site characteristics.
Prevention. All activities that interrupt the dispersal of new invasive plant species into a
geographic area or specific location where they were not previously found.
Propagule. A plant part, such as a bud, tuber, root, or shoot, that can be detached and is
able to grow in a new environment.
Range. The geographical area in which a plant or animal species normally lives or grows.
Risk. In species risk assessment, the probability that an adverse effect (injury, disease, or
death) will occur under exposure to a specific agent.
Risk Assessment. A process that includes the identification, assessment, management
and communication of the risks associated with a particular organism or function.
Species at risk. A species that is extirpated, endangered, threatened, or of special concern.
Target species. Invasive plant(s) that are the subject of eradication, control or
containment.
Weed Ranking System. A process to evaluate the relative threat of non-native plant
species on plants, animals and other resource values, and order species relative to their
ability to be managed.
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Weed Risk Assessment. A process for identifying and describing the ecological and
economic risks associated with the introduction or transfers of non-indigenous species
on native species, habitat, or economic values in new environments.
Weed. 1) A plant growing where it is not wanted, 2) A plant that interferes with
management objectives for a given area of land at a given point in time.
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Appendix 2. Exotic plant ranking system questions.
I. Significance of Impact
A. Current Level of Impact
1. Distribution relative to disturbance regime (10 points)
2. Abundance
a. Number of populations (stands) (5 points)
b. Arial extent of populations (5 points)
3. Effect on natural processes and character (15 points)
4. Significance of threat to park resources (10 points)
5. Level of visual impact to an ecologist (5 points)
B. Innate Ability of Species to Become a Pest
1. Ability to complete reproductive cycle in area of concern (5 points)
2. Mode of reproduction (5 points)
3. Vegetative reproduction (5 points)
4. Frequency of sexual reproduction for mature plant (5 points)
5. Number of seeds per plant (5 points)
6. Dispersal ability (5 points)
7. Germination requirements (5 points)
8. Competitive ability (5 points)
9. Known level of impact in natural areas (10 points)
II. Feasibility of Control or Management
A. Abundance Within Park
1. Number of populations (stands) (5 points)
2. Aerial extent of populations (5 points)
B. Ease of Control
1. Seed banks (15 points)
2. Vegetative regeneration (10 points)
3. Level of effort required (15 points)
4. Abundance and proximity of propagules near park (15 points)
C. Side Effects of Chemical/Mechanical Control Measures (15 points)
D. Effectiveness of Community Management (10 points)
E. Biological Control (10 points)
Urgency (High, Medium, Low)
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Appendix 3. Northern Prairie alien plant ranking system
questions.
Significance of Threat Impact (Site Characteristics)
1. Distribution relative to disturbance regime. ( 10 points)
2. Arial extent of populations (answer in per cent or hectares). (15 points)
3. Numerical dominance of species within a community. (15 points)
4. Association with native community. (10 points)
5. Hybridization with native species. (5 points)
6. Degree of threat and impact. (10 points)
7. Effects on management goals. (10 points)
Innate Ability to be a Pest (Species Characteristics)
8. Mode of reproduction. (4 points)
9. Vegetative reproduction. (4 points)
10. Frequency of sexual reproduction for mature plant. (5 points)
11. Number of seeds per plant. (5 points)
12. Dispersal ability. (5 points)
13. Germination requirements. (4 points)
14. Seed banks. (10 points)
15. Competitive ability. (4 points)
16. Ecological effects (select all that apply). (4 points)
17. Known level of impact in natural areas. (10 points)
Difficulty of Control
18. Likelihood of successful control. (10 points)
19. Saturation of surrounding region. (5 points)
20. Effectiveness of community management. (10 points)
21. Vegetative regeneration. (10 points)
22. Biological control. (10 points)
23. Side effects of control measures. (5 points)
Total Points (175)
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Appendix 4. Virginia ranking invasive exotic plant species
questions and scoring.
