Some research on the impact of feral cats on wildlife populations.
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Cats and Wildlife...
A Conservation Dilemma
By John S. Coleman, Stanley A. Temple
and Scott R. Craven
Observation of free-ranging domestic cats shows that some individuals can kill over 1000 wild animals per year [7], although smaller numbers are more typical. Some of the data on kills suggest that free-ranging cats living in small towns kill an average of 14 wild animals each per year. Rural cats kill many more wild animals than do urban, or suburban cats [8]. Several studies found that up to 90% of free-ranging rural cats' diet was wild animals, and less than 10% of rural cats killed no wild animals [9]. Recent research [10] suggests that rural free-ranging domestic cats in Wisconsin may be killing between 8 and 217 million birds each year. The most reasonable estimates indicate that 39 million birds are killed in the state each year. Nationwide, rural cats probably kill over a billion small mammals and hundreds of millions of birds each year. Urban and suburban cats add to this toll. Some of these kills are house mice, rats and other species considered pests, but many are native songbirds and mammals
whose populations are already stressed by other factors, such as habitat destruction and pesticide pollution.
Despite the difficulties in showing the effect most predators have on their prey, cats are known to have serious impacts on small mammals and birds. Worldwide, cats may have been involved in the extinction of more bird species than any other cause, except habitat destruction. Cats are contributing to the endangerment of populations of birds such as Least Terns, Piping Plovers and Loggerhead Shrikes. In Florida, marsh rabbits in Key West have been threatened by predation from domestic cats [11]. Cats introduced by people living on the barrier islands of Florida's coast have depleted several unique species of mice and woodrats to near extinction [12, 13].
Not only do cats prey on many small mammals and birds, but they can outnumber and compete with native predators. Domestic cats eat many of the same animals that native predators do. When present in large numbers, cats can reduce the availability of prey for native predators, such as hawks [14] and weasels [15].
Free-ranging domestic cats may also transmit new diseases to wild animals. Domestic cats have spread feline leukemia virus to mountain lions [16] and may have recently infected the endangered Florida Panther with feline panleukopenia (feline distemper) and an immune deficiency disease [17]. These diseases may pose a serious threat to this rare species. Some free-ranging domestic cats also carry several diseases that are easily transmitted to humans, including rabies and toxoplasmosis [18].
Domestic cats vs. native predators
Although cats make affectionate pets, many domestic cats hunt as effectively as wild predators. However, they differ from wild predators in three important ways: First, people protect cats from disease, predation and competition, factors that can control numbers of wild predators, such as bobcats, foxes, or coyotes. Second, they often have a dependable supply of supplemental food provided by humans and are, therefore, not influenced by changes in populations of prey. Whereas populations of native predators will decline when prey becomes scarce, cats receiving food subsidies from people remain abundant and continue to hunt even rare species. Third, unlike many native predators, cat densities are either poorly limited or not limited by territoriality [19]. These three factors allow domestic cats to exist at much higher densities than native predators. In some parts of rural Wisconsin, densities of free-ranging cats reach 114 cats per square mile. In these areas, cats are several times more abundant than all mid-sized native predators (such as foxes, raccoons, skunks) combined. With abundant food, densities can reach over 9 per acre, and cats often form large feeding and breeding "colonies" (81 cats were recorded in one colony, and colonies of over 20 are not uncommon) [20, 21]. Unlike some predators, a cat's desire to hunt is not suppressed by adequate supplemental food. Even when fed regularly by people, a cat's motivation to hunt remains strong, so it continues hunting [22].
