Biological Invasions 2: 187–205, 2000.
© 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Regional and landscape-scale patterns of shrub invasion
in tropical savannas
A.C. Grice∗ , I.J. Radford & B.N. Abbott
CSIRO Tropical Agriculture, Queensland 4814, Australia; ∗ Author for correspondence: CSIRO Sustainable
Ecosystems, Private Bag, P.O. Aitkenvale, Queensland 4814, Australia (e-mail: [email protected];
fax: +61-7-4753 8600)
Received 25 May 1998; accepted in revised form 13 September 2000
Key words: Australian savanna woodlands, Carissa, Cryptostegia, invasion, landscape, shrubs, Ziziphus
Abstract
The shrubby vine Cryptostegia grandiflora and the shrub Ziziphus mauritiana were both introduced to northern
Australia over 100 years ago and have become invasive in savanna woodland environments. Data from a land
resource survey were used to examine regional- and landscape-scale distribution patterns of these species in the
Dalrymple Shire, an area of over 6 21 million hectares in northeast Queensland. Each species was present at <10%
of the 2362 sites examined and most frequent and abundant close to Charters Towers, the major settlement of the
regions. C. grandiflora was recorded at 50% of sites within 20 km of the town and in 14 out of 21 of the region’s
major sub-catchments. Z. mauritiana was recorded at 32% of sites within 20 km of Charters Towers, but in only
three sub-catchments. Little of the variation in frequency and abundance of C. grandiflora and Z. mauritiana was
accounted for by landscape factors, including geology, soils, or vegetation. While survey results do not absolutely
distinguish between history, habitat and disturbance in explaining the weed’s current distributions within the region,
a strong influence of historical factors is suggested. Both exotic species were much less abundant than Carissa spp.,
a native taxon that has purportedly increased in the region in recent decades. In spite of their current prominence as
weeds, there is potential for further increase by both C. grandiflora and Z. mauritiana. This increase could include
expansion from the zone of high abundance and proliferation within that zone. While the results of such surveys
must be interpreted with caution, they can yield useful information about regional patterns of plant invasion.
Abbreviations: DEM – Digital Elevation Model; DLRS – Dalrymple Land Resources Survey; GPS – Global
Positioning System
Introduction
Even invasive plant species that are highly abundant
or that have rapidly expanding ranges are patchily distributed at continental, regional, landscape and finer
scales (e.g. Auld and Coote 1981; Mack 1981; Auld
et al. 1982/1983). This is due to a combination of ecological and historical factors. Ecological factors dictate
that the range of an invasive species will contain some
areas that are highly suitable for the plant and others
that are less suitable. Moreover, habitat suitability may
change over time due, for instance, to intermittent
disturbance. Historical factors are important because
invasive species are not introduced uniformly across
the invaded range and because range expansion is more
rapid in some areas than others. This has been recognized in simple models of the spread of invasive species
(Skellam 1951; Williamson and Brown 1986; Auld
et al. 1979; Auld and Coote 1980; Moody and Mack
1988).
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The spatial patterns of invading species have important practical implications for their management. At the
simplest level, a sound knowledge of where a species
occurs at a point in time facilitates the targeting of
control measures. It is also important to understand
how the various component populations of an invading
species are likely to contribute to the invasion process
and its relationship with landscape. These challenges
are especially critical for species invading large areas
subject to low-intensity land uses.
We analyse broad-scale spatial patterns in the distribution of two non-native weed species, using the results
of a land resources survey that had been conducted to
provide a basis for regional and local planning. The
weeds are the shrubby vine Cryptostegia grandiflora
(Roxb.) R. Br. (rubber vine), and the shrub Ziziphus
mauritiana Lam. (Indian jujube, chinee apple), both of
which are invasive in northern Australian woodlands.
Comparison is made with the co-occurring indigenous
increaser shrub Carissa spp.
We first provide ecological and historical backgrounds that are relevant to understanding how the
three taxa function at regional and landscape scales.
We then compare their distributions and abundance
across the Dalrymple Shire in northeastern Australia.
