Download Invasive species

•••••
Invasive species
Invasive species have been described as the second-greatest
extinction threat in the world today, behind only habitat loss
(Wilcove et al. 1998). Is this true? Are invasive species a major
cause of animal extinctions, or has the extinction threat of
invasive species been exaggerated? By what mechanisms have
invasive species driven animal species to extinction? Are
certain animal groups more threatened by invasive species
than others? Do certain environments increase the vulnerability of animal species to invasive species? Before these
questions can be answered, it is necessary to define what is
meant by the term invasive species.
Definition of invasive species
In the 1980s most ecologists used the term invader to
describe any species that colonized a territory or ecosystem in
which it had never occurred before (Mack 1985; Mooney and
Drake 1989). In the latter 1990s ecologists and policymakers
began to distinguish between nonnative species that did and
did not cause harm, with the term invasive being reserved for
only those nonnative species that cause harm. For example, in
former president Bill Clinton’s 1999 executive order on
invasive species, invasive species were defined as nonnative
species whose introduction causes, or is likely to cause, harm
to the economy, the environment, or human health. Since
about 2000, this has been the most common usage of the term
invasive species, both in the fields of ecology and conservation
and in most national and international doctrines and policies
addressing problems caused by nonnative species. In this
entry, the term invasive species refers to nonnative species
that have been deemed harmful by humans.
Are invasive species a major cause of animal
extinctions?
Invasive species are known to have caused many animal
extinctions. The brown tree snake (Boiga irregularis) was
accidentally introduced into Guam following World War II
(1939–1945). Because the native animals of Guam lacked
predator defenses against snakes, they were easy prey for this
new predator. Within several decades, these snakes had
caused the extirpation (localized extinction) of 12 of the 22
native bird species. For similar reasons, introduced rats and
Grzimek’s Animal Life Encyclopedia
cats have also caused many island bird and small mammal
species to go extinct. Often experiencing little predation
themselves, the rat and feral cat populations can grow mostly
unchecked following their introduction, resulting in large
numbers of novel predators that can drive island prey species
to extinction in decades, or even years. Particularly vulnerable to rat and cat predation are nestlings of oceanic birds
such as puffins, shearwaters, and petrels that live entirely in
the open ocean, except when they come to shore to breed.
These birds typically breed in large dense colonies, with
nesting pairs often numbering into the hundreds of
thousands or even millions. Adult birds, although susceptible
to predation while brooding the eggs or chicks, are not
nearly as vulnerable to predation by the rats and cats as are
their flightless nestlings, which are essentially defenseless.
Perhaps responding innately to the desperate behavior of
thousands of defenseless prey, whose cries and futile efforts
to escape inundate the predators’ senses, the cats and rats
often kill far more chicks than they can possibly eat. As a
result, even modest numbers of rats and cats can decimate
entire breeding colonies.
The Nile perch (Lates niloticus), a large freshwater fish
(individuals can exceed 6.5 feet [2 m] in length and weigh
more than 440 pounds [200 kg]) that is native to many of
the large African rivers, was introduced into Lake Victoria in
the early 1950s to enhance the local fisheries. Prior to the
introduction, Lake Victoria was home to hundreds of species
of fish, many of them found nowhere else in the world.
These included more than 300 species in the family
Cichlidae. While the introduction succeeded in substantially
boosting Lake Victoria’s commercial fishing industry, the
large introduced predator is believed to have caused the
extinction of more than 100 of the lake’s endemic cichlids. In
several Russian lakes a number of native amphipod species
(small crustaceans) are believed to have been extirpated,
replaced by an introduced amphipod from Lake Baikal,
Gmelinoides fasciatus, which had been introduced intentionally into many lakes to enhance fish production. It is thought
that the most likely cause of the extirpations of the native
amphipods has been predation by G. fasciatus on the juveniles
of the native species.
Introduced predatory snails, such as Euglandina rosea, have
driven many native land snails to extinction on Pacific islands.
