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The invasion ecology of mammals: A global perspective


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Of the extant species of land mammals, 124 (2.6%) can be classed as 'successful invaders', whereas 1038 (21.6%) are classed as threatened, according to the 2006 IUCN Red List. Relatively high proportions of successful invaders are found among Artiodactyla, Carnivora, Lagomorpha and Perissodactyla. Compared with other organisms, mammals seem relatively likely to become established when introduced outside their natural range. Studies of determi-nants of invasion success indicate that the number of individuals released, the size of the natural range of the introduced species, and the temperateness of climate in the new range can all increase the probability of establishment of introduced mammals. Negative impacts of invasive mammals on native biodiversity include direct effects such as predation, browsing and competition, but can extend to disruption of patterns of nutrient flow, and trophic cascades. Eradication of several species of invasive mammals from increasingly large areas is now possible. In this context, it is important to better under-stand ecological interactions between such mammals (and between them and other species) to avoid unwanted consequences such as mesopredator or competitor release, after the removal of particular species. Finally, it is increasingly apparent that research is needed on the behaviour of dispersing and invading individuals, to improve the early detection of new mammal invasions or reinvasions.
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Wildlife Research, 2007, 35, 180–184
The invasion ecology of mammals: a global perspective
Mick N. CloutA,B and James C. RussellA
ACentre for Biodiversity and Biosecurity, School of Biological Sciences, University of Auckland,
Private Bag 92019, Auckland 1142, New Zealand.
BCorresponding author. Email:
Abstract. Of the extant species of land mammals, 124 (2.6%) can be classed as ‘successful invaders’, whereas 1038
(21.6%) are classed as threatened, according to the 2006 IUCN Red List. Relatively high proportions of successful
invaders are found among Artiodactyla, Carnivora, Lagomorpha and Perissodactyla. Compared with other organisms,
mammals seem relatively likely to become established when introduced outside their natural range. Studies of determi-
nants of invasion success indicate that the number of individuals released, the size of the natural range of the introduced
species, and the temperateness of climate in the new range can all increase the probability of establishment of introduced
mammals. Negative impacts of invasive mammals on native biodiversity include direct effects such as predation, browsing
and competition, but can extend to disruption of patterns of nutrient flow, and trophic cascades. Eradication of several
species of invasive mammals from increasingly large areas is now possible. In this context, it is important to better under-
stand ecological interactions between such mammals (and between them and other species) to avoid unwanted
consequences such as mesopredator or competitor release, after the removal of particular species. Finally, it is increasingly
apparent that research is needed on the behaviour of dispersing and invading individuals, to improve the early detection of
new mammal invasions or reinvasions.
The deliberate or accidental introduction of species beyond their Methods
native range has been a historical consequence of human migra- We searched the comprehensive book by Long (2003) for
tion and trade, and has accelerated greatly in recent times. mammal species that have been recorded as being introduced
Several introduced species (including most crops and domestic and establishing a self-sustaining wild population in at least one
livestock) are largely beneficial, but many others have become location outside their natural range. Such species were classed
weeds or pests. Some have proved to be highly invasive, modi- as ‘successful invaders’, and included probable ancient intro-
fying natural ecosystems and threatening native species, ductions and introductions for conservation purposes outside
The process of biological invasion is now widely accepted to the natural range. We excluded unconfirmed prehistoric intro-
consist of a series of stages: introduction (or transport), estab- ductions, reintroductions for conservation purposes within the
lishment, and spread (Mack et al. 2000; Kolar and Lodge 2001). natural range, scientific trials, and all instances where there is
Terminology surrounding the phenomenon of biological inva- doubt (according to Long 2003) that a self-sustaining wild
sions has, however, been a subject of recent debate (Colautti and population was established. We included cases where a self-
MacIsaac 2004). sustaining wild population was subsequently eradicated by
Mammals were among the first organisms to be introduced human actions, but excluded those where populations died out
by humans: as livestock (sheep, goats, cattle), as companions naturally or where Long (2003) expresses any doubt about their
(dogs) or as commensals (rodents). More recently, many other fate. For analysis of the proportions of the various mammal
species of mammals have been deliberately introduced as sport- groups that are successful invaders, we used the global list of
ing animals, for novelty reasons, and (ironically) to control pre- wild mammals compiled by Ceballos et al. (2005), which incor-
viously introduced species that had proved to be pests. Overall, porates data from the IUCN Red List. For completeness, we
mammals are perhaps the best documented of all introduced added to this list eight domesticated mammal species – goat
organisms and have been the subject of two global compilations (Capra hircus), sheep (Ovis aries), horse (Equus caballus),
(Lever 1985; Long 2003) and some excellent regional records of
attempted and successful introductions (Thomson 1922; Myers donkey (Equus asinus), cattle (Bos taurus), water buffalo
1986; Jeschke and Strayer 2005). (Bubalus bubalis), dog (Canis familiaris), and cat (Felis catus)
The purpose of our review is first to examine differences – that are recorded by Long (2003) as having established truly
between mammal groups in their propensity to contain suc- unmanaged wild (feral) populations. For comparison with the
cessful invaders or threatened species. We then review the lit- proportions of the various mammal groups that are classed as
erature on determinants of invasion success by mammals and threatened (i.e. vulnerable, endangered or critical, according to
consider overall implications for the management of invasive 2006 IUCN Red List criteria), we also followed the global
mammals. mammal list compiled by Ceballos et al. (2005). For both
© CSIRO 2008 10.1071/WR07091 1035-3712/08/030180
The invasion ecology of mammals: a global perspective
analyses we excluded marine mammals (Cetacea, Pinnipedia,
Sirenia) and all extinct species.
