Ecologists, conservation biologists, and wildlife managers often accept that the invasion of exotic species leads to the extinction of native species. One reason for their consensus is that native species declines often occur simultaneously and in the same place as exotic invasions. However, the data supporting invasion as a leading cause of extinction is often ambiguous and is based on limited observation. This has prompted some to question the validity of the claim, as well as to suggest ways to clarify the role of invasive species in extinctions.
Robyn Perovich, Cichlid, 2006
Certains droits réservés.
In the September 2004 issue of “Trends in Ecology and Evolution,”Jessica Gurevitch and Dianna K. Padilla criticise the commonly accepted association between exotic invasions and widespread native species extinctions (1). They argue that severe habitat disturbance, the decline or extinction of native species, and an increase in exotic species commonly coincide, and it is often unclear whether exotic species are causing the decline of native species, or whether habitat alteration is facilitating the decline of natives and proliferation of exotics simultaneously.
The ability to distinguish between species decline due to invasion or habitat loss is important in determining whether removing all invasive species to prevent extinctions is necessary, or largely a waste of time and money. If it turns out that many invaders are not a primary cause of extinction, but are merely correlated with other problems, then we might be better off focusing our energies on more effective ways of protecting endangered species (1).
The first step in prioritizing our responses to the threat of invaders is distinguishing between the relative importance of different functional groups that cause the extinction of species (1). Theory and observational data often reveal that exotic predators and pathogens are more likely to cause extinctions than exotic competitors, which more often cause displacement of native species and changes within the community (2).
Even within functional groups, a few species play a disproportionate role in extinction. For example, the introduced brown tree snake of Guam has caused, through predation, the extirpation or serious reduction in most of the island’s 25 resident bird species, including 17 of 18 native species (3). Likewise, feral pigs, a few widespread rat species, and a finite list of other invaders might be responsible for most of the extinction risk posed by invasive species (1).
In other well documented cases of species extinction, the link between invasion and extinction is not as clear. The Nile perch is generally blamed for the extinction of half of the 500 or more species of endemic cichlids in Lake Victoria, East Africa. However, a study in 2002 showed that increases in phytoplankton production from 1930 onwards paralleled human population growth and agricultural activity in the drainage basin. These phytoplankton blooms led to a decline in oxygen in the deeper waters, which enabled the decimation of cichlids by Nile perch by eliminating the deep water refuges that had protected the fishes from excessive predation (4). Similarly, the zebra mussel is charged with accelerating the regional extinction rates of North American freshwater mussels by a factor of ten. Of the 12% of 297 species that are presumed to be extinct, however, no extinctions have been directly attributed to zebra mussels. Instead, these extinctions are linked to widespread habitat degradation including pollution, siltation, and river channel modification (5).
Though such case studies show that specific invaders are at least a co-contributor to extinction of native species, until recently, data on the causes in the decline and extinction of species have been limited. Therefore it has been nearly impossible to assess general patterns of threats to endangered species (1). Major recent efforts to collect and compile data on threats have been made to correct this problem. Two of the most important sources include a paper by Wilcove et al. discussing threats to US species (6), and the World Conservation Union (IUCN) Red List (7). Both sources cite habitat loss as the leading cause of extinction, affecting 85% of US species listed by Wilcove et al., and 33% of IUCN listed species respectively (the percentages are often higher within specific taxonomic groupings), while 50% of US species and 6% of IUCN listed species are threatened by exotic invaders.
Examining only the data related to the threat of invasive species on native species reveals specific trends. Firstly, native species inhabiting islands seem to be particularly vulnerable to the threats posed by invasive species. Nearly 100% of native Hawaiian birds and plants are affected by invasive species versus 50% of continental US birds and 30% of continental US plants (6). Further analysis of Wilcove et al’s data by Gurevitch and Padilla found that of the 602 US plants listed as affected by invasive species, 40% of those were Hawaiian endemics affected by feral pigs, goats, and invasive plants. Of 68 bird species listed by Wilcove et al., 34% were Hawaiian birds threatened by invasive pathogens (1). The enhanced susceptibility of island species to invaders is not surprising when we consider the geographic isolation of islands. Because few organisms are able to cross large expanses of water, island communities generally include few large grazers or predators, and native species have therefore lost or never developed defenses against such enemies (8).
