17 December 2007

Now stop blaming the cats for everything

Mesopredator release hypothesis predicts that on an oceanic island where a native species is the common prey of two or more introduced predators, the eradication of the top predator that also feeds on the smaller mesopredators will increase the predation rates of the mesopredators on the native species. This leads to the conclusion that as long as both the top and mesopredators are present, the introduced top predators, by controlling the numbers of mesopredators, could actually be beneficial to the native prey.

An almost direct test of this hypothesis has been carried out on Little Barrier Island off New Zealand and published in the Proceedings of the National Academy of Sciences*. The mesopredator Pacific rat (Rattus exulans) and the top predator feral cat (Felis cattus) were introduced to Little Barrier Island in the 13th and the late 19th centuries, respectively. The rats preyed on the native burrowing seabird the Cook's petrel (Pterodroma cookii), while the cats preyed on both the rats and the birds. The cats were eradicated in 1980 and the rats in 2004.

Rayner et al. provided data that show that the removal of cats from the Little Barrier Island ecosystem actually led to a decrease in the breeding success of Cook's petrel at high-altitude sites. Although apparently no data are available demonstrating directly that following the removal of cats, the numbers of rats and the numbers of birds preyed on by rats both increased, the authors seem to be able to discount other explanations for the available data. However, it is not clear why the presence or absence of rats had almost no impact on the birds at low altitudes.

Breeding success (in terms of chicks per burrow) of Cook's petrel at high-altitude (squares and circles) and low-altitude (triangles) sites on Little Barrier Island before and after the sequential removal of its predators. Fig. 1 from Rayner et al.

We are usually too quick and eager to rely on simple solutions to remedy negative impacts of complicated phenomena. Domesticated cats and their feral cousins are often blamed for otherwise unexplained population declines of birds and small mammals. Likewise, overpopulations of deer have been blamed for various troubles our forests are facing, including the supposed declines in snail populations even when there is no reliable scientific support for such claims. We need to realize that complex phenomena usually have complex causes that cannot be explained away by creating scapegoats. Perhaps, a paraphrase of Leslie Orgel's dictum is needed: ecological processes are cleverer than you are.

(Via A DC Birding Blog.)

*Matt J. Rayner, Mark E. Hauber, Michael J. Imber, Rosalie K. Stamp, and Mick N. Clout. Spatial heterogeneity of mesopredator release within an oceanic island system
Published online before print December 14, 2007.


Roger B. said...

Since our neighbours moved away two months ago, our garden has been vitually cat-free for the first time in about 5 years and we have noticed a marked increase in birds visiting the garden feeders.

The downside is that the garden now seems to be home to a small but active population of brown rats (Rattus rattus).

We never saw rats in the garden whilst it was patrolled daily by the neighbours' cat. But then again, we never saw many birds either.

You can't win!

Anonymous said...

Perhaps you are a bit biased because you are a cat owner?