The taxonomic distribution of mammalian invaders
Our dataset contains 4816 species of extant land mammals,
including domesticated species. Of these extant land mammals,
124 (2.6%) can be classed as ‘successful invaders’, according to
our criteria and the data compiled by Long (2003), whereas
1038 (21.6%) are classed as threatened, according to the 2006
IUCN Red List. Overall proportions of successful invaders
range from 0% (e.g. Chiroptera) to 14.7% (Artiodactyla),
whereas proportions of threatened species range from 0% (e.g.
Edentata) to 100% (Proboscidea) (Fig. 1). The orders
Artiodactyla (pigs, camels, deer, cattle, sheep, goats and ante-
lope), Carnivora (canids, bears, mustelids and cats),
Lagomorpha (rabbits and hares), and Perissodactyla (equines)
all contain high proportions of successful invaders. The
mammal family with the highest proportion of successful
invaders is Cervidae (29.2%). Only 1.8% of the order Rodentia
can be classed as successful invaders, but 13% of the genus
Rattus have this dubious distinction.
Wildlife Research 181
The absence of any successful invaders among over 1000
species of bats, compared with (for example) 32 successful
invaders among 217 species of Artiodactyla suggests that there
are differences in the propensity of different groups to be trans-
ported and establish outside their natural range. Contrasts in the
proportion of successful invaders between major taxonomic
groups seem to be largely due to differences in propensity for
deliberate introduction of species valued for hunting, as fur-
bearers, as domestic animals, as biological control agents, or as
easily transported novelties. For example, bats and insectivores
are less favoured than deer and rabbits. Beyond this initial
human ‘filter’, other biological determinants of success
become important.
A relatively small number of mammal species have success-
fully established at more than 30 locations around the world
(Long 2003). They include feral domestic animals (horses,
sheep, goats, cattle, pigs, donkeys, cats, dogs), European rabbits
(Oryctolagus cuniculus), red deer (Cervus elaphus), American
mink (Mustela vison), Indian mongoose (Herpestes javanicus)
and six species of rodents. The rodents include two deliberately
introduced species – coypu (Mycocastor coypus) and muskrat
Fig. 1. Percentage of successful invaders, threatened species and those in neither category, for each mammalian order. The number
of species in each order is shown above each bar. Nomenclature follows Long (2003) for orders Edentata, Insectivora and Marsupialia.
182 Wildlife Research M. N. Clout and J. C. Russell
(Ondatra zibethicus) – and four very widespread commensal
species that have been accidentally introduced to many locations
(Rattus rattus, R. norvegicus, R. exulans and Mus musculus).
Determinants of invasion success
Although less than 3% of the world’s mammals can be classed
as ‘successful invaders’, the data compiled by Long (2003)
reveal that many of these species have also been the subject of
unsuccessful introductions. Some other mammal species
(excluding those reintroduced for conservation purposes) have
never been successfully introduced, despite efforts to do so.
These observations raise the question of what determines inva-
sion success, a topic that has been the subject of several recent
From an analysis of the invasion success of mammals, fish
and birds in Europe and North America, Jeschke and Strayer
(2005) demonstrate that introduced vertebrates have a relatively
high probability (~50%) of establishing and spreading. They
conclude that the ‘tens rule’ of Williamson (1996) (that ~10% of
species will make each of these transitions) does not apply to
vertebrates. They also show that there is no clear difference in
invasion success of vertebrates introduced from Europe to
North America or vice versa.
The excellent records kept by Acclimatisation Societies in
New Zealand (Thomson 1922) have provided a fertile source of
data for analyses of factors affecting introduction success. In a
study of the outcomes of introductions of 14 ungulate species,
Forsyth and Duncan (2001) showed that the 11 successful
species had shorter maximum life spans and were introduced in
greater numbers than the unsuccessful ones. For all independent
introductions there was a highly significant relationship
between the number of individuals introduced and introduction
success, with an apparent threshold introduction size of about
six individuals, above which success was likely. A more recent
analysis by the same authors (Duncan and Forsyth 2006) exam-
ined population persistence of 164 introductions of six mammal
species (rabbit, goat, sheep, cat, pig and brushtail possum) to
85 islands in New Zealand. They show that small populations
were initially at greatest risk of extinction; those that survived
for 25 years were likely to persist for much longer; and the
probability of mammal populations persisting on islands
declined markedly with increasing latitude. A study of the
species richness of 17 large and small introduced mammals on
297 islands in New Zealand (Russell et al. 2004) revealed that
human transportation, facilitated by wharves and permanent
inhabitants, increased the likelihood of introduced mammals
being present, although small mammals were also dispersing by
swimming to many islands.
Forsyth et al. (2004) analysed data on 40 mammal species
introduced to mainland Australia, 23 of which successfully
established. They found that successful species had a greater
area of climatically suitable habitat available to them, had larger
overseas ranges, and were introduced more times. Overall
results were similar to those obtained for introduced birds in the
same region (Duncan et al. 2001). In both groups, established
species that had spread to occupy a large range in Australia also
tended to be species of small body masses with more offspring
per year. Among mammals, these more widespread species
tended to be carnivores or omnivores, rather than herbivores
(Forsyth et al. 2004). Burbidge and Manly (2002) demonstrated
a relationship between native mammal extinctions on Australian
Islands and the presence of introduced European foxes (Vulpes
vulpes) and feral cats.