779
Invasive species
Extinction
that had become a serious crop pest. Introduced flatworms
are also thought to have caused the extinction of some land
snails. For example, Platydemus manokwari, a flatworm
native to New Guinea has been introduced, both intentionally and unintentionally, to many Pacific islands where
they have fed on endemic snails and are believed to be the
primary cause of extinction for some of these species. Like
E. rosea, P. manokwari was sometimes introduced to control
the invasive African snail A. fulica but ended up becoming
invasive itself.
The brown tree snake, Boiga irregularis, drove most of the native bird species
of Guam to extinction on the island following the snake’s introduction in
the middle of the twentieth century. Photo By Martin Cohen Wild About
Australia/Lonely Planet Images/Getty Images.
Ironically, E. rosea, native to the southeastern United States,
was introduced to Hawaii as a biological control agent in the
1950s in an effort to reduce the abundance of another
invasive snail, Achatina fulica, an African herbivorous snail
Introduced diseases are another major cause of animal
extinctions. Avian malaria and avian pox virus, along
with their introduced mosquito vectors, are believed to
have been the primary causes of extinctions of many
Hawaiian native bird species. The pathogen currently
threatening the most species with extinction is likely
Batrachochytrium dendrobatidis, a chytrid fungus that is
lethal to many amphibians. This fungus is believed to have
originated in South Africa and to have been transported
around the world during the twentieth century via the
international trade in the African clawed frog (Xenopus
laevis), a frog species commonly used for research purposes
in developmental biology laboratories. Now found on
all continents except Antarctica, this chytrid fungus has
The Nile perch (Lates niloticus), a voracious predator, is believed to have caused the extinction of more than 100 species of cichlid fish in Lake Victoria
following its twentieth century introduction into this African lake. © Tom McHugh/Photo Researchers, Inc.
780
Grzimek’s Animal Life Encyclopedia
Extinction
Invasive species
The predatory snail, Euglandina rosea, has driven many native land snails to extinction on Pacific islands. It is shown here attacking a native Hawaiian
snail, Anchatinella vulpina. © Photo Resource Hawaii/Alamy.
already caused the extinction of many frog species, and
it is thought that this single pathogen may be one of
the primary causes of the ongoing worldwide decline in
amphibians.
The chytrid fungus, Batrachochytrium dendrobatidis, is believed be one
of the primary causes of the ongoing worldwide decline in amphibians.
Shown is a dead wood frog (Rana sylvatica) in early spring, a possible
victim of chytrid fungus. © John Cancalosi/Alamy.
Grzimek’s Animal Life Encyclopedia
A different fungus, Geomyces destructans, is currently devastating bat populations in the northeastern United States and
adjacent Canadian provinces. Infecting the skin of the bats
and causing a white growth around their noses (which is the
basis for the disease’s name: white-nose syndrome), this
fungus has killed more than one million bats since it was
first identified in bats from a cave in New York state in
2006. The origin of this disease is still not definitively
known, but most research has suggested a possible
European origin. The fungus is found in Europe but does
not have the lethal effect there it is having in North
America. This suggests European bat species have evolved
some immunity to this particular pathogen. The fungus is
believed to disrupt the bats’ winter roosting—the time
when they enter a state of torpor to reduce energetic
demands. By waking the bats repeatedly over the winter,
the fungus causes the bats to use up all their stored energy
so they end up starving to death before the insects emerge
in the spring. Although no bat species has yet gone extinct
as a result of the fungus, populations have been extirpated.
Given the virulence of the fungus and its apparent ability to
be spread widely and quickly, there is concern that at least
regional extinction of some species could be possible in
upcoming decades.
781
Invasive species
Extinction
vulnerability of the species to environmental change. Simply
because of chance, small populations can experience a
significant skew in the ratio of males to females, which can
seriously reduce subsequent reproductive output. Small
populations are also more vulnerable than large populations
to natural catastrophes and extreme weather events.
The combined effect of these different processes is to
create a positive feedback loop that forces the population into
an extinction vortex. In a phenomenon that ecologists call the
Allee effect, a species in the extinction vortex exhibits a
negative growth rate, meaning that the death rate exceeds the
birth rate. Under these conditions, and without sufficient
immigration to compensate for the low birth rate, the
population is doomed. It is only a matter of time until it
goes extinct. Because of the isolation of their environments,
populations inhabiting islands and lakes are often smaller to
begin with, compared with their continental and marine
counterparts. This means that it is more likely that population
declines on islands and in lakes will be susceptible to the Allee
effect, and hence populations resident in these environments
will be more likely to go extinct.