For each species we analyse data from the survey for
the potential of various factors to explain observed patterns. Finally, we discuss the results, their limitations
and practical implications.
Ecological and historical background
C. grandiflora
C. grandiflora was introduced to Australia as an ornamental from its native range in Madagascar. It grows
as a free-standing shrub or as a scrambling vine that
may smother trees 15–20 m high. Plants produce very
large numbers of wind dispersed seeds that germinate
readily. There is no evidence of long-lived soil seedbanks (Grice 1996). High seedling densities occur in
the immediate vicinity of reproductive plants but most
of these seedlings die in the first dry season after germination. Established plants are vulnerable to fire; over
80% of plants less than 2 m high can be killed by fire
(Grice 1997a,b).
C. grandiflora is unevenly distributed in Australia.
At the continental scale it has not fully occupied
its potential range, which extends across northern
Australia to areas well outside the current Queensland
distribution (Tomley 1998). At the regional scale,
some areas within the current range are more heavily
infested than others. The only published distribution
map of C. grandiflora shows thirteen disjunct areas
that are occupied by the species (MacFadyen and
Harvey 1990). At the landscape scale C. grandiflora
shows clear habitat preferences, being most abundant
in riparian zones and other areas of relatively high soil
moisture. The vine growth form is most common in
riparian zones while the free-standing shrub prevails in
drier parts of the landscape.
The invasion history of C. grandiflora is not well
documented. The species has been present in Australia
since the 1870s, being widely planted in northern and
eastern Queensland. It was declared a noxious plant
in Queensland in 1955, under the Stock Routes and
Rural Lands Protection Act, but unpublished reports
of the Queensland Department of Agriculture and
Stock (cited by Dale 1980) indicate that it had been
recognized as a problem since at least the 1940s.
Crude estimates of the area occupied by C. grandiflora
in Queensland were made in 1973 and 1989 (Grice
1997a). These suggest that there may have been a rapid
increase in the abundance and range of the species
since the early 1970s. This may be linked to increased
cattle numbers and a reduced prevalence of fire (Grice
1997a).
This information suggests the following framework
whereby C. grandiflora has become a major weed:
1. While documentation of introduction and early
spread is poor, C. grandiflora probably spread
from many sites after it was planted around early
settlements and homesteads in northern and eastern Queensland. Evidences for this are that (i)
the Australian populations of C. grandiflora were
derived from plants used as ornamentals; (ii) early
reports (e.g. in Dale 1980) of C. grandilfora came
from widely separated areas; (iii) the current distribution of the species is disjunct (McFadyen and
Harvey 1990).
2. The species became naturalized in riparian and similarly mesic parts of the landscape. This is where
establishment occurs most readily.
3. Spread of C. grandiflora from its initial foci was
largely by wind dispersal. Tomley (1998) suggests
that dispersal by water also occurs. While this is
reasonable, there is no evidence indicating how
important it may be. The current prevalence of
C. grandiflora in riparian areas is probably more
a function of habitat preferences than of dispersal
mechanisms.
189
4. The rate of spread of C. grandiflora increased
in recent decades (Tomley 1998; Grice 1997b),
although there are insufficient data to identify any
discontinuity in the time course of spread. A discontinuity would indicate a change in the factors
governing rates of increase and spread. Such discontinuities could: (i) depend on climatic events with
periods of more rapid population growth and range
expansion coinciding with conditions favourable
for recruitment; (ii) arise from relatively improbable stochastic events such as the transfer of viable
propagules to previously unoccupied catchments;
(iii) result from changes in the environment that are
linked to land use practices. Diminished incidence
of fire between the 1960s and the 1990s favoured
proliferation (Grice 1997a; Grice and Brown 1997).
5. A combination of historical and habitat factors gave
rise to considerable landscape scale heterogeneity in
the abundance of C. grandiflora. Spread was relatively rapid, though episodic, in favourable, mesic
habitats, but relatively slow in other parts of the
landscape.