I. Impact On Native Species, Habitats and Ecosystem
1. Ability to Invade Natural Systems
2. Impact on Ecosystem Processes
3. Impact on Natural Community Structure
4. Impact on Natural Community Composition
5. Conservation Significance of the Natural Area(s) and Native Species Threatened
II. Biological Characteristics and Dispersal Ability
6. Biological Characteristics
7. Other Regions Invaded
8. Speed of Spread
9. Current Trend in Total Range Within United States
10. Potential to be Spread by Human Activity
III. Distribution and Abundance In Virginia and the United States
11. Approximate Number of Distinct Natural Areas or Other Wildlands Infested
12. Extent of the Species US Range Where it has been Identified as a Problem
13. Potential Cover of the Species in Strata Where it Occurs
IV. Management Potential
14. Difficulty to Control Species
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Appendix 5. Australian weed assessment guide.
I. Invasiveness
1. What is the Weed’s Ability to Establish Amongst Existing Plants (3 points)
2. What is the Weed’s Tolerance to Average Weed Management Practices in the Land use (3
points)
3. What is the Reproductive Ability of the Weed in the Land use (3 points)
4. How Likely is Long-distance Dispersal (>100m) by Natural Means (3 points)
5. How Likely Long-distance Dispersal (>100m) by Human Means (3 points)
II. Impacts
6. Does the Weed Reduce the Establishment of Desired Plants (3 points)
7. Does the Weed Reduce the Yield or Amount of Desired Vegetation (4 Points)
8. Does the Weed Reduce the Quality of Products or Services Obtained from the Land use? (3
points)
9. Does the Weed Restrict the Physical Movement of People, Animals, Vehicles, Machinery
and/or Water? (3 points)
10. Does the Weed Affect the health of Animals and/or People? (3 points)
11. Does the Weed Have Major, Positive or Negative Effects on Environmental Health? (3
points)
III. Potential Distribution
12. What Area of the Land use is Suitable for the Weed? (10 points)
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Appendix 6. NatureServe questions and scoring.
Screening Questions
1. Is this a non-native species in the region of interest and is it currently established outside
cultivation? If yes, proceed to question 2; if no Stop.
2. Does this species occur in conservation areas or other native species habitats or is it highly
probable that it will do so? If yes, proceed to criteria below; if no Stop, rank as Insignificant.
I. Ecological Impact (5 questions; 50% of I-Rank Score)
1. Impact on Ecosystem Processes and System-Wide Parameters (33 points)
2. Impact on Ecological Community Structure (18 points)
3. Impact on Ecological Community Composition (18 points)
4. Impact on Individual Native Plant or Animal Species (9 points)
5. Conservation Significance of the Communities and Native Species Threatened (24 points)
II. Current Distribution and Abundance (4 questions; 25% of I-Rank Score)
6. Current Range Size in Region (15 points)
7. Proportion of Current Range Where Species Is Negatively Impacting Biodiversity (15 points)
8. Proportion of Region’s Biogeographic Units Invaded (3 points)
9. Diversity of Habitats or Ecological Systems Invaded in Region (3 points)
III. Trend in Distribution and Abundance (7 questions; 15% of I-Rank Score)
10. Current Trend in Total Range Within Region (18 points)
11. Proportion of Potential Range Currently Occupied (3 points)
12. Long-Distance Dispersal Potential Within Region (9 points)
13. Local Range Expansion or Change in Abundance (18 points)
14. Inherent Ability to Invade Conservation Areas and Other Native Species Habitats (6 points)
15. Similar Habitats Invaded Elsewhere (9 points)
16. Reproductive Characteristics (9 points)
IV. Management Difficulty (4 questions; 10% of I-Rank Score)
17. General Management Difficulty (18 points)
18. Minimum Time Commitment (15 points)
19. Impacts of Management on Native Species (15 points)
20. Accessibility of Invaded Areas (3 points)
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Appendix 7. Alaska weed ranking project questions and scoring.
A. Climatic Comparison
1. This species is present, or may potentially establish, in the following eco-geographic regions:
1) South Coastal; 2) Interior-Boreal; 3) Arctic-Alpine.