Implications for the management of invasive mammals
Some introduced mammals have not only successfully estab-
lished self-sustaining populations, but have also spread and have
caused significant impacts in the ecosystems that they have
invaded. Such species can be classed as invasive (‘widespread
and locally dominant’), according to the definition of Colautti
and MacIsaac (2004). Examples of indisputably invasive
mammals are the three widespread Rattus species (Amori and
Clout 2003), European rabbits, feral cats, several species of
deer, several mustelids and viverrids, and some other top preda-
tors such as the European fox and Arctic fox (Alopex lagopus).
Further introductions (or range extensions) of such invasive
species should be prevented wherever possible, and their eradi-
cation undertaken where feasible. The eradication of introduced
mammals has progressed substantially in recent decades (Veitch
and Clout 2002), especially from islands in countries such as
New Zealand (Clout and Russell 2006). Removal of a range of
mammals from increasingly large areas is now possible using a
range of techniques. Successful eradications have now included
the removal of rats from islands as large as 11 300 ha (Towns and
Broome 2003), and the eradication of coypu from a large area of
south-eastern England (Gosling and Baker 1989).
Changes to ecosystem processes
Losses of native biodiversity (including the extinction of
endemic species) as a consequence of invasion by new mammal
species have now been widely documented, especially in iso-
lated ecosystems such as New Zealand (Atkinson 2006) and
other oceanic archipelagos. It is clear that invasive mammals are
capable of causing radical change to ecosystems that they
invade, not only by extinguishing native prey species and alter-
ing plant communities, but also by disrupting patterns of nutri-
ent flow. An example of such effects is the trophic cascade
generated by the introduction of Arctic foxes to the Aleutian
archipelago (Croll et al. 2005). By preying on seabirds and
greatly reducing their abundance, the foxes also reduced the
input of marine nutrients (imported by seabirds to their breed-
ing colonies) to invaded islands. This subsequently changed the
soil nutrient regime and caused vegetation to change from grass-
land to tundra. Similar indirect effects have been obtained in
studies of the impacts of Rattus species on offshore islands of
New Zealand (Fukami et al. 2006). Comparison of rat-free and
rat-invaded islands revealed that predation of seabirds by rats
reduced forest soil fertility by disrupting nutrient transport by
the seabirds. This fertility reduction led, in turn, to effects on
below-ground organisms and the ecosystem processes driven by
them. Carbon sequestration in live plants was also indirectly
enhanced on the rat-invaded islands (Wardle et al. 2007). In a
similar vein, Wardle (2006) has suggested that introduced
browsing mammals not only directly affect vegetation but also
indirectly alter the decomposer subsystem.
The ultimate ecological impacts of the many introductions of
species that have occurred worldwide in recent centuries and
decades have yet to be fully felt or understood (Strayer et al.
The invasion ecology of mammals: a global perspective
Fig. 2. Eradications and reinvasions of rats on New Zealand islands,
1960–2005. Data extracted from Clout and Russell (2006).
2006). Many introduced mammals are yet to attain the full
extent of their distributions, and global climate change may
influence the potential range that species could occupy. Local
genetic adaptation and even divergent evolution are also possi-
ble over long periods (Gleeson et al. 2006). It is nevertheless
clear that the risk of new invasions should be minimised and the
removal of existing introduced mammals should be seriously
considered where possible.
Interactions between invaders
A topic that merits greater attention when planning eradications
is ecosystem responses to species removals. Ecosystems that
contain multiple invaders, have lost native species along with
their functional roles, or have suffered long-term change to
soils, can respond to eradications in unexpected ways (Zavaleta
2002). Interactions between introduced mammals and other
invasive species may generate positive feedbacks whereby inva-
sion by different species is enhanced (e.g. Pitman et al. 2005), a
process known as ‘invasional meltdown’ (Simberloff 2006).
Interactions between introduced mammals need to be properly
understood before eradication or control of one or all of the
introduced mammal species at a site is undertaken (Courchamp
et al. 2003). For example, ‘mesopredator release’ (Courchamp
et al. 1999) may become evident if a top predator is removed
whilst leaving other predators over which this predator may have
exerted some previous control. ‘Competition release’ may simi-
larly occur when an introduced competitor is removed from a
system (Caut et al. 2007). More research is needed to under-
stand such potential effects.
Risks of reinvasion
A final subject that merits more attention in relation to planning
the eradication of introduced mammals is the risk of reinvasion.
For example, along with recent successes in eradicating rodents
from islands there have been several reinvasions. This is illus-
Wildlife Research 183
trated by data extracted from Clout and Russell (2006), plotting
the number of rat eradications from islands in New Zealand
alongside the number of rat reinvasions (Fig. 2). Eradications
peaked in the early 1990s, but reinvasions have risen since then,
with an apparent lag of ~10 years. Given the current substantial
investment in eradications of introduced mammals from islands
around the world, it is vital that we achieve better understanding
of how mammals (especially highly dispersive species such as
rodents) may be transported or disperse back to sites from which
they have been cleared. It is particularly crucial that we research
the behaviour and detectability of individual dispersers that are
likely to be the founders of new populations. Some experimental
work has been undertaken on rats in this context (Russell et al.
2005, 2008) but more is needed.
Although only a small proportion of the world’s land mammals
can be classed as ‘successful invaders’, their ecological impacts
are sometimes immense. Many introduced species are yet to
attain the full extent of their global and regional distributions.
