Introduced predatory snails, such as Euglandina rosea, have driven many native
land snails to extinction on Pacific islands. Grzimek's Animal Life Encyclopedia.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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