2. This species is unlikely to establish in any region in Alaska.
B. Invasiveness Ranking
1. Ecological Impact (40%)
1. Impact on Ecosystem Processes (10 points)
2. Impact on Natural Community Structure (10 points)
3. Impact on Natural Community Composition (10 points)
4. Impact on Higher Trophic Levels (cumulative impact on animals, fungi, microbes, and other
organisms in the community (10 points)
2. Biological Characteristics and Dispersal Ability (25%)
5. Mode of Reproduction (3 points)
6. Innate Potential for Long-Distance Dispersal (3 points)
7. Potential to be Spread by Human Activities (3 points)
8. Allelopathic (2 points)
9. Competitive Ability (3 points)
10. Forms Dense Thickets, Climbing or Smothering Growth Habit, or Otherwise Taller Than the
Surrounding Vegetation (2 points)
11. Germination Requirements (3 points)
12. Other Species in the Genus Invasive in Alaska or Elsewhere (3 points)
13. Aquatic, Wetland, or Riparian Species (3 points)
3. Ecological Amplitude and Distribution (25%)
14. Is the Species Highly Domesticated or a Weed of Agriculture? (4 points)
15. Known Level of Impact in Natural Areas (6 points)
16. Role of Anthropogenic and Natural Disturbance in Establishing (5 points)
17. Current Global Distribution (5 points)
18. Extent of the Species in US Range and/or Occurrence of State or Provincial Listing (5 points)
4. Feasibility of Control (10%)
19. Seed Banks (3 points)
20. Vegetative Regeneration (3 points)
21. Level of Effort Required (4 points)
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Appendix 8. IAPP species prioritization algorithm.
Note to User: Only input information into yellow fields to maintain integrity of embedded
formulas. This ensures the auto-population of an Overall Score.
Criteria
Subtotal Score
Prorate Factor
Prorated Score
Biological
3.8
x 1.3
5.0
Impact
5.0
x 2.0
10.0
Controllability
5.0
x 1.5
7.5
Containability
5.0
x 1.5
7.5
Grand Subtotal
30.0
Overall Score
10.0
Common Name:
Scientific Name:
Biological Criteria
Precede down this list answering each Yes/No question. If the answer
is Yes to any question, then move the appropriate points for that
question over to the right and into the Score column. The Subtotal
will be calculated automatically.
Points
Score
5
5
5
3
4
4
5
3
4
4
5
5
3
3
5
5
5
3
3
5
5
5
Is this species:
New to the area (e.g. Within the last 25 years) and suited to thrive in the local
climate?
Adapted to thrive in an aquatic, riparian or sensitive ecosystem?
Tolerant a wide range of soil conditions?
Able to live in excess of 5 years?
An annual or biennial with high reproductive success?
Able to quickly dominate a site without disturbance and form a dense
monoculture? Or
Capable of slow domination of a site, but in patches without disturbance?
Shade tolerant at some stage of its life cycle?
Geophytic? (it has underground storage organs - bulbs, corms, or tubers)
A producer of seed whose viability exceeds 5 years? (if unknown, then score 2)
Capable of hybridizing naturally?
Apomictic or self-fertilizing?
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Able to reproduce by vegetatively, by stolons, rhizomes, bulbils or other
asexual means?
Seed, spore or cuttings adapted to dispersal by birds/animals, water or wind?
A prolific seed producer @ 5,000 or more seeds per plant? Or
A moderate producer of seed @ 1,000 to 5,000 seeds per plant?
Capable of setting seed after its first year?
Stimulated by mutilation, cultivation, or fire?
A plant woody (including stems or roots)?
5
5
4
5
4
5
4
3
5
4
3
5
Subtotal:
3.8
Impact Criteria
Select the single most applicable impact and place the associated
points into the bottom Subtotal cell.
Points
Score
5
5
Does this species negatively affect:
Human health and/or safety?
- Lethal or toxic to ingest
- Causes pain or discomfort
- Obstructs visibility along transportation routes
- Degrades or poses risk to public infrastructure e.g. Fire hazard
Animal health (domestic or wild)?
4
- Lethal or toxic to ingest
- Causes pain or discomfort
Natural or agricultural environments?
4
- Taints forage crop or is unpalatable
- Reduces crop yields
- Increased erosion or restricted water flow in riparian areas
Native plant communities?
3
- Alleopathic (alters soil chemistry preventing other plants to establish)
- Smothers or climbs on native plants
- Reduces biodiversity
Recreation, and/or human or animal migration?