A little brown bat (Myotis lucifugus) in Greeley Mine, Vermont, showing
symptoms of white-nose syndrome (WNS). WNS is caused by a fungus,
Geomyces destructans, which is thought to have killed more than one
million bats since it was first identified in a cave in New York state in
2006. Courtesy of U.S. Fish and Wildlife Service.
Do certain environments increase the
vulnerability of animal species to extinctions?
As the reader may have noticed, most of the examples of
extinctions and extirpations caused by invasive species that have
been presented involve the introduction of new species to
islands or freshwater systems. With few exceptions, it is difficult
to find examples of invasive species that have driven native
species to extinction on continents or in marine systems. Thus,
most documented extinctions caused by invasive species have
occurred in isolated environments.
While introduced enemies may be able to reduce the size
of local populations of continental and marine species greatly,
and sometimes even cause extirpations, it is rare for
introduced enemies to drive continental or marine species to
extinction. The native continental and marine species
generally are able to escape total eradication by persisting in
parts of their range that are unoccupied by the introduced
enemies.
Although in certain instances the last remaining individual
of an island or lake species may meet its demise at the hands
(or jaws) of the introduced enemy, it is likely that the last
individuals probably die for other reason(s). Introduced
enemies can cause extinctions of native species without having
to kill every last individual. Once the invaders have driven
population sizes to very low levels, other factors come into
play that increase the probability of extinction. This is because
small populations are at much greater risk to various random
processes. For example, genetic diversity can be lost due to
chance when populations are very small, increasing the
782
Pathogens are the one type of introduced species that do
seem to have the capability to cause extinctions on continents.
Fungal infections in particular have demonstrated this
potential, as exhibited by the devastating effects of the
white-nose fungus on North American bats and the chytrid
fungus on frogs worldwide. Both of these fungi infect only the
skin, but they damage the skin’s structural integrity and
disrupt various vital physiological processes, eventually
causing the death of the bat or frog. An important aspect of
the biology of these two fungal pathogens is that they do not
require a host to persist in an infected region. Most pathogens
become less abundant as the density of their hosts decline,
thereby representing less of an infection risk when host
numbers are low. When not infecting frogs or bats, however,
the chytrid fungus lives in water and the white-nose fungus
lives in the soil, respectively. This means that even after they
have killed large numbers of frogs or bats in an area, these
fungi are able to persist and continue to infect remaining
individuals and/or immigrants. This may explain why both
fungi have been able to drive continental populations of their
animal hosts to extinction so quickly.
By what mechanisms do invasive species
cause extinctions?
Predation and disease have been the primary causes of
animal extinction by invasive species. This indicates that
disease and top-down effects (effects coming from a higher
trophic level, that is, from predators) are stronger extinction
forces, and threats, than other processes such as competition
and bottom-up effects (effects stemming from changes in food
type and abundance). For example, although changes in
vegetation can cause local declines and even the disappearance
of particular herbivores because of a diminished food supply
(such as during secondary succession or when an introduced
plant species displaces preferred native food plants), there are
few examples of animals actually being driven to extinction by
invasive plants. The primary exception to the general absence
Grzimek’s Animal Life Encyclopedia
Extinction
of extinctions by invasive species based on bottom-up causes
involves species that are feeding specialists or host specialists.
Obviously, the extinction of a particular plant or animal
species on which one or more other species are dependent for
their own survival (such as specialist herbivores or hostspecific parasites) will necessarily result in the extinction of
these other species as well.
Prior to colonization by humans, many islands and lakes
lacked predators or diseases that were present on continents or
marine systems. This often meant that animal species that had
lived for long periods of time on islands or in lakes had not
evolved effective defenses against these new enemies. Ecologists
have argued that prey naïveté among island animals probably
has contributed to their extinctions by introduced predators.