Z. mauritiana
Z. mauritiana is a shrub or small tree that is native
to southern Asia and eastern Africa. It was introduced to Australia for ornamental and horticultural
purposes sometime during the late 1800s. It spread
to form numerous disjunct populations, particularly
in northeast Queensland. It has been declared a noxious plant in various regions of Australia (Parsons and
Cuthbertson 1992). The species produces fleshy fruits
that, in Australia, are dispersed by domestic, native
and feral mammals and possibly some birds (Grice
1996).
The following framework is proposed to describe the
processes whereby Z. mauritiana has become a weed
in northern Australia and highlights some important
differences between it and C. grandiflora.
1. As for C. grandiflora, invasion by Z. mauritiana
was from many sites of introduction but the latter
exhibits somewhat broader environmental tolerances and is noted as being drought tolerant (Pareek
1983). Z. mauritiana is often abundant in dry as well
as riparian parts of the landscape.
2. Spread of Z. mauritiana was mainly by mammalian
vectors, notably domestic livestock, wallabies and
feral pigs (Grice 1996).
3. There is no evidence for recent changes in the rate
of spread of Z. mauritiana. This observation is
compatible with the resilience of individuals of this
species to fire (Grice 1997a).
4. The rate of spread of Z. mauritiana has been
much slower than that of C. grandiflora. This is
suggested by the fact that, even though the two
species have been in Australia for approximately
the same period of time, the area currently occupied by Z. mauritiana is much lower (James 1995).
Probably, the species has spread out quite slowly
from numerous sites where it was planted, possibly
with establishment being promoted by high levels
of disturbance in the vicinity of settlements.
Carissa spp.
Carissa lanceolata R. Br. and C. ovata R. Br. (currant
bush) are widespread, indigenous Australian shrubs.
Jessop (1981) suggests that C. lanceolata is probably
best regarded as a sub-species of C. ovata. The two are
treated as a single taxon in the analyses presented in
this paper. Three important points of comparison with
the two exotic species are:
1. Unlike the exotic species, Carissa spp. have not
undergone range expansion and there is no evidence
that Carissa spp. have moved into parts of the landscape that they did not occupy before European
settlement.
2. The fruit of Carissa spp. is a berry that is probably
dispersed by birds.
3. Anecdotal evidence suggests that Carissa spp. have
increased in abundance in recent decades, perhaps
as a result of reduced intensity and frequency of fire.
They are regarded by pastoralists as ‘woody weeds’
(Anderson 1993).
Methodology
Survey area
The Dalrymple Shire, centred on Charters Towers
(20◦ 50 S, 146◦ 170 E), is an area of approximately
6,791,300 ha located in the seasonally wet–dry tropics of north Queensland, Australia (Figure 1). Average
annual rainfall ranges from 500 mm in the southwest to
3200 mm in the northeast but variation between years
is high. Most rain falls between November and April.
Most of the area, which forms 50% of the catchment
190
Figure 1. Dalrymple Shire showing the locations of survey sites and the road network.
191
of the Burdekin River, is flat to gently undulating. The
vegetation is dominated by open eucalypt woodlands
with a grassy understorey. Over 90% of the Dalrymple
Shire is used for extensive beef production (De Corte
et al. 1991; Rogers et al. 1999). C. grandiflora has been
present in the region since at least 1910 (White 1923)
and is now widespread and abundant. Z. mauritiana
occurs at many locations throughout the Shire, mostly
in the vicinity of townships. Carissa spp. are abundant
understorey shrubs.
Land Resources Survey
The Dalrymple Land Resources Survey (DLRS) was
conducted between April 1990 and September 1994.
It was designed to map the soils and landforms of the
region at a scale of 1 : 250000 to provide a tool for
regional and local land use planning and management.
Subsidiary issues addressed by the survey included pasture decline, weed invasion, vegetation regrowth and
salinity (De Corte et al. 1991; Rogers et al. 1999).
Sampling involved 2559 × 1 ha sites. Sampling
was stratified according to major geological groups,
landforms, and vegetation types that occur within the
Dalrymple Shire. Sites were selected using the free
traverse technique (Gunn et al. 1988) and described
using terminology detailed in McDonald et al. (1990).
