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Invasive species

Invasive species have been described as the second-greatestextinction threat in the world today, behind only habitat loss(Wilcove et al. 1998). Is this true? Are invasive species a majorcause of animal extinctions, or has the extinction threat ofinvasive species been exaggerated? By what mechanisms haveinvasive species driven animal species to extinction? Arecertain animal groups more threatened by invasive speciesthan others? Do certain environments increase the vulnera-bility of animal species to invasive species? Before thesequestions can be answered, it is necessary to define what ismeant by the term invasive species.

Definition of invasive speciesIn the 1980s most ecologists used the term invader to

describe any species that colonized a territory or ecosystem inwhich it had never occurred before (Mack 1985; Mooney andDrake 1989). In the latter 1990s ecologists and policymakersbegan to distinguish between nonnative species that did anddid not cause harm, with the term invasive being reserved foronly those nonnative species that cause harm. For example, informer president Bill Clinton’s 1999 executive order oninvasive species, invasive species were defined as nonnativespecies whose introduction causes, or is likely to cause, harmto the economy, the environment, or human health. Sinceabout 2000, this has been the most common usage of the terminvasive species, both in the fields of ecology and conservationand in most national and international doctrines and policiesaddressing problems caused by nonnative species. In thisentry, the term invasive species refers to nonnative speciesthat have been deemed harmful by humans.

Are invasive species a major cause of animalextinctions?

Invasive species are known to have caused many animalextinctions. The brown tree snake (Boiga irregularis) wasaccidentally introduced into Guam following World War II(1939–1945). Because the native animals of Guam lackedpredator defenses against snakes, they were easy prey for thisnew predator. Within several decades, these snakes hadcaused the extirpation (localized extinction) of 12 of the 22native bird species. For similar reasons, introduced rats and

cats have also caused many island bird and small mammalspecies to go extinct. Often experiencing little predationthemselves, the rat and feral cat populations can grow mostlyunchecked following their introduction, resulting in largenumbers of novel predators that can drive island prey speciesto extinction in decades, or even years. Particularly vulnera-ble to rat and cat predation are nestlings of oceanic birdssuch as puffins, shearwaters, and petrels that live entirely inthe open ocean, except when they come to shore to breed.These birds typically breed in large dense colonies, withnesting pairs often numbering into the hundreds ofthousands or even millions. Adult birds, although susceptibleto predation while brooding the eggs or chicks, are notnearly as vulnerable to predation by the rats and cats as aretheir flightless nestlings, which are essentially defenseless.Perhaps responding innately to the desperate behavior ofthousands of defenseless prey, whose cries and futile effortsto escape inundate the predators’ senses, the cats and ratsoften kill far more chicks than they can possibly eat. As aresult, even modest numbers of rats and cats can decimateentire breeding colonies.

The Nile perch (Lates niloticus), a large freshwater fish(individuals can exceed 6.5 feet [2 m] in length and weighmore than 440 pounds [200 kg]) that is native to many ofthe large African rivers, was introduced into Lake Victoria inthe early 1950s to enhance the local fisheries. Prior to theintroduction, Lake Victoria was home to hundreds of speciesof fish, many of them found nowhere else in the world.These included more than 300 species in the familyCichlidae. While the introduction succeeded in substantiallyboosting Lake Victoria’s commercial fishing industry, thelarge introduced predator is believed to have caused theextinction of more than 100 of the lake’s endemic cichlids. Inseveral 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 intentional-ly into many lakes to enhance fish production. It is thoughtthat the most likely cause of the extirpations of the nativeamphipods has been predation by G. fasciatus on the juvenilesof the native species.

Introduced predatory snails, such as Euglandina rosea, havedriven many native land snails to extinction on Pacific islands.

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Ironically, E. rosea, native to the southeastern United States,was introduced to Hawaii as a biological control agent in the1950s in an effort to reduce the abundance of anotherinvasive snail, Achatina fulica, an African herbivorous snail

that had become a serious crop pest. Introduced flatwormsare also thought to have caused the extinction of some landsnails. For example, Platydemus manokwari, a flatwormnative to New Guinea has been introduced, both inten-tionally and unintentionally, to many Pacific islands wherethey have fed on endemic snails and are believed to be theprimary cause of extinction for some of these species. LikeE. rosea, P. manokwari was sometimes introduced to controlthe invasive African snail A. fulica but ended up becominginvasive itself.

