Using aliens to explore how our planet works
As humans have colonized the far-flung reaches of the globe, they have generally not traveled alone. Deliberately or accidentally, they have taken with them a wide range of species whose geographic distributions did not originally encompass the destination, some of which have then found the destination to their liking. These alien species can be bad news for the natives with which they are brought into contact, which, surprisingly, often seem to be at a disadvantage on their home turf. Aliens may outcompete the natives (1), eat them (2), infect them with terminal diseases (3), or hybridize with them (4). All of these processes can lead to extinction of the native species, and indeed many species are at risk of extinction because of the presence of alien species (5).
A classic example of the potential negative consequences of alien species is provided by the introduction of exotic predatory mammals to oceanic islands. Before humans gave them a helping hand, predatory mammals had never made it to these islands, and so they were home to assemblages of native animals with no experience of how to behave on first contact. Endemic island birds have been particularly hard hit. Approximately 90% of historical bird extinctions concern island endemic species (2), and predation by alien mammals is implicated in many of these losses (6). Moreover, the more alien mammal predator species that have been introduced to an island, the greater the proportion of native bird species that have subsequently been lost (7). As a result, many island endemic bird species only persist on islands where mammal predators are absent, and the eradication of such predators is a high conservation priority to prevent further extinctions (8, 9).
A crumb of comfort given the negative impacts of many alien species is that at least some can be exploited for positive benefit. One way is to use them as probes into how ecological systems work. The processes underlying the form and structure of ecological assemblages are incompletely understood and remain controversial (10–12). Alien species have infiltrated a wide range of assemblages and have positive practical advantages as probes (13). They allow us to study change processes in real time, rather than infer past events; records of the time, place, and characteristics of alien species introductions allow rate processes to be measured; and alien species derive from so many different taxa, and occur in so many different environments, that the full panoply of biological questions is open to investigation using them. Classic insights from such studies include evidence for sympatric speciation (14), that species are generally limited in their distribution by dispersal (15), and that species are not optimally adapted for their environment (16).
To these examples can now be added evidence for mesopredator release. This comes from an opportunistic study by Rayner et al. (17) in a recent issue of PNAS that exploits a serendipitous combination of harmful alien mammalian predators on an island, the conservation action that they prompted, and changes that resulted in the native community. The study revolves around the managed eradication of alien mammals from Little Barrier Island, an island nature reserve in the Hauraki Gulf near Auckland, New Zealand.
Alien species have infiltrated a wide range of assemblages and have positive practical advantages as probes.
Mesopredator release is the situation where the elimination of a top predator from a food chain can lead to apparently paradoxical declines in prey species, because the elimination allows populations of smaller, intermediate predators to increase in abundance and effect. Little Barrier Island lost its top predator in 1980 when the Department of Conservation (DOC) completed the successful eradication a large population of alien cats (Felis catus) from the island (18). Little Barrier is also home to 99% of the world's breeding population of Cook's petrel (Pterodroma cookii), a globally endangered burrow-nesting seabird. Cats were a major predator of Cook's petrel adults and chicks, and hence a major threat to the continued existence of this species (6). Surveys on the breeding success of Cook's petrels on Little Barrier starting in 1971 allowed Rayner et al. to quantify the impact of cat eradication on this success. Data from two high-altitude sites on the island showed that eradicating cats actually caused breeding success in Cook's petrel to decline, from 0.32 ± 0.03 chick per burrow in the years leading up to eradication, to only 0.09 ± 0.04 chick per burrow in years following.
The decline in breeding success witnessed for these petrels may be consistent with mesopredator release, because Little Barrier was also home to a second species of alien mammal predator, the Pacific rat or kiore (Rattus exulans). These also fell prey to the cats, and although there are no data to confirm it, it is likely that kiore increased in abundance on Little Barrier after cat eradication. Kiore are known to prey on the eggs and chicks of ground-nesting seabirds (19) and have previously been implicated in the extinction of such species on New Zealand islands (20–22). If kiore were indeed responsible for the decline in the breeding success of Cook's petrels, then an obvious prediction is that eradication of the kiore would result in that breeding success increasing, and indeed increasing to levels above those observed before the eradication of either predator. Rayner et al. were able to test these predictions because, in 2004, DOC successfully eradicated kiore from Little Barrier. Cook's petrel breeding success at the two high-altitude sites subsequently increased to 0.59 ± 0.03 chick per burrow, significantly higher than before the eradication of rats and of cats and in line with the predictions of mesopredator release.
Rayner et al. consider other possible explanations for the observed changes in petrel breeding success, including long-term (climate change) and short-term (El Niño–Southern Oscillation) climatic fluctuations, and depredation by native tuatara (Sphenodon punctatus) or owls (Ninox novaeseelandiae). However, climatic fluctuations do not coincide with changes in success, tuatara are very rare and mainly present only at low altitudes, and owls cannot access petrel burrows to depredate petrel eggs and chicks (the main cause of breeding failure). Thus, none of explanations provides a convincing alternative to mesopredator release, leaving Rayner et al. to conclude that this is the most likely explanation for changes in petrel breeding success.
However, there is one result from the survey data that mesopredator release has a hard time explaining. Surveys of Cook's petrel breeding success started in 2003 at a low-altitude site on the Little Barrier and revealed no significant change in breeding success after kiore eradication. Petrels here raised 0.66 ± 0.04 chick per burrow when kiore were present but 0.59 ± 0.05 after kiore removal. These productivities are similar to those from high-altitude sites in the absence of alien mammal predators but are not consistent with the idea that kiore were depressing breeding success here. Rayner et al. speculate that kiore may have different nutritional requirements at low- and high-altitude sites or that vegetable matter may be a more important element of the kiore diet at the former. Neither of these explanations is overly convincing, and Rayner et al. confess to puzzlement at the result. It does suggest at least one avenue for future study.
Introducing alien mammals to oceanic islands has been an unmitigated disaster as far as the native birds are concerned. Their losses have been ours too because we now have no possibility to study and enjoy the unique bird assemblages that evolved on these islands over millennia of isolation. However, the losses have not been total. Some island species have persisted in the face of the alien invasions. Conservationists are striking back against these invaders with remarkable and increasing success, such that hundreds of alien mammal populations have been cleared from islands across the globe (8). As Rayner et al. show, ecologists and conservation biologists can learn valuable lessons about how natural systems work from these eradications—large-scale experimental manipulations that we could never in conscience advocate for native species.
Footnotes
- *E-mail: tim.blackburn{at}ioz.ac.uk
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Author contributions: T.M.B. wrote the paper.
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The author declares no conflict of interest.
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See companion article on page 20862 in issue 52 of volume 104.
- © 2008 by The National Academy of Sciences of the USA