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introduced mammals interact with each other and with other
components of invaded ecosystems, is of fundamental scientific
interest (Sax et al. 2005; Cadotte et al. 2006). Such research is
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Manuscript received 12 July 2007, accepted 15 October 2007
... They also substantially reduce the resilience of natural ecosystems and thus increase the vulnerability of these ecosystems to the impact of climate change (Mainka and Howard 2010). Mammals introduced into new environments also establish themselves relatively easily and are able to efficiently and rapidly colonise the new environment, especially in temperate regions (Clout and Russell 2007). In the mammalian fauna of the Prioksko-Terrasnyi Biosphere Reserve in Russia, 13 out of 60 species can be classified as invasive (Bobrov et al. 2008). ...
... New knowledge about behaviour of these two non-native and invasive species in Central Europe can contribute to more effective management of these animals. It could also help to mitigate the adverse effects on the native biodiversity of the region and subsequent maintenance of the beneficial conditions of its natural habitats and ecosystems (Clout andRussell 2007, Bobrov et al. 2008). Understanding daily activities represents one of the primary steps in studying autecology and the subsequent development of appropriate management measures to control both of these invasive species. ...
The activity of small invasive mammals, the American mink (Neovison vison) and the coypu (Myocastor coypus), was monitored in western part of Slovakia. Camera traps were located at 9 localities where these animals occur and were monitored throughout all four seasons. The activity of these two invasive species was analysed with regard to the habitat type and environment and, but especially, to the relationship to season, daily period, part of the day and activities. The following animal activities were observed: environmetal exploration, movement, swimming, stationary, grooming, play, fee, feeding, change of environment, mating behaviour and territorial marking. In case of the coypu, crepuscular and nocturnal activities were prevalent. Activity during daytime occurred mostly during winter days with low temperatures. On the other hand, American minks were mostly diurnal. The shift in behaviour compared to American minks in their native environment could be a sign of its adaptation to a new environment. Our research also showed seasonal changes in activity of both invasive mammals. This research could serve as a basis for management schemes to combat the presence and dispersal of these two invasive mammal species.
... Additionally, not all invasive species demonstrate this 'genetic paradox' such as those that are preadapted to conditions in introduced ranges or a founded by multiple introductions (Estoup et al. 2016). Some taxa, such as cervids, appear particularly adept at successfully colonizing new areas and forming self-sustaining, invasive populations (Long 2003, Clout andRussell 2008). ...
... For this reason, mammals have been globally considered the most pernicious species, especially on islands (Lever 1994, Courchamp et al. 2003, Long 2003, Doherty et al. 2016. Practically all of them have been introduced by humans as livestock, companion animals or as commensals; species belonging to Artiodactyla, Perissodactyla, Lagomorpha, Rodentia and Carnivora were the most successful invaders (Clout and Russell 2007). Of these, six species: ship rats, cats, dogs, pigs, goats (Capra aegagrushircus) and cattle (Bostaurus), have been included as the seven most pernicious species worldwide (Bellard et al. 2016). ...
... Biological invasions are the introduction, establishment, and spread of nonnative organisms (Mack et al. 2000, Kolar and Lodge 2001, Clout and Russell 2007, and wild pigs (Sus scrofa) are a prime example of a biological invader in North America. In the United States, wild pigs cost producers and landowners an estimated US$1.5 billion in yearly agricultural damage and control (Pimentel 2007). ...
Wild pig (Sus scrofa) eradication in demographically open populations has seemed an impossible feat for managers, but more recently, whole sounder removal (WSR) has been proposed as a trapping strategy that has the potential to be successful in eradicating wild pigs from an area. However, little empirical data exist concerning implementation of whole sounder removal strategies. Our objective was to implement and describe wild pig management using WSR. We established a 27‐km2 area (northwest section) where sounders were removed using WSR and a 29‐km2 area (southeast section) where GPS collars were deployed on Lowndes Wildlife Management Area in Alabama. Prior to implementing WSR, we used game cameras over bait at a density of one camera/km2 in November 2014 and counted 65 and 100 individuals in our northwest and southeast sections, respectively. We began WSR July 2015 and by May 2016, we reduced the estimated population by 90%. However, due to births and seasonal movements of pigs in the periphery of the study area, the population fluctuated between 10–20 individuals from May 2016 to December 2017. In December 2017, we removed the last known remaining sounder, and using game cameras, observed no sounders in the northwest section for the following 7 months, the remaining length of the study. We determined that using the WSR approach can lead to a delay before inevitable recolonization. Therefore, WSR can be a successful tool to significantly reduce a wild pig population and potentially provide managers a pig‐free area. Wild pigs are considered one of the top vertebrate, terrestrial, invasive pests in North America, and trapping efforts to eliminate wild pigs have met limited success. Here we describe a trapping strategy (whole sounder removal) and its success in eliminating wild pigs on a wildlife management area in central Alabama.
... Many pet shop owners indicated that species escape from enclosures but are later found inside their pet shops. However, this may pose an invasion risk if some small mammal species escape unnoticed as they typically can tolerate a wide range of climatic conditions, have high reproductive rate, catholic diets, are commensal, and predators are mostly ineffective in controlling them (Clout & Russell, 2008;Langton et al., 2001;Latham et al., 2017;Meyer, 2008 (Hagen & Kumschick, 2018;Shivambu et al., 2020b). ...