Introduced diseases are another major cause of animalextinctions. Avian malaria and avian pox virus, alongwith their introduced mosquito vectors, are believed tohave been the primary causes of extinctions of manyHawaiian native bird species. The pathogen currentlythreatening the most species with extinction is likelyBatrachochytrium dendrobatidis, a chytrid fungus that islethal to many amphibians. This fungus is believed to haveoriginated in South Africa and to have been transportedaround the world during the twentieth century via theinternational trade in the African clawed frog (Xenopuslaevis), a frog species commonly used for research purposesin developmental biology laboratories. Now found onall continents except Antarctica, this chytrid fungus has

The brown tree snake,Boiga irregularis, drovemost of thenative bird speciesof Guam to extinction on the island following the snake’s introduction inthe middle of the twentieth century. Photo By Martin Cohen Wild AboutAustralia/Lonely Planet Images/Getty Images.

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 Victoriafollowing its twentieth century introduction into this African lake. © Tom McHugh/Photo Researchers, Inc.

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Invasive species Extinction

already caused the extinction of many frog species, andit is thought that this single pathogen may be one ofthe primary causes of the ongoing worldwide decline inamphibians.

A different fungus, Geomyces destructans, is currently deva-stating bat populations in the northeastern United States andadjacent Canadian provinces. Infecting the skin of the batsand causing a white growth around their noses (which is thebasis for the disease’s name: white-nose syndrome), thisfungus has killed more than one million bats since it wasfirst identified in bats from a cave in New York state in2006. The origin of this disease is still not definitivelyknown, but most research has suggested a possibleEuropean origin. The fungus is found in Europe but doesnot have the lethal effect there it is having in NorthAmerica. This suggests European bat species have evolvedsome immunity to this particular pathogen. The fungus isbelieved to disrupt the bats’ winter roosting—the timewhen they enter a state of torpor to reduce energeticdemands. By waking the bats repeatedly over the winter,the fungus causes the bats to use up all their stored energyso they end up starving to death before the insects emergein the spring. Although no bat species has yet gone extinctas a result of the fungus, populations have been extirpated.Given the virulence of the fungus and its apparent ability tobe spread widely and quickly, there is concern that at leastregional extinction of some species could be possible inupcoming decades.

The predatory snail, Euglandina rosea, has driven many native land snails to extinction on Pacific islands. It is shown here attacking a native Hawaiiansnail, Anchatinella vulpina. © Photo Resource Hawaii/Alamy.

The chytrid fungus, Batrachochytrium dendrobatidis, is believed be oneof the primary causes of the ongoing worldwide decline in amphibians.Shown is a dead wood frog (Rana sylvatica) in early spring, a possiblevictim of chytrid fungus. © John Cancalosi/Alamy.


Extinction Invasive species

Do certain environments increase thevulnerability of animal species to extinctions?

As the reader may have noticed, most of the examples ofextinctions and extirpations caused by invasive species that havebeen presented involve the introduction of new species toislands or freshwater systems.With few exceptions, it is difficultto find examples of invasive species that have driven nativespecies to extinction on continents or in marine systems. Thus,most documented extinctions caused by invasive species haveoccurred in isolated environments.

While introduced enemies may be able to reduce the sizeof local populations of continental and marine species greatly,and sometimes even cause extirpations, it is rare forintroduced enemies to drive continental or marine species toextinction. The native continental and marine speciesgenerally are able to escape total eradication by persisting inparts of their range that are unoccupied by the introducedenemies.

Although in certain instances the last remaining individualof an island or lake species may meet its demise at the hands(or jaws) of the introduced enemy, it is likely that the lastindividuals probably die for other reason(s). Introducedenemies can cause extinctions of native species without havingto kill every last individual. Once the invaders have drivenpopulation sizes to very low levels, other factors come intoplay that increase the probability of extinction. This is becausesmall populations are at much greater risk to various randomprocesses. For example, genetic diversity can be lost due tochance when populations are very small, increasing the

vulnerability of the species to environmental change. Simplybecause of chance, small populations can experience asignificant skew in the ratio of males to females, which canseriously reduce subsequent reproductive output. Smallpopulations are also more vulnerable than large populationsto natural catastrophes and extreme weather events.

