by Greg Mayer

It’s long been known that house cats, which are introduced to most of the places they occur (the wild members of the species are found in Europe, North Africa, and western Asia), can wreak havoc on native wildlife, perhaps the most infamous case being that of the Stephens Island Wren (Xenicus lyalli). It has often been said that the wren was exterminated by the lighthouse keeper’s cat, but the story is both a bit more complex, and much more tragic: many cats were involved, not just one, and not just the Wren, but the entire Stephens Island land bird fauna was decimated.

A new study by Scott Loss, Tom Will and Peter Marra in Nature Communications makes new estimates of total mortality of wildlife due to house cats, and they are quite high: median estimates of 2.4 billion birds and 12.3 billion mammals annually in the United States. Money quote:

We estimate that free-ranging domestic cats kill 1.4–3.7 billion birds and 6.9–20.7 billion mammals annually. Un-owned cats, as opposed to owned pets, cause the majority of this mortality. Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals. Scientifically sound conservation and policy intervention is needed to reduce this impact.

They are particularly incensed by programs that trap feral cats, but then return them to the wild after neutering them. I must say this seems to be a crazy idea– why in the world would you put the offending predators back into the ecosystem?

The most striking thing to me was their estimate that well over 2/3 of the mortality was due to “un-owned” (i.e. feral or some slight variation thereof) cats, so that cat owners taking steps to insure that their pets do not become destructive predators, while helpful, would leave most of the problem unaddressed.

Media coverage of the study can be found at the New York Times and the BBC.

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Buller, W.L. 1895. On a new species of Xenicus form an island off the coast of New Zealand. Ibis 7:236-237.

Galbreath, R. & D. Brown. 2004. The tale of the lighthouse-keeper’s cat: Discovery and extinction of the Stephens Island wren (Traversia lyalli). Notornis 51:193-200. (pdf)

Loss, S.R., T. Will & P.P. Marra. 2013. The impact of free-ranging domestic cats on wildlife of the United States. Nature Communications (pdf)

Medway, D.G. 2004. The land bird fauna of Stephens Island, New Zealand in the early1890s, and the cause of its demise. Notornis 51:201-211. (pdf)