The sale of live non‐native animals has become a social norm and is of global concern. The pet trade industry has become one of the main pathways where non‐native small mammals are introduced worldwide. We conducted a questionnaire survey in South African pet shops from September 2018 to September 2019 to gain insights into non‐native small mammalian species trade in South Africa. We also investigated whether the pet shop owners were aware of the South African National Environmental Management: Biodiversity Act (NEM: BA; No. 10 of 2004), which regulates and provides management and conservation of the country's biodiversity. A total of 111 pet shop owners/managers responded to the survey, with 26.6% of the owners reporting the sale of birds, 25.1% of fish and 22.5% of mammals. A total of 16 non‐native small mammalian species were reported sold, with European rabbits (Oryctolagus cuniculus), Norwegian rats (Rattus norvegicus) and house mice (Mus musculus) being the most commonly sold pets. We found that breeders, animal rescues and pet shops were the major suppliers of small mammal pets, and in terms of the regulation, most respondents (67.8%) were aware of NEM: BA. However, despite the knowledge of the regulations, some of the traded species pose a serious invasion threat. As a result, we recommend increased regulation, monitoring and public awareness to prevent the potential negative impacts associated with non‐native mammal species in South Africa. La vente d'animaux vivants non indigènes est devenue une norme sociale et constitue une préoccupation mondiale. L'industrie du commerce des animaux de compagnie est devenue l'une des principales voies d'introduction de petits mammifères non indigènes dans le monde. Nous avons mené une enquête par questionnaire dans des animaleries sud‐africaines de septembre 2018 à septembre 2019 pour avoir un aperçu du commerce des espèces de petits mammifères non indigènes en Afrique du Sud. Nous avons également cherché à savoir si les propriétaires d'animaleries connaissaient la loi sud‐africaine sur la gestion environnementale nationale : Biodiversity Act (NEM : BA ; n° 10 de 2004), qui réglemente et assure la gestion et la conservation de la biodiversité du pays. Au total, 111 propriétaires/gestionnaires d'animaleries ont répondu à l'enquête. 26.6 % des propriétaires ont déclaré vendre des oiseaux, 25.1 % des poissons et 22.5 % des mammifères. Au total, 16 espèces de petits mammifères non indigènes ont été déclarées vendues, les lapins européens (Oryctolagus cuniculus), les rats norvégiens (Rattus norvegicus) et les souris domestiques (Mus musculus) étant les animaux de compagnie les plus vendus. Nous avons constaté que les éleveurs, les ONG de protection des animaux et les animaleries étaient les principaux fournisseurs de petits mammifères à titre d'animaux de compagnie. En termes de réglementation, la plupart des répondants (67.8%) étaient au courant de la NEM : BA. Cependant, malgré la connaissance de la réglementation, certaines des espèces commercialisées représentent une menace sérieuse de prolifération. Par conséquent, nous recommandons de renforcer la réglementation, la surveillance et la sensibilisation du public afin de prévenir les impacts négatifs potentiels associés aux espèces de mammifères non indigènes en Afrique du Sud.
... Many animal species were introduced for human use (e.g., food, labor, pets, hunting) and are now widespread (Long, 2003), and plants introduced for horticulture, forestry, or agriculture (Zenni, 2014) are the majority of invasive alien species in natural areas (Pyšek et al., 2012). Invasive alien species disrupt organism interactions and ecosystem processes (Blackwell, 2005;Clout and Russell, 2007) and are associated with more than half of the contemporary species extinctions worldwide (Doherty et al., 2016;Pyšek et al., 2012). ...
The trade in a wildlife species is driven by a unique combination of economic, cultural, and societal motivations, which fluctuate over time and space. Although the wildlife trade is vital for the livelihood of millions of people worldwide, it can bring serious consequences for the environment, economy, and human health when it is not well managed or regulated. In addition, loss of biodiversity and ecosystem services, spread of invasive alien species, and zoonoses and other diseases can be connected to the wildlife trade in its illegal or unsustainable form. Here, we present some purposes and drivers of the trade, the actors and legislation involved, and some trends and patterns of one of the most relevant challenges in conservation.
... Sleeper populations of introduced species can persist at low abundance for decades before being triggered by an environmental factor to become abundant and problematic (Spear et al. 2021). Deer are among the most successful biological invaders globally (Clout, Russell 2008;Davis et al. 2016), and six species established wild populations in Australia in the mid-1800s (Bentley 1998;Davis et al. 2016). As is common among biotic invasions (Crooks 2005), these deer populations initially remained small and localised in ranges reflecting the idiosyncratic pattern of release rather than optimal habitat (Caley et al. 2011). ...
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Sleeper populations of non-native species can remain at low abundance for decades before irrupting. For over a century, fallow deer (Dama dama) in the island state of Tasmania, Australia, remained at low abundance and close to the region in which they were released. Recently, there are indications the population has increased in abundance and distribution. Here, we spatially quantify the population change using a time series of annual spotlight counts from 1985 to 2019 (up to 172 transects annually, totalling of 5756 transect counts). Next, we predict the potential for further range expansion, using global occurrences to characterise the species’ climatic niche, and remote-camera surveys (3225 camera sites) to model fine-grained habitat suitability. Spotlight counts of fallow deer increased by 11.5% annually, resulting in a 40-fold increase from 1985 to 2019. The core distribution increased 2.9-fold during this 35-year period, and now spans c. 27% of Tasmania’s land area. Satellite populations have established in locations where farmed deer have escaped or been released, suggesting that humans have facilitated range expansion via new introduction events. Based on climate and habitat suitability, our models predict that 56% of Tasmania is suitable under the current climate. This suggests range expansion is likely to continue unless the population is actively managed, which could include the eradication of satellite populations and containment of core populations. This case study cautions that despite over a century of slow population growth, sleeper populations of non-native species can abruptly increase.
