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Cats hunt small prey, and both feral and domesticated cats prey on wildlife. This is sometimes seen as a desirable phenomenon, such as in the case of barn cats and other cats kept for the purposes of pest control. In other cases, the effect of cats on wildlife is met with concern.
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Mice and rats
For thousands of years, cats have been known for their ability to hunt mice and rats and keep their populations under control. This ability is understood as the reason cats became domesticated. The relationship was more of convenience (or mutualistic) than dependence: "Cats killed mice and rats, and humans provided lots of mice and rats to kill since mice and rats lived in human settlements." A 2014 study examining 5,300 years of cat remains in an agricultural village of Quanhucun, China, provides early evidence of this dynamic, where cats protected grain stores by eating rodents.
If they are well-fed, farm cats are more dependable as effective ratters, as they are less likely to stray or hunt further afield. Cats are wary of adult rats, given their size, but are particularly adept at hunting young rats.
In 2002, feral cats introduced to a flower market in Los Angeles, California, were noted to have helped lower rat populations. In Chicago's 47th Ward, feral cats were introduced in 2012 to help the city deal with the rat problem there.
Efforts to eradicate feral cats in Ventura, California, were noted in 2002 to have resulted in increasing numbers of rats, which were being monitored for health problems such as bubonic plague.
Birds
A 2013 study by Scott R. Loss and others of the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service suggested that free-ranging domestic cats (mostly unowned) are the top human-caused threat to wildlife in the United States, killing an estimated 1.3 to 4 billion birds and 6.3 to 22.3 billion mammals annually. These figures were much higher than previous estimates for the U.S. Unspecified species of birds native to the U.S. and mammals including mice, shrews, voles, squirrels and rabbits were considered most likely to be preyed upon by cats.
Perhaps the first U.S. study that pointed to predation by cats on wildlife as a concern was ornithologist Edward Howe Forbush's 1916 report for the Massachusetts State Board of Agriculture, The Domestic Cat: Bird Killer, Mouser and Destroyer of Wildlife: Means of Utilizing and Controlling It.
U.K. biologist and cat behaviour expert Roger Tabor states that "studies from all around the world have found that cats catch relatively few birds compared to small mammals." The Royal Society for the Protection of Birds states that there is no scientific evidence that cat predation "is having any impact on bird populations UK-wide." Moreover, city cats have smaller ranges. In his research, Tabor found "the average annual catch of the average London cat to be two items instead of the fourteen of a village cat." Tabor comments about some of the challenges of stalking birds for cats: "From the cat's point of view not only do birds not play fair by flying and having eyes that can see beyond the back of their heads, but they can positively cheat by using loud alarm calls and throw the cat's chances of catching any others."
Island settings pose particular challenges for wildlife. A 2001 study identified cats alone as responsible for the plight of some island bird species, such as the Townsend's shearwater, socorro dove, and the Marquesan ground dove. The same study identified the greatest cause of endangerment of birds as habitat loss and degradation, with at least 52% of endangered birds affected, while introduced species on islands, such as domestic cats, rats and mustelids, affected only 6% of endangered birds. Other studies caution that removing domestic cats from islands can have unintended consequences, as increasing rat populations can put native bird and mammal species at risk.
Australia
Cats in Australia have been found to have European origins. This is important to note because of their effect on native species. Feral cats in Australia have been linked to the decline and extinction of various native animals. They have been shown to cause a significant impact on ground nesting birds and small native mammals.
Feral cats have also hampered any attempts to re-introduce threatened species back into areas where they have become extinct as the cats have hunted and killed the newly released animals. Numerous Australian environmentalists claim the feral cat has been an ecological disaster in Australia, inhabiting most ecosystems except dense rainforest, and being implicated in the extinction of several marsupial and placental mammal species.