Accessibility was a major consideration in site selection as indicated by the close association between
the locations of sites and roads (Figure 1). Locations
of sites were recorded within 100 m using a global
positioning system (GPS).
Each site was described in terms of its geology,
landform and soil morphology. The depth and width
of gully, rill and sheet soil erosion was recorded by
visual estimates. The dominant grass, shrub and tree
species were identified. Projected foliage cover of the
dominant species and exotic and native weed species
was recorded by visual estimate using broad quantitative categories (Table 1) (Rogers et al. 1999). Data on
native and exotic weeds were recorded for 2362 of the
sites. The sampling unit of 1 ha appeared appropriate
given the broad land type categories used and lack of
evidence that C. grandiflora, Z. mauritiana and Carissa
spp. were patchily distributed at this scale. Each species
is readily detectable in the relatively open vegetation of
Dalrymple Shire. The DLRS data were not specifically
collected for the analyses presented in this paper.
Data management and analysis
Information from the DLRS was stored in the WARIS
database (Rosenthal et al. 1986). Data for sites for
which information on weeds was recorded were transferred from this database to the Arcview package for
spatial analysis (ESRI 1996). Geological, topographic,
vegetation, erosion and soil type categories that were
recorded at fewer than 5 sites were excluded from the
analyses.
Six approaches were used to examine spatial
patterns:
1. The distributions of C. grandiflora, Z. mauritiana
and Carissa spp. across the region were examined
using presence/absence data for the 2362 sites.
Table 1. Fields and categories used to describe each site (Rogers et al. 1999).
Shrub extent
(% cover)
Geology
Soil
(no. soil types)
Landform element
(McDonald et al. 1990)
Vegetation
Erosion depth
and width
0
0–10
11–20
21–30
31–40
41–50
51–60
61–70
71–80
81–90
91–100
Alluvial
Basalt
Cainozoic
Granodiorite
Igneous
Metamorphic
Sedimentary
Limestone
Granite
Acid Volcanic
8
7
16
4
12
8
9
1
1
1
Plain
Fan
Hillslope
Hillcrest
Footslope
Levee
Summit
Drainage depression
Valley flat
Terrace flat
Bank
Terrace
Plain
Pediment
Swamp
Tree basal area
Distribution of upper stratum spp.
Distribution of mid-stratum spp.
Distribution of lower stratum spp.
Grass basal area
Sheet erosion
Rill erosion
Gully erosion
Scalds
192
2. Five concentric zones around Charters Towers were
defined and the percentages of sites at which each
weed was present were calculated. The zones used
were 0–20, 20–40, 40–60, 60–120 and >120 km.
3. The Arcview Spatial Analyst Extension (ESRI
1996) was used to define stream networks from
the AUSLIG 90 -gridded Digital Elevation Model
(DEM). This network was then used to describe
sub-catchments in relation to primary and secondary
streams using Strahler’s ordering method (Strahler
1952) for the Burdekin and Cape Rivers and their
major tributaries. The majority of the region was
allocated to one of 21 sub-catchments. The proportions of survey sites in each sub-catchment at which
C. grandiflora, Z. mauritiana and Carissa spp. were
present were calculated.
4. The density function of Arcview Spatial Analyst
Extension was used to map weed distributions. For
each sampling site, this spatial analysis considers
the proximity of other sites, the quantity at those
sites, and how many other cells share a portion of the
measured quantity (ESRI 1996). The searching distance around each sampling site was 20 km and the
Kernal option was chosen to give smooth rendition
of the surface over the sampling points (Silverman
1986).
5. The Inverse Distance Weighted Interpolation function of the Arcview Spatial Analysis Extension
was used to calculate an interpolated surface that
describes the magnitude (weed abundance) of each
sampling site. This method assumes that each input
point has a local influence that diminishes with distance. The 15 nearest neighbours of each site were
used to determine an interpolated surface for the
entire Dalrymple Shire (ESRI 1996).
