JUST, Vol. V, No. 1, 2017
Trent University
There Were Cats and Rats and Ungulates: Island
Colonisation Success of Mammals
Tessa Sarah McBurney
Abstract
Islands are particularly susceptible to the severe ecological consequences wrought from the dispersal of invasive
species, and introduced mammals have exceeded other vertebrate groups when it comes to posing challenges for
native populations. The purpose of this study was to determine what factors affect a mammalian species’ ability
to invade islands by comparing how two taxonomically and physiologically unique mammalian orders, Rodentia
and Artiodactyla, invade islands and resist eradication. Using data provided by the Threatened Island Biodiversity
Database and the Database of Island Invasive Species Eradications, the results of multiple Wilcoxon rank-sum
tests demonstrated that rodents and even-toed ungulates are equally capable of invading islands. However,
rodent populations were found to be superior in surviving eradication attempts (W = 24, p − value < 0.05). More
research is required to understand what makes a species a successful invader to assist in mitigating the negative
impacts of invasive species.
Keywords
Rodents — Even-toed Ungulates — Artiodactyla — Invasive Species — Island Biogeography — Eradications
Gzowski College
1. Introduction
At a time when biological invasions have reached their highest rate in history, it is crucial to consider how alien species
impact native species populations (Reaser et al. 2007). Currently, biological invasions are the second leading cause of
biodiversity loss, and are a major factor in the extinctions of
many species (Courchamp et al. 2003). Islands in particular
have been experiencing severe ecological consequences from
the dispersal of invasive species (Reaser et al. 2007), and
introduced mammals have posed more challenges than any
other vertebrate group (Courchamp et al. 2003). For an alien
or introduced species to be considered invasive it must have
established a self-sustaining population and spread rapidly,
or had an impact on native organisms (Ricciardi et al. 2013).
The difference between an alien and an introduced species,
which are both not native to the area in question, is that the
latter has dispersed via human actions.
1.1 Island Biogeography
As stated in the MacArthur-Wilson Theory of Island Biogeography, immigration, speciation, and extinction are affected by two major environmental factors: isolation and
area (MacArthur and Wilson 1967; Whittaker et al. 2008).
MacArthur and Wilson (1967) predicted that the rate of immigration will decline as isolation, or the physical distance from
the mainland to the island, increases, and the rate of extinction
will decline as the area of the island increases (Whittaker et
al. 2008). The relative geographic isolation of islands has
allowed for the evolution of a high number of novel and en-
demic species, the likes of which are not found in continental
areas (Jamie et al. 2007). The physical distance from the
mainland to the island, across oceanic environments, is what
limits the type of species that are able to colonise an island
(Jamie et al. 2007). This means few large mammals are native
to isolated islands (Jamie et al. 2007). However, mammals of
varying size have been both intentionally and unintentionally
introduced outside of their native range by human dispersal
and trade (Clout and Russell 2008). Mammals were some of
the earliest animals to be introduced to islands by humans for
the purpose of livestock or companionship, or accidentally
through commensal relationships (a relationship that was beneficial to the animals and not adverse for the humans) (Clout
and Russell 2008; Nogueira et al. 2009; Spear and Chown
2009).
1.2 Rodentia and Artiodactyla General Traits
Variation of body masses between different taxonomic groups
may be fundamental to explaining physiological, ecological,
and phylogenetic constraints (Smith et al. 2003). Order Artiodactyla is a taxon that encompasses even-toed ungulates,
or hooved animals with an even number of toes, such as
deer, boar, cows, and goats. Order Rodentia is the taxonomic
group for rodents and includes: mice, rats, lemmings, shrews,
squirrels, and agouti, among other rodents species. Based on a
dataset of taxonomic body mass created by Smith et al. (2003),
members of order Artiodactyla generally have a considerably
larger body mass than those of order Rodentia. As smaller
organisms, rodents tend to have high reproductive output but
die young (Sibly and Brown 2007). Larger mammals, such
There Were Cats and Rats and Ungulates: Island Colonisation Success of Mammals — 2/5
as even-toed ungulates, have slower metabolic production per
unit body mass and so produce offspring at a lower rate (Sibly
and Brown 2007). In this regard, body mass is an essential
factor in the invasive capabilities of mammals. Successful
establishment of a new colony relies on the number of new
recruits added to the population, which is in turn partially
dependent on the reproductive output of the species.