The combined effect of these different processes is tocreate a positive feedback loop that forces the population intoan extinction vortex. In a phenomenon that ecologists call theAllee effect, a species in the extinction vortex exhibits anegative growth rate, meaning that the death rate exceeds thebirth rate. Under these conditions, and without sufficientimmigration to compensate for the low birth rate, thepopulation is doomed. It is only a matter of time until itgoes extinct. Because of the isolation of their environments,populations inhabiting islands and lakes are often smaller tobegin with, compared with their continental and marinecounterparts. This means that it is more likely that populationdeclines on islands and in lakes will be susceptible to the Alleeeffect, and hence populations resident in these environmentswill be more likely to go extinct.

Pathogens are the one type of introduced species that doseem to have the capability to cause extinctions on continents.Fungal infections in particular have demonstrated thispotential, as exhibited by the devastating effects of thewhite-nose fungus on North American bats and the chytridfungus on frogs worldwide. Both of these fungi infect only theskin, but they damage the skin’s structural integrity anddisrupt various vital physiological processes, eventuallycausing the death of the bat or frog. An important aspect ofthe biology of these two fungal pathogens is that they do notrequire a host to persist in an infected region. Most pathogensbecome less abundant as the density of their hosts decline,thereby representing less of an infection risk when hostnumbers are low. When not infecting frogs or bats, however,the chytrid fungus lives in water and the white-nose funguslives in the soil, respectively. This means that even after theyhave killed large numbers of frogs or bats in an area, thesefungi are able to persist and continue to infect remainingindividuals and/or immigrants. This may explain why bothfungi have been able to drive continental populations of theiranimal hosts to extinction so quickly.

By what mechanisms do invasive speciescause extinctions?

Predation and disease have been the primary causes ofanimal extinction by invasive species. This indicates thatdisease and top-down effects (effects coming from a highertrophic level, that is, from predators) are stronger extinctionforces, and threats, than other processes such as competitionand bottom-up effects (effects stemming from changes in foodtype and abundance). For example, although changes invegetation can cause local declines and even the disappearanceof particular herbivores because of a diminished food supply(such as during secondary succession or when an introducedplant species displaces preferred native food plants), there arefew examples of animals actually being driven to extinction byinvasive plants. The primary exception to the general absence

A little brown bat (Myotis lucifugus) in Greeley Mine, Vermont, showingsymptoms of white-nose syndrome (WNS). WNS is caused by a fungus,Geomyces destructans, which is thought to have killed more than onemillion bats since it was first identified in a cave in New York state in2006. Courtesy of U.S. Fish and Wildlife Service.



of extinctions by invasive species based on bottom-up causesinvolves species that are feeding specialists or host specialists.Obviously, the extinction of a particular plant or animalspecies on which one or more other species are dependent fortheir own survival (such as specialist herbivores or host-specific parasites) will necessarily result in the extinction ofthese other species as well.

Prior to colonization by humans, many islands and lakeslacked predators or diseases that were present on continents ormarine systems. This often meant that animal species that hadlived for long periods of time on islands or in lakes had notevolved effective defenses against these new enemies. Ecologistshave argued that prey naïveté among island animals probablyhas 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 preynaïveté for many of these island species. Continental terrestrialprey are generally not as likely to exhibit naïveté to anintroduced predator, because it is unlikely that any newpredator would represent a new predatory archetype. This isnot the case, however, for continental aquatic systems, in whichthe isolation of many freshwater systems is believed to havesimilar 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 easternmosquitofish (Gambusia holbrooki) into Australia have causedmajor reductions in native fish and amphibians. Prey naïveté isbelieved to have played a role in these reductions (Hamer et al.2002). Although a number of freshwater extinctions thatresulted from the introduction of a predator have beendocumented, there are few examples of recent extinctions ofmarine species caused by an introduced predator, a finding thatis consistent with the hypothesis of increased prey naïveté infreshwater systems.

