Domestic cats (Felis sylvestris catus) are believed to have been first domesticated in Egypt some 4000 years ago, making them one of the most recently domesticated of species. Possibly (semi-)domesticated cats existed earlier but the close similarity in the wild species (F. sylvestris) and the domestic version complicates assigning skeletal remains associated with older human artifacts to subspecies. Appearance-wise, the “tabby” pattern may represent the persistence of wildcat pelage in most modern domesticated cat breeds and underscore the persistent “wildness” of modern domestic cats. Apparently cat domestication was facilitated by the relative docility of the local (Egyptian) wild sub-species, F. s. lybica, and the predilection of human Egyptians for taming wild animals. The wildcat of Europe, F. s. sylvestris, in contrast, is considered to be “virtually untameable” (Sundquist and Sundquist 2002).
Domesticated or not, small cats are opportunistic predators, adept at stalking or ambushing their prey. They can live indoors year-round, or outdoors, supplementing their human-supplied food with prey or offal. Most breeds have little trouble surviving as wild or feral animals. In this sense they differ from most dog breeds which through artificial selection, have lost the ability to survive without human support. Cats also are more widely distributed than are dogs, having successfully colonized a wide range of terrestrial habitats on all the continents except Antarctica as well as on islands (from tropical to sub-arctic), and in agricultural and urban areas. Basically the domestic cat, like the gray rat (Rattus rattus), is associated with man wherever he goes. The estimated number of cats in the U. S., 70 million (compared to 56 million dogs), suggests that even if a large proportion of these are pampered house pets that never roam, the number of prey animals at risk from wandering pets and feral animals could be large and locally damaging to wildlife.
Determining what roaming cats are actually doing when not sleeping can be difficult. One innovative approach is the “kitty cam” of the University of Georgia, a light weight tracking unit affixed to a freely roaming cat. It’s got a breakaway collar and a radio tracking device that can be used to record the nocturnal activity of the cats. In a study involving 55 free-roaming cats in a Georgia county, observations revealed that 44% of the cats hunted wildlife and that the most common prey captured were anoles (lizard) and woodland voles, both native species . Capture rates were low- averaging 2 animals per 7 days of roaming. Of the total of 39 prey captured in the study, 13% were birds, 26% mammals, 41% reptiles/amphibians and 8% invertebrates. Unfortunately all but one of the vertebrate prey were native species. Cat age, sex and time outside did not significantly influence hunting behavior. Interestingly, only 28% of the captured prey were eaten. Of the killed but uneaten prey, nearly half (49%) were left behind and 23% brought home. Thus if you are assessing your pet’s effects on wildlife, don’t fool yourself that counting dead mice or birds on your doorstep gives an accurate number!
Most studies find that feral cats rely principally on small mammals as prey, with adult rabbits and adult Norway rats (R. norvegicus, which are larger than the more widely distributed R. rattus) approaching the upper limit of prey size. Sunquist and Sunquist (2002) say that despite accusations to the contrary, birds are not important in the diets of cats except on islands. Here they can be devastating to birds and are responsible for the extinction or near elimination of seabirds and reptiles on numerous islands, although sometimes as in the case of the marine iguana of the Galapagos, other alien species like pigs, rats and dogs share responsibility. In the Galapagos, however, native rodents exist only on islands without cats.
One recent estimate of the total annual mortality in the U.S. due to house cats falls within 1.4 to 3.7 billion birds and 6.9 to 20.7 billion mammals (Loss et al. 2013). These figures seem huge and if, for example there were trillions or even many billions of birds or small mammals, there might be less cause for concern. But there are apparently far fewer. One estimate for birds is 10-20 billion in the U.S., arrived at from volunteer birding projects like the Christmas Bird Count and the national breeding bird survey (see ebird.org and the Cornell Lab of Ornithology site) and on review of the scientific literature. From 7 to 34 % of the wild bird population of the U.S. would be lost annually to cats based on these estimates. By any estimation these are ecologically significant loss rates especially when recognizing the growing habitat loss in both nesting and wintering ranges of many bird species. Far fewer data are available for mammals, leaving us without credible estimates of their total numbers, but cats are likely to be important influences on population sizes in at least some areas. Cat depredations are likely the single greatest source of human-caused mortality of birds and mammals in the U.S., with feral cats the worst offenders. Fortunately, cat prey undoubtedly includes some nuisance animals like starlings, rats and house mice, depending on local availability. In fact, nuisance animals in urban areas might comprise most of the prey.