Long-term isolation from certain predatory archetypes (e.g.,
snakes and ground mammals) is believed to be the cause of prey
naïveté for many of these island species. Continental terrestrial
prey are generally not as likely to exhibit naïveté to an
introduced predator, because it is unlikely that any new
predator would represent a new predatory archetype. This is
not the case, however, for continental aquatic systems, in which
the isolation of many freshwater systems is believed to have
similar effects as the isolation of oceanic islands. For example,
the introductions of the European brown trout (Salmo trutta)
into South America and New Zealand and the eastern
mosquitofish (Gambusia holbrooki) into Australia have caused
major reductions in native fish and amphibians. Prey naïveté is
believed to have played a role in these reductions (Hamer et al.
2002). Although a number of freshwater extinctions that
resulted from the introduction of a predator have been
documented, there are few examples of recent extinctions of
marine species caused by an introduced predator, a finding that
is consistent with the hypothesis of increased prey naïveté in
freshwater systems.
The type of naïveté just described is evolutionary naïveté,
in which the species has not evolved recognition abilities for
certain predator types, as opposed to ontogenetic naïveté,
which refers to the lack of individual exposure to a particular
predator type during the prey’s lifetime. In species where
learning plays a large role in predator defense, animals can
lose effective predator defenses rather quickly. Tammar
wallabies (Macropus eugenii), which had been introduced in
the late 1800s onto Kawau Island, New Zealand, which was
free of large wallaby predators, have been reported to have
lost some of their predator-recognition abilities. In a 2001
study, Joel Berger, Jon E. Swenson, and Inga-Lill Persson
found that native North American moose that have lived for
multiple generations in the absence of predators, such as
wolves and grizzly bears, exhibited prey naïveté when these
predators were reintroduced. Berger and his colleagues also
found, however, that predator recognition and avoidance
behavior in the moose developed quite quickly through
learning, leading the researchers to conclude that it was highly
unlikely that the moose would experience a predation
“blitzkrieg” because of these predator introductions. Given
the life span of the moose, as well as the rapidity with
which they regained their predator-avoidance behavior, the
change almost certainly resulted from individual moose
learning through experience. In other instances, though, the
Grzimek’s Animal Life Encyclopedia
Invasive species
acquisition of antipredator responses to a novel predator may
involve natural selection and genetic changes. For example,
the red-legged frog (Rana aurora), an endangered California
species, is reported to have developed recognition abilities
(chemical cues) and antipredator responses to the introduced
American bullfrog (Rana catesbeiana)—changes that are
believed to have a genetic component to them (Kiesecker
and Blaustein 1997).
While these findings provide some hope for prey species
threatened by extinction from introduced predators, prey
need time to develop defenses against a new predator
archetype. As shown by some of the examples of island
extinctions, some novel predators are simply too effective and
the prey are extinguished before they have time to develop or
evolve effective defenses. Even if a new predator does not
represent a new predatory archetype—and hence the prey
does not suffer from naïveté—this does not mean the new
predator cannot drastically reduce the size of the prey
population, or even cause its extinction. If the predator is
highly efficient, prey populations can be substantially reduced
even if the prey recognizes the new species as a predator and
tries to take evasive action. Examples of this phenomenon
include the very heavy predation on the European water vole
(Arvicola terrestris) by the introduced American mink (Mustela
vison; Macdonald and Harrington 2003), the predatory impact
of the red fox (Vulpes vulpes) on eastern gray kangaroos
(Macropus giganteus; Banks, Newsome, and Dickman 2000),
and the Nile perch on cichlid species in Lake Victoria (as well
as human hunters using modern technology on just about any
species).
In species where learning plays a large role in predator defense, animals
can lose effective predator defenses rather quickly. Tammar wallabies
(Macropus eugenii), which had been introduced in the late 1800s onto
Kawau Island, New Zealand—which was free of large wallaby
predators—have been reported to have lost some of their predatorrecognition abilities. © Jose Gil/ShutterStock.com.