6. Stepwise regressions were used to estimate the contributions of various factors recorded during the survey to variation in weed abundance. Analyses were
performed both on the full 2362 sites from which
weed data were collected, and separately for each
of the concentric zones defined above. Stepwise
regressions, excluding sites with zero weed abundance, were used to consider factors that could
influence abundance but not presence of weeds.
Riparian zones were defined as those areas within
100 m of primary and secondary streams. The proportion of the Dalrymple Shire that is riparian was
calculated by converting the DEM stream network to
vector form.
Results
Presence/absence data show that the two exotic weed
taxa occupied only a small proportion of sites within
Dalrymple Shire. C. grandiflora, Z. mauritiana and
Carissa spp. were recorded at 10%, 5% and 50%
of sites respectively. Differences between concentric
zones around Charters Towers in the frequency of
occurrence of each species were statistically significant (Table 2). C. grandiflora was present at 50% of
sites in the 0–20 km zone, but only 5% of sites in the
60–120 km zone (Table 2). For Carissa spp., the only
significant differences were between the >120 km zone
and each of the other zones (Table 2).
The 21 sub-catchments varied greatly in the proportion of surveyed sites at which C. grandiflora, Z.
mauritiana and Carissa spp. were present. C. grandiflora was recorded in 14 of the 21 sub-catchments.
In three sub-catchments, C. grandiflora was present at
>30% of sites (Figure 2a). Z. mauritiana was recorded
in only three of the 21 sub-catchments, the highest
proportion of sites at which it was recorded was 13% in
sub-catchment 11 (Fanning River) (Figure 2b). Carissa
spp. were recorded in all 21 catchments with the species
present at 3–100% of sites depending on the catchment
(Figure 2c).
The interpolated surfaces indicating frequency of
occurrence highlighted the uneven distribution of each
of the three species at the regional scale. C. grandiflora occurred most frequently in areas just to the
east, north and northwest of Charters Towers though
there may be additional small areas of relatively high
frequency further from Charters Towers (Figure 3a).
Z. mauritiana occurred most frequently to the east of
Charters Towers (Figure 3b). Carissa spp. was rather
more evenly spread across the Shire but with some areas
of higher frequency particular to the south of Charters
Table 2. Percentage (%) of sites within concentric zones around
Charters Towers at which C. grandiflora, Z. mauritiana and Carissa
spp. were present.
Species
C. grandiflora
Z. mauritiana
Carissa spp.
Distance from Charters Towers (km)
0–20
20–40
40–60
60–120
>120
50a
32a
60a
41a
12b
63a
25b
10b
58a
5c
5c
52a
0d
0d
44b
Within each species, values followed by different superscripts are
significantly different (P < 0.05).
193
Figure 2. Frequency (% of sites) of (a) C. grandiflora, (b) Z. mauritiana and (c) Carissa spp. in 21 sub-catchments of the Dalrymple Shire. The
main identified catchments are: 1. The upper Burdekin River; 2. Running River; 3. Clarke River; 4. Star River; 5. Allingham Creek; 6. Basalt
River; 7. Keelbottom Creek; 8. Burdekin River and adjacent low lying areas; 9. Fletcher and Lolworth Creeks; 10. Hann Creek; 11. Fanning
River and lower Burdekin River; 12. Cape River; 13. Campaspe River; 14. Rollston River; 15, 16 & 18. Numerous small tributaries of the Cape
River; 17. Broadly Creek; 19. Big Creek; 20. Natal Creek; 21. Amelia Creek.
194
Figure 2. Continued.
195
Figure 2. Continued.
196
Towers (Figure 3c). Carissa spp. are less likely to be
encountered along the northern–northeastern boundary, and in the central west and far south of the Shire.
Maps of weed abundance, as determined by Inverse
Weighted Distance Interpolation, showed similar patterns to frequency of occurrence but suggest that, even
within zones where a species occurs with a relatively
high frequency, there are areas where abundance is low
or zero (Figure 4).
