Extinction of mammals on islands by introduced domestic and feral cats

When the predation of wildlife by feral and domestic cats is raised as a topic the devastating impact of cats on islands is invariably a major issue. Wildlife species on islands are particularly vulnerable because they are isolated from many of the diseases, predators and parasites that plague mammals on the mainland.

Stewart Island - Photo: Getty Images / tsvibrav

Dr. Bradshaw states that island species account for 83% of all documented extinctions of mammals. However, scientists can only implicate feral cats in the destruction of wildlife on these islands in about 15% of such extinctions. And further, within that 15% of such extinctions to which the blame is only the feral cat, other introduced predators should take their share of the responsibility according to Bradshaw in his book Cat Sense.

He says that mongooses, cane toads and especially rats are equally if not more devastating than feral cats on wildlife predation. Black rats a.k.a. ship rats, it is claimed, can do more damage than any other introduced predator. It is ironic, therefore, that it may be beneficial or there are at least some benefits to not slaughtering feral cats because cats are reasonably effective hunters of black rats according to Dr. Bradshaw.

If you attempt to exterminate feral cats (as is currently the objective of Australian legislatures) you might find that the outcome is far worse than imagined in terms of the population of black rats. He cites the example of Stewart Island off the coast of New Zealand. On that island feral cats have existed for more than 200 years with an endangered flightless parrot called the kakapo (owl parrot Strigops habroptilus). The cats mainly fed on the introduced species of the brown and black rat. Those species of rat have been held responsible for the extinction of several other species of birds in the same area.

Removing the cats in these places might lead to an increase in the rat population which in turn might lead to the extinction of the kakapo.

That's just an example because sometimes eradicating cats from an island can lead to a dramatic recovery in the population of certain vertebrate species. Bradshaw cites examples such as iguanas on Long Cay in the West Indies. Also, deer mice on Coronado Island in the Gulf of California.

Although, there is no doubt that the sheer number of feral cats in most places must have a significant impact on wildlife. The difficulty, as I see it, is quantifying that impact and the way that ornithologists and their organisations latch onto biased or estimated predation rates in rather poor studies to further their agenda which is to in effect kill large numbers of cats.