783
Invasive species
Extinction
Although one often hears claims that invasive species
threaten to drive native species to extinction by outcompeting
them, there are very few documented examples of extinctions
caused by competition (Davis 2003). The belief that
competition from invasive species represents a major extinction threat is grounded in traditional niche theory, which
holds that resident species have partitioned up the environment so that each species uses a unique set of resources,
thereby minimizing competition among species. The notion
that communities could be saturated with species is implied by
this niche-based argument. In a species-saturated environment, species would have partitioned the resources in the
environment to the maximum extent possible, with any more
partitioning resulting in insufficient resources to support a
species. If many communities are species saturated, then
either a species introduction must fail because the new species
cannot gain access to resources already monopolized by the
residents, or if the species successfully establishes, it must be a
better competitor than one or more of the resident species.
Because the community is species saturated, this means that
the establishment of the new species must be accompanied by
the extirpation of one or more of the native species through a
process known as competitive exclusion. The introductions of
species throughout the world have provided a test of this
niche-based perspective of how communities are maintained.
This natural experiment has consistently shown that communities have not been species saturated and that, more often
than not, communities are able to accommodate new species
without any accompanying extinctions or extirpations of
native species (Davis 2009).
If competition is a relatively weak threat, extinctions caused
by competition should take longer than those caused by
predation and habitat loss. This raises the possibility that so
few competition-driven extinctions have been documented
because not enough time has passed for competitive exclusion
to occur. If this is the case, it has been suggested that more
competition-driven extinctions may be observed in the future.
Yet, the increased time needed for these extinctions to occur
also provides more time for other factors to disrupt the
competitive asymmetry between the new and long-term
resident species, thereby reducing the likelihood that such
extinctions would ever occur. These possible factors include
events and processes that would reduce the abundance of the
new species, such as disturbances, disease, environmental
fluctuations, or even a new introduced species. For example,
in a 1999 study, Michael P. Marchetti concluded that
although the Sacramento perch (Archoplites interruptus) is
threatened by the aggressive dominance of an introduced
bluegill (Lepomis macrochirus), competitive exclusion of the
perch may never occur because of fluctuating environmental
conditions.
A longer time frame also means that the resident species
may have time to adapt to the new competition pressure in its
environment and thereby reduce the intensity of competition
to a level that permits coexistence. For example, the introduction of more than 250 new fish species into the Mediterranean Sea following the completion of the Suez Canal has
resulted in only a single extinction (Por 1978). This has been
attributed to the ability of the long-term residents to respond to
784
During the years following the completion of the Suez Canal in 1869,
more than 250 new fish species dispersed into the Mediterranean
Sea. Despite this large number of species introductions, the new
fish are believed to have resulted in only a single extinction of a
native Mediterranean fish species. © MAPS.com/Corbis.
Grzimek’s Animal Life Encyclopedia
Extinction
the competitive interactions with the Red Sea species by
adjusting their foraging depths. This niche adjustment enabled
the long-term residents, which prefer to feed in the warmer
surface waters of the Mediterranean, to accommodate the
introductions.
Are certain animal groups more threatened by
invasive species than others?
Birds, mammals, amphibians, reptiles, fish, and invertebrates (e.g., mollusks) have all been driven to extinction by
invasive species. Thus, there does not seem to be any
particular taxonomic group of animals that are inherently
more vulnerable than other groups to extinctions caused by
invasive species. Any generalizations from data that can be
made are more likely geographic than taxonomic. Specifically,
as described above, animal species living in isolated environments (e.g., actual or ecological islands) are far more
vulnerable to extinction than are species living on continents
or in marine environments. Beyond this geographic generalization, when it comes to causing animal extinctions, invasive
predator and pathogen species seem to be an equal-opportunity
destroyer.
Have extinction threats by invasive species been
overstated?