The type of naïveté just described is evolutionary naïveté,in which the species has not evolved recognition abilities forcertain predator types, as opposed to ontogenetic naïveté,which refers to the lack of individual exposure to a particularpredator type during the prey’s lifetime. In species wherelearning plays a large role in predator defense, animals canlose effective predator defenses rather quickly. Tammarwallabies (Macropus eugenii), which had been introduced inthe late 1800s onto Kawau Island, New Zealand, which wasfree of large wallaby predators, have been reported to havelost some of their predator-recognition abilities. In a 2001study, Joel Berger, Jon E. Swenson, and Inga-Lill Perssonfound that native North American moose that have lived formultiple generations in the absence of predators, such aswolves and grizzly bears, exhibited prey naïveté when thesepredators were reintroduced. Berger and his colleagues alsofound, however, that predator recognition and avoidancebehavior in the moose developed quite quickly throughlearning, leading the researchers to conclude that it was highlyunlikely that the moose would experience a predation“blitzkrieg” because of these predator introductions. Giventhe life span of the moose, as well as the rapidity withwhich they regained their predator-avoidance behavior, thechange almost certainly resulted from individual mooselearning through experience. In other instances, though, the

acquisition of antipredator responses to a novel predator mayinvolve natural selection and genetic changes. For example,the red-legged frog (Rana aurora), an endangered Californiaspecies, is reported to have developed recognition abilities(chemical cues) and antipredator responses to the introducedAmerican bullfrog (Rana catesbeiana)—changes that arebelieved to have a genetic component to them (Kieseckerand Blaustein 1997).

While these findings provide some hope for prey speciesthreatened by extinction from introduced predators, preyneed time to develop defenses against a new predatorarchetype. As shown by some of the examples of islandextinctions, some novel predators are simply too effective andthe prey are extinguished before they have time to develop orevolve effective defenses. Even if a new predator does notrepresent a new predatory archetype—and hence the preydoes not suffer from naïveté—this does not mean the newpredator cannot drastically reduce the size of the preypopulation, or even cause its extinction. If the predator ishighly efficient, prey populations can be substantially reducedeven if the prey recognizes the new species as a predator andtries to take evasive action. Examples of this phenomenoninclude the very heavy predation on the European water vole(Arvicola terrestris) by the introduced American mink (Mustelavison; Macdonald and Harrington 2003), the predatory impactof 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 wellas human hunters using modern technology on just about anyspecies).

In species where learning plays a large role in predator defense, animalscan lose effective predator defenses rather quickly. Tammar wallabies(Macropus eugenii), which had been introduced in the late 1800s ontoKawau Island, New Zealand—which was free of large wallabypredators—have been reported to have lost some of their predator-recognition abilities. © Jose Gil/ShutterStock.com.



Although one often hears claims that invasive speciesthreaten to drive native species to extinction by outcompetingthem, there are very few documented examples of extinctionscaused by competition (Davis 2003). The belief thatcompetition from invasive species represents a major extinc-tion threat is grounded in traditional niche theory, whichholds that resident species have partitioned up the environ-ment so that each species uses a unique set of resources,thereby minimizing competition among species. The notionthat communities could be saturated with species is implied bythis niche-based argument. In a species-saturated environ-ment, species would have partitioned the resources in theenvironment to the maximum extent possible, with any morepartitioning resulting in insufficient resources to support aspecies. If many communities are species saturated, theneither a species introduction must fail because the new speciescannot gain access to resources already monopolized by theresidents, or if the species successfully establishes, it must be abetter competitor than one or more of the resident species.Because the community is species saturated, this means thatthe establishment of the new species must be accompanied bythe extirpation of one or more of the native species through aprocess known as competitive exclusion. The introductions ofspecies throughout the world have provided a test of thisniche-based perspective of how communities are maintained.This natural experiment has consistently shown that commu-nities have not been species saturated and that, more oftenthan not, communities are able to accommodate new specieswithout any accompanying extinctions or extirpations ofnative species (Davis 2009).

If competition is a relatively weak threat, extinctions causedby competition should take longer than those caused bypredation and habitat loss. This raises the possibility that sofew competition-driven extinctions have been documentedbecause not enough time has passed for competitive exclusionto occur. If this is the case, it has been suggested that morecompetition-driven extinctions may be observed in the future.Yet, the increased time needed for these extinctions to occuralso provides more time for other factors to disrupt thecompetitive asymmetry between the new and long-termresident species, thereby reducing the likelihood that suchextinctions would ever occur. These possible factors includeevents and processes that would reduce the abundance of thenew species, such as disturbances, disease, environmentalfluctuations, or even a new introduced species. For example,in a 1999 study, Michael P. Marchetti concluded thatalthough the Sacramento perch (Archoplites interruptus) isthreatened by the aggressive dominance of an introducedbluegill (Lepomis macrochirus), competitive exclusion of theperch may never occur because of fluctuating environmentalconditions.

