December 11, 2007
Observatory
When Removing One Predator Harms the Prey
By HENRY FOUNTAIN
The predator-prey relationship is simple, right? If a predator is around, that is bad for the prey, and if the predator is removed, that is good for the prey.
Ecological theory, however, suggests that isn't always the case, particularly if there is more than one predator species around and they share the same prey. In that case, elimination of the top predator may allow the midlevel predator to thrive, and a result may actually be worse for the prey.
Matt J. Rayner of the University of Auckland and colleagues found such a case on Little Barrier Island, a wildlife sanctuary in New Zealand. They studied the impact of two predators, feral cats and kiore, or Pacific rats, on a small burrowing seabird, Cook's petrel.
Kiore were introduced to the island hundreds of years ago, and cats were introduced in the 1870s. Both preyed on the petrels, with the cats also preying on the rats. Both were eventually eradicated, the cats in 1980, the rats in 2004.
The researchers analyzed data on petrel chick survivability from 1972 to 2007. As they report in The Proceedings of the National Academy of Sciences, removing the cats actually made life worse for the petrels, since that left more kiore to prey on them.
The researchers also found that the effect of the rats on petrels was worse at higher elevations than at lower ones. They suggest that at higher, colder elevations less variety of foods was available to the rats.
Only when the rats were eliminated did petrel breeding success increase, to a level above that when both predators were around. The researchers say the findings reinforce the idea that an understanding of an entire ecosystem is crucial to proper management of introduced species.
http://www.nytimes.com/2007/12/11/science/11obprey.html?th=&emc=th&pagewanted=print
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