Feral and domesticated livestock introduced by humans are also a highly significant threat to endangered animals and plants. Feral pigs, goats, and cattle threaten 64% of all 930 US species listed by Wilcove et al. as affected by invaders. This includes 92% of all listed US plants and 31% of listed US birds. Data compiled by the IUCN in 2003 shows that three and half times as many plants are affected by livestock as by non-domesticated invasive species. Furthermore, farming of livestock threatened almost as many animals as did invasive predators (1).
Finally, it is important to clarify that very few native species are threatened solely by exotic invaders. Of the 602 US plants listed by Wilcove et al. as affected by invaders, 20% of those are also affected by habitat loss. Habitat destruction also affects 71% of 68 bird species listed as threatened by invasive species (1). Many of the species listed as threatened by exotics by the IUCN in 2003 are also threatened by other factors such as habitat destruction and exploitation (7).
Large data sets such as these are obviously useful in helping to understand patterns and causes of decline in threatened or extinct species, but they are not without their limitations. Since most species at risk face multiple threats, it can be difficult to disentangle the various causes of decline, not to mention the interactions between them. Despite these limitations, data sets can help to identify patterns more effectively than generalizing from specific case studies, thus providing a starting point for prioritizing our responses to threats.
Such recommendations follow Gurevitch and Padilla’s observations that not all invasions are created equal, not all invaders pose equal threats, and not all communities are equally threatened (1). We need to be strategic so we can deal with threats of extinction directly rather than spreading our resources too thin trying to combat all exotic invaders. However, there is an inherent problem in using global extinction as the standard for classifying the importance of exotic invasions as Gurevitch and Padilla do in their article. Any given species can be made up of many genetically distinct populations. If we conduct analyses focusing only on global extinctions by invasive species we ignore a very significant loss of biodiversity. By narrowing our focus to the population level, we gain an even deeper understanding of the impact of invasive species on biodiversity loss, since a native species might lose many unique populations without becoming globally extinct.
References
(1)Gurevitch, J., and D.K. Padilla. “Are invasive species a major cause of extinctions?” Trends in Ecology and Evolution 19 (2004): 470-474.
(2)Davis, M.A. “Biotic globalization: does competition from introduced species threaten biodiversity?” Bioscience 53 (2003): 481-489.
(3)Wiles, G.J., Bart, J., Beck JR, R.E., and C.F. Aguon. “Impacts of the brown tree snake: patterns of decline and species persistence in Guam’s avifauna.” Conservation Biology 17 (2003): 1350-1360.
(4)Verschuren, D., Johnson, T.C., Kling, H.J., Edgington, D.N., Leavitt, P.R., Brown, E.T., Talbot, M.R., and R.E. Hecky. “History and timing of human impact on Lake Victoria, East Africa.” Proceedings of the Royal Society of London, Series B, Biological Sciences 269 (2002): 289-294.
(5)Ricciardi, A., Neves, R.J., and J.B. Rasmussen. “Impending extinctions of North American freshwater mussels (Unionoida) following the zebra mussel (Dreissena polymorpha) invasion.” Journal of Animal Ecology 67 (1998): 613-619.
(6)Wilcove, D.S., Rothstein, D., Dubow, J., Phillips, A., and E. Losos. “Quantifying threats to imperiled species in the United States.” Bioscience 48 (1998): 607-615.
(7) IUCN 2006. “2006 IUCN Red List of Threatened Species.” Jun. 2007 < http://www.iucnredlist.org>.
(8) Primack, R.B. “Overexploitation, Invasive Species, and Disease.” Essentials of Conservation Biology 4th ed. Ed. Richard Primack. Massachusetts: Sinauer Associates, Inc. (2006): 215-242.