... Both factors may interact multiplicatively, making their simultaneous study relevant (Brook et al. 2008). In the case of family Cervidae or cervids, one of the mammalian taxa with the highest proportion of successful invaders globally (Clout and Russell 2008), several species preferentially utilize plantations or disturbed forests rather than other natural habitats (Lantschner et al. 2013;Tejeda-Cruz et al. 2009). ...
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Axis deer (Axis axis), an introduced invasive species of growing concern around the globe, have rapidly expanded through the southern cone countries in South America. Despite increasing culling efforts over 14 years, axis deer remained abundant at El Palmar National Park in north-eastern Argentina. We tested whether this continued abundance possibly reflected control failures as a result of adjacent plantation forests providing a safe-haven refuge for deer. We carried out a cross-sectional survey of deer faecal pellet groups and tracks in 77 matched pairs of 25 m2 plots deployed at random over the park–plantation interface and assessed the presence of deer trails along the 14.2-km wire fence between both land-use types. The relative odds of having at least one pellet group (occupancy) were 4.5 (95% CI 1.5 to 18.3) times higher among park plots than plantation plots. Using generalized linear mixed models, the relative odds of occupancy decreased significantly with increasing distance to the main permanent water course, but it was 83% lower in plantation plots than in the park plots. Principal component analysis of shrub cover, plant structure and plant height revealed greater shelter within the park. Deer trails were spatially aggregated up to 2300 m and were directly associated with deer occupancy. These results indicate that, in El Palmar National Park interface, plantation forests do not provide a refuge or selected habitat, and suggest instead that the environmental characteristics and diversity of habitats within the protected area are relevant for the effectiveness of the exotic ungulate management program.
The exclusion or local extirpation of native species by exotic or introduced carnivores is a burgeoning issue for conservation. Exotic carnivores may indeed present a serious threat as they have the potential to negatively influence and/or interact with native wildlife via exploitative or interference competition, intraguild predation and/or transmission of pathogens. So far, studies investigating co‐occurrence have failed to include both a spatial and temporal component which is likely to lead to improper inference. Here, we used a novel approach to investigate the relationship between native and exotic carnivores across both space and time and provide insight on the spatial exclusion of the native spotted fanaloka, Fossa fossana (listed as Vulnerable by the IUCN), by the exotic small Indian civet, Viverricula indica , across Madagascar's eastern rainforest ecosystem. We combined both spatial (single‐species and two‐species occupancy analyses) and temporal (kernel density estimation) analyses to investigate potential spatio‐temporal interactions across the landscape, comparing degraded and non‐degraded forests. We found that the exotic Indian civet negatively influenced spotted fanaloka occupancy, which resulted in a strong decrease in occupancy across degraded forests. Further, spotted fanaloka occupancy decreased by 40% at sites where Indian civet were present, resulting in a strong lack of co‐occurrence between these two species. Finally, we recorded strong spatio‐temporal overlap during the nocturnal time period within degraded, patchy forests. As a result, we suggest that this reveals evidence of spatial exclusion of the spotted fanaloka. This novel approach provides a unique investigation across both space and time – allowing us to identify more accurately the precise locations where co‐occurring carnivores are potentially interacting – and has wide‐ranging implications for conservation managers working to address the negative impacts of exotic species on native wildlife.
Successful species introductions are not homogeneously distributed over the globe, which points to the need to understand why some have succeeded, yet others failed. We summarized information on small carnivore introductions worldwide and assessed whether introduction outcomes (success or failure) supported one or more of the following hypotheses: climate‐matching, propagule pressure, inherent superiority, island susceptibility and Darwin's naturalization hypotheses. Using the literature, we summarized: number of individuals released, mean body size, mean litter size, consumer type, latitude difference, ecoregions difference, congener presence, and mainland or island release. We generated generalized linear models and ranked them using Akaike's Information Criterion and Akaike's weights. We identified 253 documented introduction events of 24 species from five families, with two thirds of them involving the northern raccoon, Procyon lotor , the American mink, Neovison vison , and the small Indian mongoose, Urva [= Herpestes] auropunctata . Overall introduction success was high, with a success rate > 70% for four of the five represented families. We found support for climate‐matching, inherent superiority, and Darwin's naturalization hypotheses. Likelihood of success increased with matching climatic conditions that allow survival, a greater body size together with smaller litter size, a carnivorous diet, and the absence of congeners in the area of introduction. Islands were not more susceptible than the mainland, and the number of individuals introduced did not influence success. As biological invasions become increasingly widespread, understanding the biological and environmental factors affecting introduction success is important for conservation and management.
