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Domestic Cat Predation and Biodiversity

Cat tracks.

Tracks from a domestic cat (Felis sylvestris)

 

These fresh tracks in the snow reveal that a cat (Felis silvestris) was here, most likely on the hunt. Domestic cats are highly effective predators that often hunt if they are allowed to roam freely. Studies indicate that more than 90% of free roaming pet cats hunt, and on average each kills about 14 wild animals per year (Coleman et al, 1997). A typical free roaming cat's diet consists of about 70% small mammals, 20% wild birds, and the remaining 10% reptiles, amphibians, fish, and arthropods; but they will eat what is most available. A recent study on Hawaii found that over 70% of the diet of feral cats was small songbirds (USGS). The impact of cat predation on endangered bird and small mammal populations is a growing concern.

Even if only 20% of an average cat's diet consists of birds, the impact on bird populations will be significant because of the sheer size of the cat population. According to Humane Society data there are at least 66 million pet cats in the United States. An estimated 40 to 60 million feral (stray or wild domesticated) cats nearly double this number. David Pimental, a scientist at Cornell University, estimates that pet cats kill 600,000 wild birds in the United States every year. Coleman et al. (1997) estimated that in Wisconsin alone at least 39 million birds (but possibly as many as 217 million birds) were killed by cats. Another study showed that cats were significant predators at bird feeders in the winter. Species in the United States that are at risk include Plover, Wood Thrush, and Black-throated Blue Warbler, and endangered species such as the Least Tern and Piping Plover.

A recent study in California showed that parks that excluded cats had twice as many ground nesting birds and far fewer exotic pest species of mice than parks that had cats. Ground nesting birds are most at risk because they are such easy prey. Just last year a colony of extremely rare Wedge-tailed Shearwaters was nearly exterminated by feral cats. In 5 days 80% of the colony was killed. The most extreme example is the extinction of the flightless Stephen Island wren, which was caused by one pet cat that was introduced onto an island off New Zealand by a lighthouse keeper (Greenway 1967).

Defenders of the practice of allowing captive cats to hunt and promoting the release of sterilized feral cat colonies argue that human impacts, mostly habitat fragmentation, are causing the declines in bird populations. Habitat fragmentation magnifies the effects of cat predation because fragments are much like islands that support only small populations of birds. Small habitat size makes it easy for an effective predator like a cat to completely extirpate populations of some birds. A recent study by Crooks and Soule (1999) in California showed that pet cats hunting in canyons adjacent to subdivisions were easily collecting birds in excess of certain species' population sizes.

But it is also important to remember that human impacts include habitat destruction, pollution, hunting, and introduced species. Domesticated cats represent an introduced species that is not native to any existing ecosystem and therefore they cannot represent anything natural in the course of nature. Most cats are vaccinated against diseases and their diets and housing are provided or augmented by humans. It is clearly in the best interest of biodiversity preservation to keep pet cats indoors and to make sure all pets are neutered or spayed. Release of any non-native species into the wild presents a risk to conservation efforts, even when those animals are sterilized. Feral cats are not wild cats and they do not belong in natural areas.

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Text by Vicki Medland

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Last updated on May 9, 2014