A longer time frame also means that the resident speciesmay have time to adapt to the new competition pressure in itsenvironment and thereby reduce the intensity of competitionto a level that permits coexistence. For example, the intro-duction of more than 250 new fish species into the Mediterra-nean Sea following the completion of the Suez Canal hasresulted in only a single extinction (Por 1978). This has beenattributed to the ability of the long-term residents to respond to

During the years following the completion of the Suez Canal in 1869,more than 250 new fish species dispersed into the MediterraneanSea. Despite this large number of species introductions, the newfish are believed to have resulted in only a single extinction of anative Mediterranean fish species. © MAPS.com/Corbis.



the competitive interactions with the Red Sea species byadjusting their foraging depths. This niche adjustment enabledthe long-term residents, which prefer to feed in the warmersurface waters of the Mediterranean, to accommodate theintroductions.

Are certain animal groups more threatened byinvasive species than others?

Birds, mammals, amphibians, reptiles, fish, and inverte-brates (e.g., mollusks) have all been driven to extinction byinvasive species. Thus, there does not seem to be anyparticular taxonomic group of animals that are inherentlymore vulnerable than other groups to extinctions caused byinvasive species. Any generalizations from data that can bemade are more likely geographic than taxonomic. Specifically,as described above, animal species living in isolated environ-ments (e.g., actual or ecological islands) are far morevulnerable to extinction than are species living on continentsor in marine environments. Beyond this geographic generali-zation, when it comes to causing animal extinctions, invasivepredator and pathogen species seem to be an equal-opportunitydestroyer.

Have extinction threats by invasive species beenoverstated?

In a 1998 study, David S. Wilcove and colleagues concludedthat invasive species are the second-greatest extinction threat tospecies in peril. This conclusion has been cited more than 1,600times since the article’s publication, as well as in countlessresearch proposals, management documents, and universityclasses throughout the world. By the first years of the twenty-first century, it had become common boilerplate for invasionliterature, the conclusion often presented as fact without anyreference at all. However, there are serious limitations andsome biases in the information that Wilcove and his colleaguesused to come to their conclusion. First, little of the informationused to declare nonnative species the second-greatest threat tospecies survival was based on actual data at all, as the authorswere careful to make very clear:

We emphasize at the outset that there are some importantlimitations to the data we used. The attribution of a specificthreat to a species is usually based on the judgment of anexpert source, such as a USFWS [US Fish and WildlifeService] employee who prepares a listing notice or a stateFish and Game employee who monitors endangeredspecies in a given region. Their evaluation of the threatsfacing that species may not be based on experimentalevidence or even on quantitative data. Indeed, such dataoften do not exist. With respect to species listed under theESA [Endangered Species Act], Easter-Pilcher (1996) hasshown that many listing notices lack important biologicalinformation, including data on past and possible futureimpacts of habitat destruction, pesticides, and alien species.Depending on the species in question, the absence ofinformation may reflect a lack of data, an oversight, or adetermination by USFWS that a particular threat is not

harming the species. The extent to which such limitationson the data influence our results is unknown. (Wilcove et al.1998, 608–609)

Second, the article dealt with species only in the UnitedStates, as its title made very clear: “Quantifying Threats toImperiled Species in the United States.” Thus, it has neverbeen justifiable to cite this article when making claims aboutglobal extinction threats. Third, the findings are dramaticallyaffected by the inclusion of Hawaii, which, while of coursepart of the United States, has a dramatically different invasionhistory than does the continental, and substantially majority,portion of the country. A similar review of extinction threatsin Canada found introduced species to be the least importantof the six categories analyzed (habitat loss, overexploitation,pollution, native species interactions, introduced species, andnatural causes, the latter including stochastic events such asstorms and factors inherent to the species, such as limiteddispersal ability; Venter et al. 2006). When the Hawaiianspecies were excluded from Wilcove and colleagues’ data, theUnited States and Canada did not differ with respect to thethreats posed by introduced species (Venter et al. 2006),meaning that nonnative species would have ranked very low onthe list of threats to the survival of species in the United States.Other studies that have examined species threats over a muchlarger global area have come to similar conclusions. Forexample, an analysis of the causes of species depletions andextinctions in estuaries and coastal marine waters concludedthat the threat of nonnative species was negligible compared tothat of exploitation and habitat destruction (Lotze et al. 2006).