In a 1998 study, David S. Wilcove and colleagues concluded
that invasive species are the second-greatest extinction threat to
species in peril. This conclusion has been cited more than 1,600
times since the article’s publication, as well as in countless
research proposals, management documents, and university
classes throughout the world. By the first years of the twentyfirst century, it had become common boilerplate for invasion
literature, the conclusion often presented as fact without any
reference at all. However, there are serious limitations and
some biases in the information that Wilcove and his colleagues
used to come to their conclusion. First, little of the information
used to declare nonnative species the second-greatest threat to
species survival was based on actual data at all, as the authors
were careful to make very clear:
We emphasize at the outset that there are some important
limitations to the data we used. The attribution of a specific
threat to a species is usually based on the judgment of an
expert source, such as a USFWS [US Fish and Wildlife
Service] employee who prepares a listing notice or a state
Fish and Game employee who monitors endangered
species in a given region. Their evaluation of the threats
facing that species may not be based on experimental
evidence or even on quantitative data. Indeed, such data
often do not exist. With respect to species listed under the
ESA [Endangered Species Act], Easter-Pilcher (1996) has
shown that many listing notices lack important biological
information, including data on past and possible future
impacts of habitat destruction, pesticides, and alien species.
Depending on the species in question, the absence of
information may reflect a lack of data, an oversight, or a
determination by USFWS that a particular threat is not
Grzimek’s Animal Life Encyclopedia
Invasive species
harming the species. The extent to which such limitations
on the data influence our results is unknown. (Wilcove et al.
1998, 608–609)
Second, the article dealt with species only in the United
States, as its title made very clear: “Quantifying Threats to
Imperiled Species in the United States.” Thus, it has never
been justifiable to cite this article when making claims about
global extinction threats. Third, the findings are dramatically
affected by the inclusion of Hawaii, which, while of course
part of the United States, has a dramatically different invasion
history than does the continental, and substantially majority,
portion of the country. A similar review of extinction threats
in Canada found introduced species to be the least important
of the six categories analyzed (habitat loss, overexploitation,
pollution, native species interactions, introduced species, and
natural causes, the latter including stochastic events such as
storms and factors inherent to the species, such as limited
dispersal ability; Venter et al. 2006). When the Hawaiian
species were excluded from Wilcove and colleagues’ data, the
United States and Canada did not differ with respect to the
threats posed by introduced species (Venter et al. 2006),
meaning that nonnative species would have ranked very low on
the list of threats to the survival of species in the United States.
Other studies that have examined species threats over a much
larger global area have come to similar conclusions. For
example, an analysis of the causes of species depletions and
extinctions in estuaries and coastal marine waters concluded
that the threat of nonnative species was negligible compared to
that of exploitation and habitat destruction (Lotze et al. 2006).
Biodiversity impacts of invasive species
As mentioned earlier, there is abundant evidence that
introduced predators and pathogens can cause extinctions,
mainly on islands and in freshwater systems. It does not
necessarily follow, however, that biodiversity is reduced in these
regions because of species introductions. Species richness in a
region will decline only if the number of species that have gone
extinct exceeds the number of new species that have been
introduced. This is not the case in most regions of the world,
where species introductions have typically exceeded extinctions,
often by a great margin.
For example, although more than 80 nonnative marine
species are believed to have established themselves in the
North Sea since the early nineteenth century, with respect to
species richness, their impact has been primarily additive, with
little evidence that they have driven any native species to
extinction (Reise et al. 2002). This may be the case with inland
seas as well. Although more than 100 nonnative species are
believed to have been introduced into the Baltic Sea since the
early nineteenth century, at least seventy of which have
become established, no extinctions of native species had been
recorded as of 2002 (Leppäkoski et al. 2002), and this was still
the case at the end of 2007 (personal communication with
Erkki Leppäkoski). Also, in their characterization of the fauna
in the Caspian Sea in a 2002 study, Nikolai V. Aladin, Igor S.
Plotnikov, and Andrei A. Filippov concluded that, while some
of the introduced species produced some undesirable effects,
they primarily contributed to the Caspian Sea’s rich biodiversity.
785
Invasive species
In a 2006 study of the impacts of nonnative species on coastal
marine environments, Karsten Reise, Stephan Gollasch, and
Wim J. Wolff reported that there was no indication that
nonnative species were causing a decline in biodiversity. On the
contrary, they concluded that, more often than not, the new
species expand ecosystem functioning by adding new ecological
traits, intensifying existing ones, and increasing functional
redundancy.
