Feral cats are often portrayed as the scourge of of island ecosystems, killing off or pushing out endemic species at an alarming rate. To an extent such a reputation is deserved, but a new study out in the Proceedings of the National Academy of Sciences reveals that the elimination of a top-level introducted predator can lead to an explosion in the numbers of other predators that were being controlled by the “top cat.”
While cats are certainly a problem in many areas where they were introduced, often preying upon adults and chicks in the nest, rats attack the birds from the other side of the reproductive cycle and consume eggs in the nests. Many bird species suffer pressure from both ends, then, but the elimination of cats might actually make things worse for certain species. This is due to something known as “mesopredator release”; when a top-level introduced predator is killed, the populations of a lower-level introduced predator may explode. Such a trend is outlined in the study by Rayner et al., the researchers reviewing 35 years-worth of data from Little Barrier Island off the coast of New Zealand, Cook’s petrel (Pterodroma cookii) being the focus of the study. Being that the bird uses burrows and crevices to nest in, its eggs and young can easily been accessed by predators.
The trends observed are very clear; the removal of cats from the island caused an explosion in the number of rats, which caused a steep decline in the breeding success of the birds. When the rats were wiped out in 2004, however, the birds were free from the two major predation factors and experienced a breeding success higher than what was seen when cats and rats were present together. There was one low-altitude site that did not fit the data, however, the birds showing little change in breeding success between when the rats were present and when they were eradicated. What this might reflect is that the rats on the island preferred different foods at warmer low altitudes where invertebrate prey and plant food were more abundant, high altitude rats turning to egg consumption due to a lack of other preferred food sources. Indeed, the rats did not wipe out the birds everywhere but instead hit nests in certain habitats harder than others, animals that breed across temperature gradients and in varying habitats perhaps having more resistance to extinction due to an explosion in a predator population than if all breeding took place at one site or only in one type of habitat. The differences between a low altitude and high altitude nest site might be slight, but apparently they’re enough to make a difference.
Rayner, M.J.; Hauber, M.E.; Imber, M.J.; Stamp, R.K.; Clout, M.N. (2007) “Spatial heterogeneity of mesopredator release within an oceanic island system.” Proceedings of the National Academy of Sciences, Vol. 104 (52) pp. 20862-20865