Considering the current impact of invasive species, especially introduced mammals, on native species populations, an
important question to ask is: What factors affect a mammalian
species’ success in invading islands? Being able to answer
this question could lead to the discovery of more effective
control measures for managing island colonisations of invasive mammals. This study sought to answer this question by
comparing how effectively the members of two taxonomically
and physiologically disparate mammalian orders, Rodentia
and Artiodactyla, colonise islands. To assess the success of invasive colonisation, effectiveness was measured as the number
of islands colonised by members of each order and the number
of human-mediated eradications of alien island colonies of
both orders. The objectives of this study were to determine if
the orders were equally:
1. Successful at colonising islands.
2. Able to withstand human-mediated eradications of alien
island colonies.
The hypothesis for the first objective was that the two
mammalian orders were not equally successful at colonising
islands. It was predicted that members of order Rodentia
would be more effective at colonising islands than members
of order Artiodactyla, due to small body size and generalist
tendencies in diet and habitat. The hypothesis for the second
objective was that the orders were not equally able to withstand eradications of alien island colonies. It was predicted
that invasive colonies of order Artiodactyla species would be
less able to withstand eradication attempts than alien colonies
of order Rodentia species, due to a generally larger body size
and more specialist dietary tendencies. If both predictions
were supported, members of order Rodentia would be considered more effective islands invaders.
2. Materials and Methods
The number of islands that have been colonised by rodents and
even-toed ungulates were recorded from the Threatened Island Biodiversity Database (Invasive Species Specialist Group
2016; Appendix: Table 1; Table 2). Only species in each order
that were found to invasively colonise islands were included
in the analysis. Before addressing the first objective, the data
were explored to ensure the assumptions of normality and homogeneity of variance were met. If one of these assumptions
was not met, then the data were natural log-transformed before
running a statistical test. An independent two sample t-test
was used to determine if there was a difference in the number
of islands colonised by rodents and even-toed ungulates. If
the natural log-transformation did not help the data to meet
the assumptions, then a non-parametric Wilcoxon rank-sum
test was used in place of the t-test.
The second objective was evaluated by comparing the
failure rates and success rates for human-mediated island
eradications of invasive rodents and even-toed ungulates. The
number of successful and failed eradications of island invaders
were collected from the Database of Island Invasive Species
Eradications (Invasive Species Specialist Group 2016; Appendix: Table 3; Table 4). These were each divided by the
total number of eradication attempts that took place for each
species in both orders to ensure that success and failure rates
were correctly measured. Although successes and failures can
typically be compared without using rates, because they sum
to the total number of attempts, this is not the case in this study.
The database included more eradication attempt statuses than
those coded as successes and failures because many eradication attempts were still ongoing or the results were unknown.
While the database did not provide a definition for success or
failure, this study assumes that a successful eradication is one
where the island has hitherto remained free of the eradicated
organism after the eradication attempt, in accordance with
Courchamp et al. (2003). A failed eradication is one where
the eradication attempt did not work or where the island was
recolonised by the invasive species. Eradication attempts that
were not listed as successes or failures were not counted in
this study and only species that have island populations where
eradications have been attempted were included in the data
set. The data were tested against the same assumptions as
the first objective, and then depending on the results, either
independent two sample t-tests or Wilcoxon rank-sum tests
were used to compare both the failure and success rates of
island eradications between the two mammalian orders.
All data were analysed using RStudio (Version 0.00.903;
Allaire 2016). Island size data were examined in order to
rule out island size as a confounding variable before either
the island or eradication data were analysed (Appendix Table
5; Table 6). For each rodent and even-toed ungulate species
that invaded an island, the median island area was calculated
using the area of the largest and smallest islands invaded by
each particular species. Then these data were explored to
see if they met the assumptions of homogeneity of variance
and normality. If the data did not meet these assumptions
they were natural log-transformed prior to statistical analysis.
Based on these results, an independent two sample t-test or a
Wilcoxon rank-sum test was used to verify that there was no
difference between the areas of islands colonised by Rodentia
species and those colonised by Artiodactyla species. An a priori significance level of 0.05 was used for all of the statistical
analyses.
3. Results
The prediction that members of order Rodentia would be
more effective at colonising islands than members of order
Artiodactyla was not supported by the results of the Wilcoxon
There Were Cats and Rats and Ungulates: Island Colonisation Success of Mammals — 3/5
Figure 1. A boxplot of the average size of islands invasively colonised
by orders Artiodactyla (red) and Rodentia (blue). The median average
island size, represented by the black line, is not significantly different
between the two orders. Whereas the Artiodactyla data is fairly
symmetrical, the Rodentia data is skewed right with a large
interquartile range. There are also outliers present in the Artiodactyla
data.
rank-sum test. The prediction that invasive colonies of order
Artiodactyla species would be less able to withstand eradication attempts than order Rodentia species was not statistically
supported by the Wilcoxon rank-sum test for success rate, but
was statistically supported for failure rate.
First, the effect of island size as a covariate was tested.