The opening of the Suez Canal in 1869 enabled many
residents of the Red Sea and the Indo-Pacific to move into the
Mediterranean Sea, a phenomenon often referred to as the
Lessepsian migration, named after the French engineer who
supervised the construction of the canal, Ferdinand de Lesseps
(1805–1894). Although there have been some local extinctions
of some native species, the primary biodiversity impact on a
regional scale has been a substantial increase in species
richness. Likewise, the species richness of European aquatic
coastal communities has been enhanced by the introductions
of nonnative species, particularly in the historically biodiversity-poor estuaries. Reise and his colleagues concluded in their
2006 study that in coastal aquatic ecosystems, there is no
support for the idea that if new species come in, others have to
go extinct.
Although animal species on islands typically have been
much more vulnerable to extinction from invasive species than
mainland species, island faunas have also usually exhibited the
most dramatic increases in species richness resulting from
species introductions. This has often been because island
fauna has lacked entire groups of animals. For example,
Hawaii, which had no terrestrial amphibian or reptile species
and only one terrestrial mammal species (a bat) before the
arrival of humans, now has a diverse terrestrial fauna of
amphibians, reptiles, and mammals, all introduced except for
the endemic bat.
While it is true that introductions of animals have
increased the animal diversity in most regions of the world,
it is also true that these introductions have caused a reduction
in the number of species at the global level. At the global
level, the rate of animal extinctions caused by invasive species
far exceeds animal speciation rates. Also, even when animal
Extinction
introductions increase regional species diversity, they also
usually homogenize regional faunas. Homogenization is the
combined result of introductions of nonnative species and
the extirpation of native species. In the United States, the
similarity in the fish faunas of the 50 states has increased
dramatically since European settlement, a finding that was
determined to have been caused primarily by widespread
introductions of game fish, with extinctions of native species
having less of an impact. Frank J. Rahel reported in 2000 that
89 pairs of states in the United States that had no species in
common prior to European settlement shared, by the end of
the twentieth century, an average overlap of 25 species.
Documented cases of animal extinctions and extirpations
caused by invasive species are numerous. In most instances,
these events have taken place in isolated environments,
particularly islands and freshwater systems. Comparatively
few animal extinctions that can be primarily attributed to
invasive species have occurred on continents or in marine
systems. Introduced predators and pathogens have been the
primary agents of animal extinction caused by invasive species
during the past few hundred years. In contrast, competitiondriven extinctions have been rare.
That invasive species seldom drive continental or marine
animals to extinction does not mean that invasive species have
little effect on these animals or their communities. Although a
species may not be eliminated by an invasive species totally, its
numbers may be so reduced that it becomes ecologically
extinct. Ecological extinction occurs when a species has been
reduced to such an extent that it has little effect on other
species or ecosystem processes. Although the species technically is still present, any role that it played in its environment
has essentially vanished.
The extinction threat posed by invasive species is real. On
continents and in marine environments, however, animals face
far more serious extinction threats than introduced species.
Habitat loss, overharvesting, and pollution are the primary
causes of animal extinction in these environments, and it is
these causes, along with climate change, that will continue to
be the primary threats for the foreseeable future.
Resources
Books
Aladin, Nikolai V., Igor S. Plotnikov, and Andrei A. Filippov.
“Invaders in the Caspian Sea.” In Invasive Aquatic Species of
Europe: Distribution, Impacts, and Management, edited by Erkki
Leppäkoski, Stephan Gollasch, and Sergej Olenin. Dordrecht,
Netherlands: Kluwer Academic Publishers, 2002.
Leppäkoski, Erkki, Sergej Olenin, and Stephan Gollasch. “The
Baltic Sea: A Field Laboratory for Invasion Biology.” In
Invasive Aquatic Species of Europe: Distribution, Impacts, and
Management, edited by Erkki Leppäkoski, Stephan Gollasch,
and Sergej Olenin. Dordrecht, Netherlands: Kluwer Academic
Publishers, 2002.
Davis, Mark A. Invasion Biology. Oxford: Oxford University Press,
2009.
