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Invasive predators more harmful to biodiversity than native predators

Alien predators are more harmful to prey populations than native predators, reports a study published in the current issue of the journal Proceedings of the Royal Society B: Biological Sciences.

Analyzing prey response across dozens of field experiments in New Zealand, Australia, and island ecosystems, an international team of researchers led by Pälvi Salo of the University of Turku in Finland found that alien predators have twice the impact as native predators on prey populations. Surprisingly, the research also showed that alien predators in mainland areas had a greater impact than in island environments, though the researchers say this finding may be biased by a large amount of data from Australia.

The authors cite a number of possible factors for the difference in effectiveness between indigenous and alien predators.

A lemur’s potential response to predators. Photo by Rhett A. Butler.
While not a subject in the Salo study, lemurs have been significantly impacted by the invasion of alien predators: most notably humans, but also Indian civet, mongoose, cats, and dogs. Dozens of species of lemurs have gone extinct since mankind first set foot on the island.

“In communities where predators and prey have coexisted for long periods, prey often respond to predatory pressure by developing behaviours or morphologies that reduce the chance of encounters with predators or enhance the chance of escape once detected,” the authors write “In contrast, prey in communities with novel alien predators are likely to be predator-naive and to lack specific avoidance behaviours. Such naiveté would facilitate greater hunting efficiency in predators and lead to greater suppressive impacts on naive prey compared with native predators hunting prey with which they have long coexisted.”

The authors look specifically at Australia, where many of their experimental studies were based.

“Recent research shows that Australia has also possessed a rich assemblage of marsupial carnivores from Miocene to recent times… and that the native fauna may not be as predator naive as previously thought,” they write. “However, Australia never had placental carnivores until they were introduced by humans, and it may be that these novel predators use tracking and hunting tactics that differ from those of their marsupial counterparts, to which native prey have little or no defence.”

The researchers say that while “alien predators have long been presumed to have greater impacts in island ecosystems when compared with the mainland ecosystems… [they] found no support for such phenomena.”

“In these analyses, Australia was considered mainland; but despite its large size, Australia’s island-like characteristics, such as geographical isolation and diversity of endemic species, may also have contributed to the profound impacts of alien predators there,” they explain.

Wallaby in Australia. Photo by Nancy Butler.

“Many native Australian prey populations are now restricted to small, island-like refugia within large tracts of unsuitable, disturbed land owing to the widespread alteration and fragmentation of habitat, and natural adaptations to stochastic environmental conditions… Compared with real island ecosystems, the large land mass of Australia may have provided more room for such refugia, which may possibly have saved some native prey populations from otherwise inevitable extinction. Although the large-scale effects of alien predators may be blunted by such refuge areas, the island-like characteristics of the refugia make them especially vulnerable to alien predators.”

Overall the results bolster the contention that invasive species can have a detrimental impact on biodiversity. Biologists elsewhere argue that alien invasives are among the greatest threats to global biodiversity.

CITATION: Pälvi Salo, Erkki Korpimäki, Peter B. Banks, Mikael Nordström, and Chris R. Dickman (2007). Alien predators are more dangerous than native predators to prey populations. Proceedings of the Royal Society B: Biological Sciences. March 20, 2007. DOI: 10.1098/rspb.2006.0444

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