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Forest dwelling browsing mammals, notably feral goats and deer, have been introduced to New Zealand over the past 220 years; prior to this such mammals were absent from New Zealand. The New Zealand forested landscape, therefore, presents an almost unique opportunity to determine the impacts of introduction of an entire functional group of alien animals to a habitat from which that group was previously absent. We sampled 30 long-term fenced exclosure plots in indigenous forests throughout New Zealand to evaluate community- and ecosystem-level impacts of introduced browsing mammals, emphasizing the decomposer subsystem. Browsing mammals often significantly altered plant community composition, reducing palatable broad-leaved species and promoting other less palatable types. Vegetation density in the browse layer was also usually reduced. Although there were some small but statistically significant effects of browsing on some measures of soil quality across the 30 locations, there were no consistent effects on components of the soil microfood web (comprising microflora and nematodes, and spanning three consumer trophic levels); while there were clear multitrophic effects of browsing on this food web for several locations, comparable numbers of locations showed stimulation and inhibition of biomasses or populations of food web components. In contrast, all microarthropod and macrofaunal groups were consistently adversely affected by browsing, irrespective of trophic position. Across the 30 locations, the magnitude of response of the dominant soil biotic groups to browsing mammals (and hence their resistance to browsers) was not correlated with the magnitude of vegetation response to browsing but was often strongly related to a range of other variables, including macroclimatic, soil nutrient, and tree stand properties. There were often strong significant effects of browsing mammals on species composition of the plant community, species composition of leaf litter in the litter layer, and composition of various litter-dwelling faunal groups. Across the 30 locations, the magnitude of browsing mammal effects on faunal community composition was often correlated with browser effects on litter layer leaf species composition but never with browser effects on plant community composition. Browsing mammals usually reduced browse layer plant diversity and often also altered habitat diversity in the litter layer and diversity of various soil faunal groups. Across the 30 locations, the magnitude of browser effects on diversity of only one faunal group, humus-dwelling nematodes, was correlated with browser effects on plant diversity. However, browser effects on diversity of diplopods and gastropods were correlated with browser effects on habitat diversity of the litter layer. Reasons for the lack of unidirectional relationships across locations between effects of browsers on vegetation community attributes and on soil invertebrate community attributes are discussed. Browsing mammals generally did not have strong effects on C mineralization but did significantly influence soil C and N storage on an areal basis for several locations. However the direction of these effects was idiosyncratic and presumably reflects different mechanisms by which browsers affect soil processes. While our study did not support hypotheses predicting consistent negative effects of browsing mammals on the decomposer subsystem through promotion of plant species with poorer litter quality, our results still show that the introduction of these mammals to New Zealand has caused far-ranging effects at both the community and ecosystem levels of resolution, with particularly adverse effects for indigenous plant communities and populations of most groups of litter-dwelling mesofauna and macrofauna.
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Over the last four decades the eradication of rats from islands around New Zealand has moved from accidental eradication following the exploratory use of baits for rat control to carefully planned complex eradications of rats and cats (Felis catus) on large islands. Introduced rodents have now been eradicated from more than 90 islands. Of these successful campaigns, those on Breaksea Island, the Mercury Islands, Kapiti Island, and Tuhua Island are used here as case studies because they represent milestones for techniques used or results achieved. Successful methods used on islands range from bait stations and silos serviced on foot to aerial spread by helicopters using satellite navigation systems. The development of these methods has benefited from adaptive management. By applying lessons learned from previous operations the size, complexity, and cost effectiveness of the campaigns has gradually increased. The islands now permanently cleared of introduced rodents are being used for restoration of island‐seabird systems and recovery of threatened species such as large flightless invertebrates, lizards, tuatara, forest birds, and some species of plants. The most ambitious campaigns have been on remote subantarctic Campbell Island (11 300 ha) and warm temperate Raoul Island (2938 ha), aimed to provide long‐term benefits for endemic plant and animal species including land and seabirds. Other islands that could benefit from rat removal are close inshore and within the natural dispersal range of rats and stoats (Mustela erminea). Priorities for future development therefore include more effective methods for detecting rodent invasions, especially ship rats (Rattus rattus) and mice (Mus musculus), broader community involvement in invasion prevention, and improved understanding of reinvasion risk management.
The ecological threat represented by invasive species is well-known, but the scientific opportunities are underappreciated. Invasion studies have historically been largely directed at the important job of collecting case studies. Invasion biology has matured and is now incorporated into the heart of ecology, and can be an extension of ecological theory. In this edited volume, global experts in ecology and evolutionary biology explore how theories in ecology elucidate the processes of invasion, while also examining how specific invasions inform ecological theory. This reciprocal benefit is highlighted in a number of scales of organization: population, community and biogeographic, while employing example invaders in all major groups of organisms and from a number of regions around the globe. The chapters in this book utilize many cutting-edge observational, experimental, analytical and computational methods used in modern ecology. By merging conceptual ecology and invasion biology the book offers a better understanding of the invasion process while also developing a better understanding of how ecological systems function.
ABSTRACT The use of simple terms to articulate ecological concepts can confuse ideological debates and undermine management efforts. This problem is particularly acute in studies of nonindigenous species, which alternatively have been called ‘exotic’, ‘introduced’, ‘invasive’ and ‘naturalised’, among others. Attempts to redefine commonly used terminology have proven difficult because authors are often partial to particular definitions. In an attempt to form a consensus on invasion terminology, we synthesize an invasional framework based on current models that break the invasion process into a series of consecutive, obligatory stages. Unlike previous efforts, we propose a neutral terminology based on this framework. This ‘stage-based’ terminology can be used to supplement terms with ambiguous meanings (e.g. invasive, introduced, naturalized, weedy, etc.), and thereby improve clarity of future studies. This approach is based on the concept of ‘propagule pressure’ and has the additional benefit of identifying factors affecting the success of species at each stage. Under this framework, invasions can be more objectively understood as biogeographical, rather than taxonomic, phenomena; and author preferences in the use of existing terminology can be addressed. An example of this recommended protocol might be: ‘We examined distribution data to contrast the characteristics of invasive species (stages IVa and V) and noninvasive species (stages III and IVb)’.