Stepwise regressions showed that little of the
observed variation in C. grandiflora, Z. mauritiana or
Carissa spp. abundance was explained by landscape
variables (Table 3). Using the total data set, less than
25% of the variation (R2 < 0.25) in weed abundance
was explained. The maximum amount of variation
explained was for Carissa spp. (20%). Similarly, a
maximum of only 44% of variation (for Carissa spp.)
was explained by combined habitat elements in each
distance interval. R2 values for relationships with weed
abundance were less than 0.5 for sites where weeds
were present. Maximum R2 values for single habitat
elements were 0.13 for C. grandiflora (presence of
alluvial soil), 0.26 for Z. mauritiana (site on plain)
0.21 for Carissa spp. (grass basal area). Maximum R2
values were all associated with zones within 60 km of
Charters Towers.
Discussion
Although C. grandiflora and Z. mauritiana are serious weeds in the Dalrymple Shire, neither occurred at
more than 10% of the sites surveyed. In contrast, the
indigenous Carissa spp., also recognized as a weed
(Anderson 1993), were found at approximately 50%
of sites surveyed.
Maps based on presence/absence and abundance
of C. grandiflora, Z. mauritiana and Carissa spp. at
surveyed sites indicate that each species is unevenly
distributed within the Dalrymple Shire. Both C. grandiflora and Z. mauritiana were most frequently encountered in the central east of the Shire, that is close to
the main settlement in the region, Charters Towers.
Carissa spp. were widespread in the Dalrymple Shire
but recorded very infrequently on the north and northeast margins, in the central west and the far south.
The north and northeastern margins of the Shire are
upland areas of higher rainfall, forest rather than woodland vegetation and of igneous geology. The extensive
area in the west of the Shire, where Carissa spp. are
infrequent, supports open woodlands on relatively fertile soils derived from basalt. In spite of the apparent
negative association of Carissa spp. with particular
geology and other habitat variables, single geological
factors account for very little of the variation in the
occurrence of this taxon.
The concentrations of C. grandiflora and Z. mauritiana around Charters Towers can be considered
in relation to three hypotheses, namely, they are
attributable to (i) historical factors, (ii) disturbance
and/or (iii) habitat. Under a ‘historical’ hypothesis,
these species occur most frequently around the major
settlement of the region because it was a focus of sites
at which they were introduced. Under a ‘disturbance’
hypothesis, levels of anthropogenic disturbance close
to Charters Towers have been higher than in other parts
of the Shire and this disturbance has facilitated the
proliferation of the weeds. Charters Towers is an old
settlement, having been established in the early 1870s
as a mining town and service centre for the region’s
cattle industry. High levels of disturbance expected as
a result of such settlement include clearing of timber,
heavy grazing and mechanical disturbance of soil. The
grazing properties in the immediate vicinity of Charters
Towers are, on average, smaller than those of the Shire
in general and small property size has been associated
with higher levels of disturbance and land degradation (Young 1985; MacLeod 1990). Under a ‘habitat’
hypothesis, the concentration close to Charters Towers
is associated with some aspect of climate, geology, soil
type or other habitat variables.
It is difficult to distinguish between the three
hypotheses using survey data because habitat, disturbance and historical factors are correlated with one
another. Most of the area around Charters Towers is
on the granodiorite geological unit (De Corte et al.
1991), has possibly experienced levels of disturbance
greater than those in other parts of the Shire, and was
probably a centre of introduction. However, stepwise
regression analysis indicates that the habitat factors
considered, including the presence of granodiorite,
explain little of the variation in the presence/absence or
abundance of either of the exotic species. If one argues
that the frequency of the exotic weeds around Charters
Towers is due to disturbance, it must be accepted
that the native Carissa spp. are responding quite differently to various types of disturbance. Increases in
Carissa spp. over recent decades have been anecdotally associated with disturbance due to high grazing
pressure (Anderson 1993) and yet Carissa spp. do not
197
Figure 3. Interpolated densities (sites/km2 ) of sites at which (a) C. grandiflora, (b) Z. mauritiana and (c) Carissa spp. occurred in the Dalrymple
Shire.
