The most obvious impact of feral cats is the predatory impact they exert on native prey populations; this has resulted in the probable local or regional decline or extinction of many species (Dickman 1996). However, unambiguous evidence of cats causing a decline in a prey species is difficult to find as other factors, such as other predator species, may also be involved in the decline (Dickman 1996). One exception to this is a study by Saunders (1991) which showed that cats killed 7% of nestlings of red-tailed cockatoos (Calyptorhynchus magnificus) over 11 breeding seasons in Western Australia. Several reintroduction programmes in Australia have failed, due to the predation pressure exerted by feral cats, often in conjunction with foxes. For example, the success of the reintroductions of the golden bandicoot (Isoodon auratus) and the burrowing bettong (Bettongia lesueur) in the Gibson Desert, Western Australia was hindered primarily by feral cat predation. In general, the predatory impact of cats primarily affects birds and small to medium-sized mammals (Dickman 1996). Endangered species around the world are threatened by the presence of cats, including the black stilt (see Himantopus novaezelandiae in the IUCN Red List of Threatened Species) (New Zealand), the Okinawa woodpecker (see Sapheopipo noguchii in IUCN Red List of Threatened Species) (Japan) and the Cayman Island ground iguana (see Cyclura lewisi in IUCN Red List of Threatened Species), to list just some of the many species effected.\r\n
Changes in island fauna after the introduction of cats can provide compelling evidence of their predatory impact. Cats have been introduced to 40 islands off the coast of Australia; seven off the coast of New Zealand and several dozen islands elsewhere in the Pacific (Dickman 1992a, Veitch 1985, King 1973 1984, in Dickman 1996). Feral cats have been implicated in the decline of at least six species of island endemic birds in New Zealand, including the Stephens Island wren, the sooty shearwater (Puffinus griseus) and the kakapo (Strigops habroptilus), as well as 70 local populations of insular birds (King 1984, in Dickman 1996). The elimination of cats often leads to an increase in the population size of prey species. For example, following removal of cats from Little Barrier Island, New Zealand, the stitchbird (Notiomystis cincta) increased from less than 500 individuals to 3000 individuals in just a few years (Griffin et al. 1988, in Dickman 1996).