Mack, Richard N. “Invading Plants: Their Potential Contribution to Population Biology.” In Studies on Plant Demography,
edited by James White. London: Academic Press, 1985.
Lafferty, Kevin D., Katharine F. Smith, Mark E. Torchin, et al.
“The Role of Infectious Diseases in Natural Communities:
What Introduced Species Tell Us.” In Species Invasions: Insights
into Ecology, Evolution, and Biogeography, edited by Dov F. Sax,
John J. Stachowicz, and Steven D. Gaines. Sunderland,
MA: Sinauer, 2005.
786
Mooney, Harold A., and James A. Drake. “Biological Invasions:
A SCOPE Program Overview.” In Biological Invasions: A
Global Perspective, edited by James A. Drake, Harold A.
Mooney, Francesco di Castri, et al. Chichester, UK:
Wiley, 1989.
Grzimek’s Animal Life Encyclopedia
Extinction
Por, Francis Dov. Lessepsian Migration: The Influx of Red Sea Biota
into the Mediterranean by Way of the Suez Canal. Berlin:
Springer-Verlag, 1978.
Reise, Karsten, Stephan Gollasch, and Wim J. Wolff.
“Introduced Marine Species of the North Sea Coasts.” In
Invasive Aquatic Species of Europe: Distribution, Impacts, and
Management, edited by Erkki Leppäkoski, Stephan Gollasch,
and Sergej Olenin. Dordrecht, Netherlands: Kluwer Academic
Publishers, 2002.
Periodicals
Banks, Peter B., Alan E. Newsome, and Chris R. Dickman.
“Predation by Red Foxes Limits Recruitment in Populations
of Eastern Grey Kangaroos.” Austral Ecology 25, no. 3 (2000):
283–291.
Invasive species
Original Habitats in South-Eastern Australia.” Oecologia 132,
no. 3 (2002): 445–452.
Kiesecker, Joseph M., and Andrew R. Blaustein. “Population
Differences in Responses of Red-Legged Frogs (Rana aurora)
to Introduced Bullfrogs.” Ecology 78, no. 6 (1997): 1752–1760.
Lotze, Heike K., Hunter S. Lenihan, Bruce J. Bourque, et al.
“Depletion, Degradation, and Recovery Potential of Estuaries
and Coastal Seas.” Science 312, no. 5781 (2006): 1806–1809.
Macdonald, David W., and Lauren A. Harrington. “The
American Mink: The Triumph and Tragedy of Adaptation
Out of Context.” New Zealand Journal of Zoology 30, no. 4
(2003): 421–441.
Marchetti, Michael P. “An Experimental Study of Competition
between the Native Sacramento Perch (Archoplites interruptus)
and Introduced Bluegill (Lepomis macrochirus).” Biological
Invasions 1, no. 1 (1999): 55–65.
Berger, Joel, Jon E. Swenson, and Inga-Lill Persson.
“Recolonizing Carnivores and Naïve Prey: Conservation
Lessons from Pleistocene Extinctions.” Science 291, no. 5506
(2001): 1036–1039.
Rahel, Frank J. “Homogenization of Fish Faunas across the
United States.” Science 288, no. 5467 (2000): 854–856.
Davis, Mark A. “Biotic Globalization: Does Competition from
Introduced Species Threaten Biodiversity?” BioScience 53, no.
5 (2003): 481–489.
Venter, Oscar, Nathalie N. Brodeur, Leah Nemiroff, et al.
“Threats to Endangered Species in Canada.” BioScience 56,
no. 11 (2006): 903–910.
Easter-Pilcher, Andrea. “Implementing the Endangered Species
Act.” BioScience 46, no. 5 (1996): 355–363.
Wilcove, David S., David Rothstein, Jason Dubow, et al.
“Quantifying Threats to Imperiled Species in the United
States.” BioScience 48, no. 8 (1998): 607–615.
Hamer, A.J., S.J. Land, and M.J. Mahony. “The Role of
Introduced Mosquitofish (Gambusi holbrooki) in Excluding the
Native Green and Golden Bell Frog (Litoria aurea) from
Grzimek’s Animal Life Encyclopedia
Mark A. Davis
787