198
Figure 3. Continued.
199
Figure 3. Continued.
200
Figure 4. Abundance (% cover) of (a) C. grandiflora, (b) Z. mauritiana and (c) Carissa spp. in the Dalrymple Shire, derived by Inverse Distance
Weighted Interpolation.
201
Figure 4. Continued.
202
Figure 4. Continued.
203
Table 3. Variance explained (R2 ) by combinations of variables (Table 1) using (i) data from all sites for the Dalrymple Shire, (ii) data from
all sites in each concentric zone around Charters Towers, and (iii) only sites where weeds were present (abundance > 0).
Distance interval (n)
C. grandiflora
Z. mauritiana
Carissa spp.
Presence/absence
Abundance
Presence/absence
Abundance
Presence/absence
Abundance
(i) All sites (2362)
0.08
0.07
0.19
0.13
0.13
0.20
(ii) Concentric zones
0–20 km (68)
20–40 km (208)
40–60 km (260)
60–120 km (979)
120 km (847)
0.23
0.23
0.19
0.07
0
0.37
0.29
0.30
0.03
0
0.44
0.21
0.34
0.23
0
0.20
0.33
0.29
0.21
0
0.25
0.26
0.44
0.13
0.21
0.44
0.38
0.41
0.24
0.27
(iii) Sites with abundance > 0
0.06
occur frequently in concentric zones closer to Charters
Towers.
The current distributions of C. grandiflora and Z.
mauritiana within the Dalrymple Shire indicate that
neither species has colonized all suitable areas within
the region. This is in spite of the fact that both have
been present in the region for perhaps one hundred
years. Moreover, the two species have apparently differed in their rates of range expansion. Z. mauritiana
is confined to a lower proportion of sites and more
tightly restricted to the area closest to Charters Towers
than is C. grandiflora. The two species use very different dispersal mechanisms. C. grandiflora is dispersed
predominantly by wind, whereas the seeds of Z. mauritiana are animal-dispersed. Cattle, the main dispersal
agents of Z. mauritiana (Grice 1996), are restricted in
their movements by fences, so that new infestations
are likely to develop only when cattle are transported.
Otherwise, spread will largely be confined within individual properties or even paddocks. C. grandiflora, on
the other hand, can freely disperse across paddock and
property boundaries even though most seeds probably
fall close to parent plants (Grice and Brown 1996). Seed
production per unit area by C. grandiflora is up to two
orders of magnitude greater than that of Z. mauritiana
(Grice 1996). This could also contribute to a greater
rate of spread by the former species.
The current distribution of C. grandiflora is consistent with the idea that Charters Towers has acted as
a centre from which range expansion has taken place.
Sites more distant from Charters Towers can be taken
as outliers from this main population. There is, however, little evidence for multiple foci of introduction
and spread at the regional scale.
In general, the survey is less informative about
finer-scale patterns in C. grandiflora. At a finer scale,
0.49
0.16
the array of occupied and unoccupied sites within a
zone can be interpreted as representing multiple foci
from which expansion can occur. For example, there
are many sites close to Charters Towers at which
C. grandiflora was not recorded. This suggests that the
species still has potential to increase even in those parts
of the region where it currently occurs most frequently.
The prevalence of C. grandiflora in riparian parts of the
landscape is not apparent from the survey or analysis.
This means it is not possible to consider the comparative rates of invasion through or away from riparian
areas.
For C. grandiflora we suggest that invasion involves
expansion from a scale- and landscape-related hierarchy of fronts. The regional-scale patterns indicate
that expansion is occurring by radiation from Charters
Towers. At a finer scale, within the zone that is most
intensely colonized, expansion is occurring from the
numerous occupied sites into adjacent habitat. These
two related processes are likely to give rise to complex
distribution patterns, particularly in a region that is as
topographically, edaphically and geologically diverse
as the Dalrymple Shire. The spatial patterns of invasion
by Z. mauritiana are, perhaps, less complex, being
dominated by relatively slow radiation from sites of
introduction. The expanding populations thus derived
have so far remained more or less independent of one
another.