Biodiversity impacts of invasive speciesAs mentioned earlier, there is abundant evidence that

introduced predators and pathogens can cause extinctions,mainly on islands and in freshwater systems. It does notnecessarily follow, however, that biodiversity is reduced in theseregions because of species introductions. Species richness in aregion will decline only if the number of species that have goneextinct exceeds the number of new species that have beenintroduced. 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 marinespecies are believed to have established themselves in theNorth Sea since the early nineteenth century, with respect tospecies richness, their impact has been primarily additive, withlittle evidence that they have driven any native species toextinction (Reise et al. 2002). This may be the case with inlandseas as well. Although more than 100 nonnative species arebelieved to have been introduced into the Baltic Sea since theearly nineteenth century, at least seventy of which havebecome established, no extinctions of native species had beenrecorded as of 2002 (Leppäkoski et al. 2002), and this was stillthe case at the end of 2007 (personal communication withErkki Leppäkoski). Also, in their characterization of the faunain the Caspian Sea in a 2002 study, Nikolai V. Aladin, Igor S.Plotnikov, and Andrei A. Filippov concluded that, while someof the introduced species produced some undesirable effects,they primarily contributed to theCaspian Sea’s rich biodiversity.



In a 2006 study of the impacts of nonnative species on coastalmarine environments, Karsten Reise, Stephan Gollasch, andWim J. Wolff reported that there was no indication thatnonnative species were causing a decline in biodiversity. On thecontrary, they concluded that, more often than not, the newspecies expand ecosystem functioning by adding new ecologicaltraits, intensifying existing ones, and increasing functionalredundancy.

The opening of the Suez Canal in 1869 enabled manyresidents of the Red Sea and the Indo-Pacific to move into theMediterranean Sea, a phenomenon often referred to as theLessepsian migration, named after the French engineer whosupervised the construction of the canal, Ferdinand de Lesseps(1805–1894). Although there have been some local extinctionsof some native species, the primary biodiversity impact on aregional scale has been a substantial increase in speciesrichness. Likewise, the species richness of European aquaticcoastal communities has been enhanced by the introductionsof nonnative species, particularly in the historically biodiver-sity-poor estuaries. Reise and his colleagues concluded in their2006 study that in coastal aquatic ecosystems, there is nosupport for the idea that if new species come in, others have togo extinct.

Although animal species on islands typically have beenmuch more vulnerable to extinction from invasive species thanmainland species, island faunas have also usually exhibited themost dramatic increases in species richness resulting fromspecies introductions. This has often been because islandfauna has lacked entire groups of animals. For example,Hawaii, which had no terrestrial amphibian or reptile speciesand only one terrestrial mammal species (a bat) before thearrival of humans, now has a diverse terrestrial fauna ofamphibians, reptiles, and mammals, all introduced except forthe endemic bat.

While it is true that introductions of animals haveincreased the animal diversity in most regions of the world,it is also true that these introductions have caused a reductionin the number of species at the global level. At the globallevel, the rate of animal extinctions caused by invasive speciesfar exceeds animal speciation rates. Also, even when animal

introductions increase regional species diversity, they alsousually homogenize regional faunas. Homogenization is thecombined result of introductions of nonnative species andthe extirpation of native species. In the United States, thesimilarity in the fish faunas of the 50 states has increaseddramatically since European settlement, a finding that wasdetermined to have been caused primarily by widespreadintroductions of game fish, with extinctions of native specieshaving less of an impact. Frank J. Rahel reported in 2000 that89 pairs of states in the United States that had no species incommon prior to European settlement shared, by the end ofthe twentieth century, an average overlap of 25 species.

Documented cases of animal extinctions and extirpationscaused by invasive species are numerous. In most instances,these events have taken place in isolated environments,particularly islands and freshwater systems. Comparativelyfew animal extinctions that can be primarily attributed toinvasive species have occurred on continents or in marinesystems. Introduced predators and pathogens have been theprimary agents of animal extinction caused by invasive speciesduring the past few hundred years. In contrast, competition-driven extinctions have been rare.