3
- Impenetrable thickets
- Turf grass invasion
Landscape aesthetics?
2
- Visual blight
Subtotal:
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BC Ministry of Forests and Range
5.0
Evaluation of Ranking Procedures for Invasive Plants
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Controllability Criteria
Select the single most relevant scenario and place the associated
points into the bottom Subtotal cell.
Points
Score
Is this species:
Extremely difficult to control?
5
5.0
- Few if any treatment options and extremely expensive
- Multiple re-treatments are necessary
4
Difficult to control?
- Options are available but expensive
- Re-treatments are necessary
Moderately difficult to control?
3
- Multiple options are available but less expensive
- Re-treatment(s) may not be necessary
Easy to kill but re-treatments are necessary?
2
- Garden escapees, annuals
Subtotal:
5.0
Containability Criteria
Select the single most relevant scenario and place the associated
points into the bottom Subtotal cell.
Points
Score
Is this species found in:
Isolated areas?
5
- Good chance of containment given that species is confined to only a few
areas with jurisdiction
- A newly arrived species
Moderate distribution?
5.0
4
- Moderate change of containment given that species is moderately distributed
throughout jurisdiction
And throughout the subject area (e.g. Is ubiquitous)?
2
- Poor chance of containment given that species is found throughout
jurisdiction
- A species with a long history in the area
Subtotal:
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BC Ministry of Forests and Range
5.0
Evaluation of Ranking Procedures for Invasive Plants
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Appendix 9. Alberta invasive species risk assessment tool
questions and scoring.
BIOLOGICAL CHARACTERISTICS
Dispersal Ability
1. Does the organism possess biological attributes that contribute to long-distance dispersal?
2. What is the rate of dispersal once the species is released or disperses into a new area?
Reproduction Characteristics
3. Is asexual reproduction (e.g. vegetative reproduction or self-fertilization) an important aspect of
this organism’s reproduction?
4. What is the frequency of sexual reproduction?
5. Is there rapid growth to reproductive maturity?
6. Is the production of offspring prolific and consistent?
7. Are the organism’s specific requirements for reproduction available in Alberta?
8. Does the opportunity to hybridize naturally with native species in Alberta exist?
9. Are there known natural predators or natural control agents in Alberta?
Habitat Suitability
10. How many natural sub-regions or watersheds have a climate suitable for the organism?
11. How many natural sub-regions have a physical habitat suitable for the organism’s
establishment?
12. Are the biological requirements of the organism available in Alberta?
INTRODUCTION
Introduction Potential
13. Is this species established in Alberta or present in areas adjacent to Alberta?
14. Do anthropogenic mechanisms for the introduction, re-introduction, or dispersal of this
organism exist?
15. How great is the potential for introduction, re-introduction, or dispersal from human
pathways?
16. How likely is the organism to survive in transit?
17. What is the probability of detecting the organism at the entry point?
18. If introduced, how many individuals are likely to be released?
ENVIRONMENTAL IMPACT
Effect On Environment
19. Is the invasive species known to compete with native species or species at risk for resources?
20. Is the organism a parasite or predator of native species, species at risk, or species of
management concern?
21. Is the species a host or vector for known diseases, parasites, or pests?
22. What is the potential impact on ecosystem processes?
ECONOMIC IMPACT
General Effects
23. What is the potential impact to infrastructure of economic importance?
Effect On Agriculture
24. What is the potential risk to agricultural operations?
Effect On Forestry
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Evaluation of Ranking Procedures for Invasive Plants
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25. What is the potential risk to forestry operations?
Effect On Animal Health
26. What is the potential risk to the health of animal species of economic importance?
SOCIAL IMPACT
Effect On Human Health And Well-Being
27. What is the likelihood of severe health risks after association with this species?
28. What is the impact on aesthetic values and social convenience?
Effect On Culture, Including Community And Traditional Knowledge
29. What is the potential of an invasive species to reduce local biodiversity (i.e. native plants)
valued by local communities or aboriginal communities?
30. What is the potential risk to culturally valued spaces and features?
CONTROL MANAGEMENT
31. Are there potential side effects of control methods to other species?
32. What level of effort is required for the control of this organism?
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