Although cats seem to avoid deep woods and swamps like the Everglades (even before the infamous rise of the invasive Burmese python!), they enter many natural habitats. In Florida where the federally endangered Lower Keys marsh rabbit is down to a total population of 100 to 300, 53% of rabbit deaths are traceable to cats according to the U.S. Fish and Wildlife Service. It is perhaps ironic that at the other end of the Florida Keys chain of islands, on Key Largo, there is a managed colony of about 1000 cats! Similarly, the domestic cat is known to be a major source of mortality in some of six sub-species of beach mice which are important in Florida beach ecosystems and are federally threatened or endangered.
Turning to indirect impacts, competition for prey between feral cats and native predators like owls, hawks and foxes, although difficult to quantify, is likely to be deleterious to the native species, in some situations perhaps very much so. Another indirect impact of cats upon other mammals, involves diseases. Cats can spread rabies to raccoons, skunks and foxes, and probably spread feline panleukopenia (distemper) to the endangered Florida panther, and feline leukemia virus to mountain lions. Humans can also contract diseases like rabies, toxoplasmosis, cat scratch fever, and encephalitis from cats, and pick up parasites like hookworms and roundworms from them.
Trapping and neutering free-ranging cats would presumably reduce numbers somewhat but many such programs avoid the ire of cat lovers by returning the neutered cats to the wild, rather than choosing euthanasia. Ideally, all cat owners would take steps to insure that their pets do not become destructive predators, but given the results of Loss and colleagues, the efforts of owners alone would not deal with the bulk of the problem.
The managed colony approach whereby local cats are sterilized, inoculated against disease and regularly fed has its advocates as it also aims to sterilize “drop-ins” that inevitably turn up. This approach was designed in response to the evident failure of traditional trap and kill programs to reduce populations. However, it is criticized by some conservation and wildlife biologists, vets and animal welfare groups, for it in effect subsidizes opportunistic predation, and depends on long-term human diligence. Considerable fault can be laid at the doorstep of irresponsible owners who abandon unwanted pets and avoid sterilizing them. Incentives to sterilize cats are small and laws are easy to ignore.
Suggestions for cat owners who care about how their feline pet(s) affect the environment include keeping their cat(s) indoors. Committing to this practice may be facilitated by remembering that indoor cats live much longer than outdoor cats. Other tips- do not feed strays; get your cat neutered and don’t release unwanted cats; “double-bell” your cat because some cats have learned to avoid ringing a single bell when hunting, and beware of sneaky cats lingering around bird feeders.
To somewhat counterbalance this bleak assessment of domestic cat impacts, I note that sometimes the effect of cats on bird populations is not straight-forward. A study of cat predation on New Island of the Falklands estimated that cats could be eating 1500-11,000 thin-billed prions per season. The thin-billed prion is a pelagic, ground-nesting seabird of conservation concern, and the New Island colony is the world’s largest. Although this may seem to be a large number, it comprises under 1% of the adult and sub-adult population of the colony of two million nesting pairs (Matias and Catry 2008). Predation on several other nesting seabird species on New Island including 2 penguin species, was insignificant. The principal prey species of cats were the house mouse, the grey rat and a rabbit species, all introduced to the island by European man. For each prion killed by cats, 1.1-1.9 rats were killed in summer, and even more in fall and winter. Thus because rats destroy eggs and young birds, cats could be seen as benefiting the prion colony by reducing rat populations. Therefore the authors did not recommend a cat control program on New Island, in the absence of successful measures to eliminate rat populations.
The following are among the references consulted:
Hatley, P. J. 2003. Feral Cat Colonies in Florida: The fur and the feathers are flying. Proc. 9th Annual Public Interest Environ. Conf.: Saving What’s Left.
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);
Matias R. and P. Catry 2008. The diet of feral cats at New Island, Falkland Islands, and the impact on seabirds. Polar Biol 31:609-616
Mayer, Greg, http://www.whyevolutionistrue.wordpress, Jan 30,2013
Sunquist, Mel and Fiona Sunquist 2002. Wild Cats of the World. Univ. of Chicago Press.