Ecology

Habitats
A black cat in snowy weather.Cats are a cosmopolitan species and are found across much of the world.[38 ] Geneticist Stephen James O'Brien, of the National Cancer Institute in Frederick, Maryland, remarked on how successful cats have been in evolutionary terms: "Cats are one of evolution's most charismatic creatures. They can live on the highest mountains and in the hottest deserts."[167 ] They are extremely adaptable and are now present on all continents except Antarctica, and on 118 of the 131 main groups of islands—even on sub-Antarctic islands such as the Kerguelen Islands.[168 ] [169 ] Feral cats can live in forests, grasslands, tundra, coastal areas, agricultural land, scrublands, urban areas and wetlands.[170 ] Their habitats even include small oceanic islands with no human inhabitants.[171 ] This ability to thrive in almost any terrestrial habitat has led to the cat's designation as one of the world's worst invasive species.[172 ] Despite this general adaptability, the close relatives of domestic cats, the African wildcat (Felis silvestris lybica) and the Arabian sand cat (Felis margarita) both inhabit desert environments,[4 ] and domestic cats still show similar adaptations and behaviors.[35 ]

Impact on prey species
Young feral cat eating a cottontail rabbit.To date, there are few scientific data available to assess the impact of cat predation on prey populations. Even well-fed domestic cats may hunt and kill, mainly catching small mammals, but also birds, amphibians, reptiles, fish and invertebrates.[133 ] [173 ] Hunting by domestic cats may be contributing to the decline in the numbers of birds in urban areas, although the importance of this effect remains controversial.[174 ] In the wild, the introduction of feral cats during human settlement can threaten native species with extinction.[171 ] In many cases controlling or eliminating the populations of non-native cats can produce a rapid recovery in native animals.[175 ] However, the ecological role of introduced cats can be more complicated. For example, cats can control the numbers of rats, which also prey on birds' eggs and young, so a cat population can protect an endangered bird species by suppressing mesopredators.[176 ]

In the Southern Hemisphere, cats are a particular problem in landmasses such as Australasia, where cat species have never been native and there were few equivalent native medium-sized mammalian predators.[177 ] Native species such as the New Zealand Kakapo and the Australian Bettong, for example, tend to be more ecologically vulnerable and behaviorally "naive" to predation by feral cats.<sup class="reference" id="cite_ref-pmid12085522_178-0">[178 ] Feral cats have had a major impact on these native species and have played a leading role in the endangerment and extinction of many animals.<sup class="reference" id="cite_ref-pmid16094805_179-0">[179 ]

Cat numbers in the UK are growing and their abundance is far above the "natural" carrying capacity, because their population sizes are independent of their prey's dynamics: i.e. cats are "recreational" hunters, with other food sources.<sup class="reference" id="cite_ref-May.2C_R..2C_1988_180-0">[180 ] Population densities can be as high as 2,000 individuals per km2<sup class="reference" id="cite_ref-Liberg_1982_181-0">[181 ] and the trend is an increase of 0.5 million cats annually.

Impact on birds
Eating a house sparrow.The domestic cat is probably a significant predator of birds. UK assessments indicate that they may be accountable for an estimated 64.8 million bird deaths each year.<sup class="reference" id="cite_ref-Woods_133-3">[133 ] Certain species appear more susceptible than others; for example, 30% of house sparrow mortality is linked to the domestic cat.<sup class="reference" id="cite_ref-Chucher_1987_182-0">[182 ] In the recovery of ringed robins (Erithacus rubecula) and dunnocks (Prunella modularis), it was also concluded that 31% of deaths were a result of cat predation.<sup class="reference" id="cite_ref-Mead_1982_183.E2.80.93186_183-0">[183 ] The presence of larger carnivores such as coyotes which prey on cats and other small predators reduces the effect of predation by cats and other small predators such as opossums and raccoons on bird numbers and variety.<sup class="reference" id="cite_ref-Crooks_1999_184-0">[184 ] The proposal that cat populations will increase when the numbers of these top predators decline is called the mesopredator release hypothesis. However a new study suggests that cats are a much greater menace than previously thought and that feral cats kill several billion birds each year in the United States.<sup class="reference" id="cite_ref-185">[185 ]

On islands, birds can contribute as much as 60% of a cat's diet.<sup class="reference" id="cite_ref-Fitzgerald_186-0">[186 ] In nearly all cases, however, the cat cannot be identified as the sole cause for reducing the numbers of island birds, and in some instances eradication of cats has caused a ‘mesopredator release’ effect;<sup class="reference" id="cite_ref-187">[187 ] where the suppression of top carnivores creates an abundance of smaller predators that cause a severe decline in their shared prey. Domestic cats are, however, known to be a contributing factor to the decline of many species; a factor that has ultimately led, in some cases, to extinction. The South Island Piopio, Chatham Islands Rail,<sup class="reference" id="cite_ref-Mead_1982_183.E2.80.93186_183-1">[183 ] the Auckland Islands Merganser,<sup class="reference" id="cite_ref-188">[188 ] and the common diving petrel<sup class="reference" id="cite_ref-189">[189 ] are a few from a long list, with the most extreme case being the flightless Stephens Island Wren, which was driven to extinction only a few years after its discovery.<sup class="reference" id="cite_ref-190">[190 ] <sup class="reference" id="cite_ref-191">[191 ]

Some of the same factors that have promoted adaptive radiation of island avifauna over evolutionary time appear to promote vulnerability to non-native species in modern time. The susceptibility inherent of many island birds is undoubtedly due to evolution in the absence of mainland predators, competitors, diseases and parasites. In addition to lower reproductive rates and extended incubation periods.<sup class="reference" id="cite_ref-dowd_192-0">[192 ] The loss of flight, or reduced flying ability is also characteristic of many island endemics.<sup class="reference" id="cite_ref-193">[193 ] These biological aspects have increased vulnerability to extinction in the presence of introduced species, such as the domestic cat.<sup class="reference" id="cite_ref-194">[194 ] Equally, behavioral traits exhibited by island species, such as "predatory naivety"<sup class="reference" id="cite_ref-195">[195 ] and ground-nesting,<sup class="reference" id="cite_ref-dowd_192-1">[192 ] have also contributed to their susceptibility