The p-value was found to be greater than the a priori set 0.05
significance level, so the null hypothesis that there was no
difference between the average sized islands colonised by the
two orders was accepted (n=74; W=647.5, p-value=0.7284;
Appendix: Fig.4).
After ruling out island area as a confounding variable,
the first objective, to determine if the orders are equally successful at colonising islands, was addressed. The F test to
compare two variances found the variances to be unequal
and the Shapiro-Wilk normality test also indicated the data
were non-normal (n=74; Artiodactyla: W = 0.4188, p-value =
2.113x10−11 ; Rodentia: W = 0.4544, p-value = 4.096x10−10 )
. The data were natural log-transformed and the natural logtransformed data were found to have equal variances. However, the data were still not normal, so a Wilcoxon rank-sum
test was used in place of a t-test with the non-normal natural
log-transformed data. The results of the Wilcoxon rank-sum
test indicated the null hypothesis should be accepted (W=707,
p-value=0.7684). Therefore, there was no evidence that there
was a significant difference between the number of islands
colonised by rodents and even-toed ungulates (Fig. 1).
For successful island eradications of invasive even-toed
ungulates and rodents, the data were found to have equal
variances, however, the order Artiodactyla data were found
to be non-normal. As a result, the data were natural logtransformed, which made the data from both orders nonnormal. Thus, a Wilcoxon rank-sum test was used as a nonparametric test with the original non-transformed data. Based
on the results of the Wilcoxon rank-sum test there was no
difference in the success rate of island eradications between
members of order Rodentia and order Artiodactyla (n=22;
Figure 2. A boxplot of the number of islands invasively colonised by
orders Artiodactyla (red) and Rodentia (blue). The median number of
colonised islands, represented by the black line, is not significantly
different between the two orders. The interquartile range is small for
both orders, although there are outliers present in both the
Artiodactyla and Rodentia data.
Figure 3. A boxplot of the success rate of eradications on islands
invasively colonised by orders Artiodactyla (red) and Rodentia (blue).
The median number of colonised islands, represented by the black
line, is not significantly different between the two orders. The
interquartile range is large for order Artiodactyla, and there is an
outlier present in the Rodentia data.
W=52, p-value=0.6805; Fig. 2).
The same statistical methods were used to test the difference between failed island eradications of invasive even-toed
ungulate and rodent species. The variances were found to be
unequal and the data were not normally distributed. After natural log-transforming the data, the variances were improved,
but still not equal, and neither the order Rodentia nor the order
Artiodactyla data were normal. A Wilcoxon rank-sum test was
conducted using the natural log-transformed non-normal data.
Rodents had a higher mean eradication failure rate (0.2244)
than even-toed ungulates (0.0108), a difference which is statistically significant (n=22; W=24, p-value=0.0084; Fig. 3).
4. Discussion
The hypothesis for the first objective, that members of orders Rodentia and Artiodactyla were not equally successful at
colonising islands, was not supported by the statistical analysis. The hypothesis for the second objective, that the orders
were not equally able to withstand eradications of alien island
colonies, was supported by the analysis of the eradication
There Were Cats and Rats and Ungulates: Island Colonisation Success of Mammals — 4/5
is possible that particular rodent species are more effective at
invasively colonising islands than even-toed ungulate species.
Figure 4. A boxplot of the failure rate of eradications on islands
invasively colonised by orders Artiodactyla (red) and Rodentia (blue).
The median number of colonised islands, represented by the black
line, is significantly different between the two orders, with order
Rodentia having a higher rate of failed eradications. The interquartile
range is small for order Artiodactyla, and there are outliers present in
the data for both orders.
failure rate data, but not the eradication success rate data. Rodents were found to have a higher average failure rate than
even-toed ungulates, which suggests that species from order
Rodentia are at least somewhat better able to withstand island
eradications than order Artiodactyla species. Although there
are no differences between the two orders in terms of island
colonisation ability and the island eradication success rate,
this suggests that overall order Rodentia is more successful at
invasive colonisation of islands than order Artiodactyla.