New Zealand
The fauna of New Zealand has evolved in isolation for millions of years without the presence of mammals (apart from a few bat species). Consequently, birds dominated the niches occupied by mammals and many became flightless. The introduction of mammals after settlement by Māori from about the 12th century had a huge effect on the indigenous biodiversity. European explorers and settlers brought cats on their ships and the presence of feral cats were recorded from the latter decades of the 19th century. It is estimated that feral cats have been responsible for the extinction of six endemic bird species and over 70 localised subspecies as well as depleting bird and lizard species.
United Kingdom
In the U.K., The Royal Society for the Protection of Birds states that there is no scientific evidence that cat predation "is having any impact on bird populations UK-wide. This may be surprising, but many millions of birds die naturally every year, mainly through starvation, disease, or other forms of predation. There is evidence that cats tend to take weak or sickly birds ... Those bird species that have undergone the most serious population declines in the UK (such as skylarks, tree sparrows and corn buntings) rarely encounter cats, so cats cannot be causing their declines. Research shows that these declines are usually caused by habitat change or loss, particularly on farmland." This evidence is despite the common practice in the U.K. of allowing owned cats access to the outdoors, which is recommended to prevent feline obesity (p. 138) and behavior problems and other health problems arising from confinement stress (p. 121).
Consequences of introduction
Many islands host ecologically naive animal species. That is, animals that do not have predator responses for dealing with predators such as cats. Feral cats introduced to such islands have had a devastating impact on these islands' biodiversity.
They have been implicated in the extinction of several species and local extinctions, such as the hutias from the Caribbean, the Guadalupe storm petrel from the Pacific coast of Mexico, and the Stephens Island wren. In a statistical study, they were a significant cause for the extinction of 40% of the species studied. Moors and Atkinson wrote, in 1984, "No other alien predator has had such a universally damaging effect."
Feral cats, along with rabbits, some sea birds, and sheep, form the entire large animal population of the remote Kerguelen Islands in the southern Indian Ocean. Although exotic mammals form the bulk of their diet, cats' impact on seabirds is very important.
Restoration
Because of the damage cats cause in islands and some ecosystems, many conservationists working in the field of island restoration have worked to remove feral cats. (Island restoration involves the removal of introduced species and reintroducing native species.) As of 2004, 48 islands have had their feral cat populations eradicated, including New Zealand's network of offshore island bird reserves and Australia's Macquarie Island.
Larger projects have also been undertaken, including their complete removal from Ascension Island. The cats, introduced in the 19th century, caused a collapse in populations of nesting seabirds. The project to remove them from the island began in 2002, and the island was cleared of cats by 2004. Since then, seven species of seabird that had not nested on the island for 100 years have returned.
In some cases, the removal of cats had unintended consequences. An example is Macquarie Island, where the removal of cats caused an explosion in the number of rabbits, rats, and mice that harm native seabirds even if the eradication was positioned within an integrated pest management framework. The removal of the rats and rabbits was scheduled for 2007 and it could take up to seven years and cost $24 million.
Hybridisation with wild felids
Feral cats have interbred with wildcats to various extents throughout the world, the first reported case occurring more than 200 years ago. The significance of hybridisation is disputed. Older documentation suggests that the wildcat was a separate species from the domestic cat, but modern genetic analysis has shown that the domestic cat is a domesticated version of the near-eastern wildcat (Felis silvestris lybica). In some locations, high levels of hybridisation have led to difficulties in distinguishing a "true" wildcat from feral domestic and domestic hybrid cats, which can complicate conservation efforts. Some researchers argue that "pure" wildcats do not exist anymore, but others dispute this.
One study in Scotland suggests that while "true" Scottish wildcats are unlikely to exist, the current wildcat population is distinct enough from domestic cats to be worth protecting, genetics research is still underway as part of the Wildcat Haven project in Scotland, which has reported success protecting wildcats from hybridisation with widespread trap-neuter-return work. Wildcat populations' notable gene introgression exists also in Italy, Hungary, Spain and Portugal. For a discussion of this issue see The Encyclopedia of Mammals, OUP, pages 656–657.