While the array of infestations of C. grandiflora in
the Dalrymple Shire conforms to the broad definition
of a metapopulation as a ‘set of local populations linked
by dispersal’ (Gillman and Hails 1997), the actual
situation is more complex than could be described
by a simple metapopulation model. A region such as
Dalrymple Shire contains habitats that vary greatly in
their suitability for the species, ranging from highly
204
suitable to unsuitable. The most suitable habitats are
the extended linear riparian areas that make up only
a small proportion (ca. 6%) of the region. The least
suitable are upland or run-off areas, some of which
may be totally unsuitable for C. grandiflora. Current
metapopulation models are too restrictive for this type
of situation.
These results are relevant to the management of
C. grandiflora and Z. mauritiana. Both species are
infrequent in large parts of the Dalrymple Shire and
possibly absent from a few entire sub-catchments. A
regional strategy for managing these weed species
should focus initially on containment by controlling
those populations most distant from Charters Towers
and those in catchments where they are infrequent [e.g
sub-catchments 2 (Running River), 3 (Clarke River), 4
(Star River) and 6 (Basalt River)]. The fact that many
sites close to Charters Towers do not currently support
either weed species suggests that containment is also
a reasonable objective for this zone. This is inspite of
their long-term presence in the area, and, in the case
of C. grandiflora, a reputation for being freely dispersed. Containment may be effectively implemented
at property and paddock scales.
The results of the survey in relation to these weeds
should be interpreted with caution for three main reasons. First, the purpose of the DLRS meant that
the overall sampling intensity was low (< 0.05% of
Shire). Second, sampling sites were unevenly distributed across the Shire, so that some parts of the
Shire were under-sampled relative to others. The area
around Charters Towers, for instance, was more intensively sampled than outlying areas of the Shire, to
account for geological complexities. This may have
led to under-estimates of frequency in some parts of
the region. Third, the sampling protocol did not specifically target riparian areas which are the primary habitat
of C. grandiflora. The survey did include ‘alluvial
landscapes’ which would have included riparian zones
(Rogers et al. 1999). A sampling protocol stratified
on the basis of fine-scale landscape position (riparian
versus non-riparian) would have more effectively dealt
with landscape-scale patterns as they would influence
species such as C. grandiflora.
Conclusion
The DLRS and our analyses demonstrate the uneven
distribution of the exotic shrubs C. grandiflora and
Z. mauritiana and the indigenous Carissa spp. at the
regional scale (tens to several hundred km). Both exotic
species occur at higher frequencies close to the main
settlement of the region and are far more frequent in
some sub-catchments than in others. The survey did
not yield a reliable picture of weed distributions at
a landscape-scale (tens to several hundred m). This
would require stratified sampling on the basis of the
main habitat types with which each weed is associated. Such an approach would also be more useful for
examining the role of habitat factors underlying the distributions. Examining the demographic characteristics
(e.g. population structure, reproductive output, establishment and mortality rates) of populations growing in
different parts of the landscape would also help explain
how they contribute to the overall invasion process.
Likewise, comparing populations central and peripheral to areas of high frequency would contribute to an
understanding of the history of weeds within regions.
Studies of spatial variation in population function could
usefully include work on patterns of dispersal and
establishment around infestations in different parts of
the landscape. Our results suggest that containment
of spread would be a reasonable objective at regional,
sub-catchment, and, perhaps, finer scales.
Acknowledgements
This research was funded by the Co-operative Research
Centre for the Sustainable Development of Tropical
Savannas. We gratefully acknowledge the assistance
of Gary Rogers and Mike Cannon in making available
data gathered during the Dalrymple Land Resources
Survey, a project involving the Queensland Department
of Natural Resources, the CSIRO, the Queensland
Department of Primary Industries and the National
Landcare Program. We thank Joel Brown, John
Ludwig, John McIvor, Gary Rogers and anonymous
referees for fruitful discussions and helpful comments
on drafts of this paper.
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