Even though rodents were found to be better overall island
invaders than even-toed ungulates, the results for the first objective were surprising based on the existing literature. Rats,
including Norway rats (Rattus norvegicus), Black rats (Rattus
rattus), and Polynesian rats (Rattus exulans), are cited to be
among the most successful invasive mammal species having
invaded 80% of islands in the world (Caut et al. 2008). Additionally, House mice (Mus musculus) are considered to be
one of the most far-reaching invasive mammals on the planet
(Angel et al. 2009). However, a list of the most successful
introduced mammals, measured by the number of introductions and the risk posed to native populations, includes: three
rat species, one mouse species, rabbits, cats, goats, pigs, and
cattle (Courchamp et al. 2003). This cites both rodents and
even-toed ungulates as top mammalian invaders, and is supported by the results of the first statistical test of this study
which indicates that the orders were equally successful at
colonising islands. This is a point of interest because the most
successful introduced mammals include various species from
several different orders which suggests that at least initial
colonising ability may be species dependent rather than a trait
at the taxonomic level of order. This is further supported by a
study conducted by Clout and Russell (2008) where only 1.8%
of rodent species were classified as successful invaders, but
13% of the genus Rattus were included in this category. This
could explain the far outliers found in the order Rodentia data
in this study. While order Rodentia in its entirety may not be
more effective at invading islands than order Artiodactyla, it
The results of the second objective, which established that
the two orders were not equally able to withstand eradication of alien island colonies, are supported by studies which
demonstrate that rodents are among the most widespread
island-colonised invasive species (Shiels and Drake 2011).
Additionally, invasive rodents are challenging to manage effectively (Shiels and Drake 2011), which is partly attributable
to the risk of reinvasion posed by rodent species (Clout and
Russell 2007). While there have been many successful eradications of rodents from islands, there have also been several
reinvasions that have occurred up to ten years after the eradication, particularly with rats (Clout and Russell 2007). This is
thought to be due to the high dispersal ability of order Rodentia species (Clout and Russell 2007). This supports the results
of both the eradication success and failure rate analyses, as
dispersal ability would not affect how susceptible a species
is to being eradicated, which would account for even-toed
ungulates and rodents having the same average success rate
of eradications. However, it could impact the average eradication failure rate as dispersal ability would allow rodents
to recolonise islands that have already undergone eradication
attempts, which may account for why rodents were found to
have a higher average number of failed eradication attempts
than even-toed ungulates. The recolonisation of islands by
rodents has been observed several times, including the recolonisation of Italian islands by Black rats post eradication
attempts (Capizzi et al. 2010). In this way, members of order
Rodentia may be better able to withstand human mediated
eradication attempts, and thus, be more successful overall at
invasively colonising islands.
Some possible limitations of this study include the availability of data concerning the details of the invaded islands.
Due to the importance of island size and isolation when it
comes to immigration and establishing populations (Whittaker et al. 2008), it was determined that these factors should
both be ruled out as potential cofounding variables when attempting to isolate the island invasion abilities of rodents and
even-toed ungulates. Island size was eliminated as a confounding variable early in the study when no difference was
found in the median island areas colonised by species in order
Rodentia and Artiodactyla. However, a better method may
be to determine whether there is a difference between the
average island size colonised by each species in each order,
and although this was not considered in the current study, it
is recommended that it be accounted for in future research.
Unlike island size, the potential effect of isolation could not
be ruled out as a confounding variable because data on the relative isolation of the islands were not included in the database
(Invasive Species Specialist Group 2016). While island isolation could be a potential confounding variable in this study, it
can be argued that it does not necessarily apply to all invasive
species. This is because many invasive species are introduced
to islands through human-mediated dispersal, which can oc-
There Were Cats and Rats and Ungulates: Island Colonisation Success of Mammals — 5/5
cur over large distances. Many species are limited in which
islands they can invade by the distance between the island and
the mainland, but this is not the case for humans who have
invented other means for travel, such as ships. As both rodents
and even-toed ungulates have travelled with humans on ships
as humans colonise islands, island isolation may not greatly
affect the invasive abilities of these mammalian species. In
this way, humans contribute to the invasiveness of species by
facilitating the initial island colonisation and by enabling the
persistence of introduced domestic populations.
The purpose of this study was to determine if traits belonging to a specific taxonomic order affect a mammalian species’
success at invading islands by comparing the invasive ability
of two distinct mammalian orders that colonise islands. The
results were mixed, as there was no evidence that members of
order Rodentia and order Artiodactyla had differing success
at colonising islands or island eradication success rates, but
it was found that rodents are better able to withstand humanmediated eradications of island colonies based on failure rates
of eradication attempts. This gives at least partial evidence
that members of order Rodentia possess particular traits that
generally make them more effective island invaders than eventoed ungulates. However, it is more likely to be genus specific,
or even species specific, traits than an order-wide ability that
make particular members of order Rodentia more efficient
colonisers. There are also other unaccounted for factors at
play, primarily the effect humans have on which mammalian
species are introduced to islands. Island species are particularly susceptible to the negative impacts of invasive species,
and as a result many endemic populations are going extinct.
It is crucial now, more than ever, to determine how exactly
invasive species are introduced and established on islands, and
how they may be successfully eradicated or controlled.
5. Acknowledgments
Originally written for Biology 4180 (Mammalogy) Dr. Joanna
Zigouris.
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7. Appendix
Raw data tables can be found at trentu.ca/just in additional
materials for this article