That invasive species seldom drive continental or marineanimals to extinction does not mean that invasive species havelittle effect on these animals or their communities. Although aspecies may not be eliminated by an invasive species totally, itsnumbers may be so reduced that it becomes ecologicallyextinct. Ecological extinction occurs when a species has beenreduced to such an extent that it has little effect on otherspecies or ecosystem processes. Although the species techni-cally is still present, any role that it played in its environmenthas essentially vanished.

The extinction threat posed by invasive species is real. Oncontinents and in marine environments, however, animals facefar more serious extinction threats than introduced species.Habitat loss, overharvesting, and pollution are the primarycauses of animal extinction in these environments, and it isthese causes, along with climate change, that will continue tobe the primary threats for the foreseeable future.

ResourcesBooksAladin, Nikolai V., Igor S. Plotnikov, and Andrei A. Filippov.

“Invaders in the Caspian Sea.” In Invasive Aquatic Species ofEurope: Distribution, Impacts, and Management, edited by ErkkiLeppäkoski, Stephan Gollasch, and Sergej Olenin. Dordrecht,Netherlands: Kluwer Academic Publishers, 2002.

Davis, Mark A. Invasion Biology. Oxford: Oxford University Press,2009.

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: Insightsinto Ecology, Evolution, and Biogeography, edited by Dov F. Sax,John J. Stachowicz, and Steven D. Gaines. Sunderland,MA: Sinauer, 2005.

Leppäkoski, Erkki, Sergej Olenin, and Stephan Gollasch. “TheBaltic Sea: A Field Laboratory for Invasion Biology.” InInvasive Aquatic Species of Europe: Distribution, Impacts, andManagement, edited by Erkki Leppäkoski, Stephan Gollasch,and Sergej Olenin. Dordrecht, Netherlands: Kluwer AcademicPublishers, 2002.

Mack, Richard N. “Invading Plants: Their Potential Contribu-tion to Population Biology.” In Studies on Plant Demography,edited by James White. London: Academic Press, 1985.

Mooney, Harold A., and James A. Drake. “Biological Invasions:A SCOPE Program Overview.” In Biological Invasions: AGlobal Perspective, edited by James A. Drake, Harold A.Mooney, Francesco di Castri, et al. Chichester, UK:Wiley, 1989.



Por, Francis Dov. Lessepsian Migration: The Influx of Red Sea Biotainto 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.” InInvasive Aquatic Species of Europe: Distribution, Impacts, andManagement, edited by Erkki Leppäkoski, Stephan Gollasch,and Sergej Olenin. Dordrecht, Netherlands: Kluwer AcademicPublishers, 2002.

PeriodicalsBanks, Peter B., Alan E. Newsome, and Chris R. Dickman.

“Predation by Red Foxes Limits Recruitment in Populationsof Eastern Grey Kangaroos.” Austral Ecology 25, no. 3 (2000):283–291.

Berger, Joel, Jon E. Swenson, and Inga-Lill Persson.“Recolonizing Carnivores and Naïve Prey: ConservationLessons from Pleistocene Extinctions.” Science 291, no. 5506(2001): 1036–1039.

Davis, Mark A. “Biotic Globalization: Does Competition fromIntroduced Species Threaten Biodiversity?” BioScience 53, no.5 (2003): 481–489.

Easter-Pilcher, Andrea. “Implementing the Endangered SpeciesAct.” BioScience 46, no. 5 (1996): 355–363.

Hamer, A.J., S.J. Land, and M.J. Mahony. “The Role ofIntroduced Mosquitofish (Gambusi holbrooki) in Excluding theNative Green and Golden Bell Frog (Litoria aurea) from

Original Habitats in South-Eastern Australia.” Oecologia 132,no. 3 (2002): 445–452.

Kiesecker, Joseph M., and Andrew R. Blaustein. “PopulationDifferences in Responses of Red-Legged Frogs (Rana aurora)to Introduced Bullfrogs.” Ecology 78, no. 6 (1997): 1752–1760.

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Mark A. Davis



extinction fm vol2 1. · predator defenses against snakes, they were easy prey for this new...

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