Major progress in understanding biological invasions has recently been made by quantitatively comparing successful and unsuccessful invasions. We used such an approach to test hypotheses about the role of climatic suitability, life history, and historical factors in the establishment and subsequent spread of 40 species of mammal that have been introduced to mainland Australia. Relative to failed species, the 23 species that became established had a greater area of climatically suitable habitat available in Australia, had previously become established elsewhere, had a larger overseas range, and were introduced more times. These relationships held after phylogeny was controlled for, but successful species were also significantly more likely to be nonmigratory. A forward-selection model included only two of the nine variables for which we had data for all species: climatic suitability and introduction effort. When the model was adjusted for phylogeny, those same two variables were included, along with previous establishment success. Of the established species, those with a larger geographic range size in Australia had a greater area of climatically suitable habitat, had traits associated with a faster population growth rate (small body size, shorter life span, lower weaning age, more offspring per year), were nonherbivorous, and had a larger overseas range size. When the model was adjusted for phylogeny, the importance of climatic suitability and the life-history traits remained significant, but overseas range size was no longer important and species with greater introduction effort had a larger geographic range size. Two variables explained variation in geographic range size in a forward-selection model: species with smaller body mass and greater longevity tended to have larger range sizes in Australia. These results mirror those from a recent analysis of exotic-bird introductions into Australia, suggesting that, at least among vertebrate taxa, similar factors predict establishment and spread. Our approach and results are being used to assess the risks of exotic vertebrates becoming established and spreading in Australia. Resumen: Recientemente se ha logrado un progreso importante en el entendimiento de invasiones biológicas al comparar invasiones exitosas y no exitosas cuantitativamente. Utilizamos ese método para probar hipótesis acerca de la adaptabilidad climática, historia de vida y factores históricos en el establecimiento y extensión posterior de 40 especies de mamíferos que han sido introducidas en Australia. En relación con especies no exitosas, las 23 especies que se establecieron tenían una mayor área de hábitat adecuado climáticamente en Australia, se habían establecido en otras partes, tenían un mayor rango y fueron introducidas más veces. Después de controlar para filogenia, estas relaciones se mantuvieron, pero las especies exitosas también fueron significativamente no migratorias. Un modelo de selección anterior incluyó a sólo dos de las nueve variables para las que teníamos datos para todas las especies: adaptabilidad climática y esfuerzo de introducción. Los ajustes para filogenia incluyeron esas dos mismas variables además del éxito de establecimiento previo. De las especies establecidas, aquellas con un mayor rango geográfico en Australia tenían un mayor área de hábitat climáticamente adecuado, tenían características asociadas con una tasa de crecimiento poblacional más rápida (tamaño corporal pequeño, menor longevidad, menor edad de destete, más crías por año), fueron no herbívoras y tenían un mayor rango de distribución fuera de Australia. Los ajustes para filogenia la importancia de la adaptabilidad climática y las características de historia de vida permanecieron significativas, pero el rango fuera de Australia ya no fue importante y especies con mayor esfuerzo de introducción tenían un mayor rango geográfico. Dos variables explicaron la variación en tamaño de rango geográfico en un modelo de selección anterior: especies con menor masa corporal y mayor longevidad tendieron a tener rangos de mayor tamaño en Australia. Estos resultados son semejantes a los de análisis recientes de introducciones de aves exóticas a Australia, lo que sugiere que, por lo menos en taxones de vertebrados, factores similares predicen el establecimiento y extensión. Nuestro método y resultados están siendo utilizados para evaluar los riesgos del establecimiento y de la extensión de vertebrados exóticos en Australia.
AimData on Australian landbridge islands were analysed to seek relationships between the extinction of mammals on islands and a number of variables related to the islands, the native mammal species that occur on them and the presence or absence of exotic mammalian predators. The data base included attributes of the mammals (mean adult body weight, diet and shelter habitat) and of the islands [area, rainfall, presence/absence of significant areas of rockpile habitat, presence/absence of European red fox Vulpes vulpes Linnaeus, 1758, feral cat Felis catus Linnaeus, 1758, and rats Rattus rattus (Linnaeus, 1758) and R. exulans (Peale, 1848)].Methods Statistical analysis of the 388 cases where a Critical Weight Range (CWR) mammal is extant and the forty-one cases where a CWR mammal is extinct was conducted using logistic regression to model the probability of extinction as a function of the island and mammal variables.ResultsFoxes and cats are correlated with CWR mammal extinctions, but cats are associated with extinctions particularly on more arid islands. Extinctions are more likely on islands with an absence of significant areas of rockpile habitat and where the native mammal is restricted to the ground's surface, and is relatively large.Conclusions An association between the presence of cats and native mammal extinctions has not previously been demonstrated for Australian islands. The introduction of exotic predators to Australian islands with native mammal species should be avoided and should any of these predators establish all means should be employed to eradicate them. For Australian continental islands the introduction of exotic predators, not habitat clearance, has been the major factor in extinctions.
1. We investigated factors hypothesized to influence introduction success and sub-sequent geographical range size in 52 species of bird that have been introduced to mainland Australia. 2. The 19 successful species had been introduced more times, at more sites and in greater overall numbers. Relative to failed species, successfully introduced species also had a greater area of climatically suitable habitat available in Australia, a larger overseas range size and were more likely to have been introduced successfully outside Australia. After controlling for phylogeny these relationships held, except that with overseas range size and, in addition, larger-bodied species had a higher probability of introduction suc-cess. There was also a marked taxonomic bias: gamebirds had a much lower probability of success than other species. A model including five of these variables explained perfectly the patterns in introduction success across-species. 3. Of the successful species, those with larger geographical ranges in Australia had a greater area of climatically suitable habitat, traits associated with a faster population growth rate (small body size, short incubation period and more broods per season) and a larger overseas range size. The relationships between range size in Australia, the extent of climatically suitable habitat and overseas range size held after controlling for phylogeny. 4. We discuss the probable causes underlying these relationships and why, in retrospect, the outcome of bird introductions to Australia are highly predictable.