Cats are predators and free ranging cats kill may large numbers of wildlife. Conservationists are concerned about the survival of endangered animals preyed upon by cats which are owned, stray or feral. Cats target mammals but birds, reptiles and insects may also be preyed upon. Cats change prey species depending on availability. The impact of cats on wildlife varies. It is influenced by the characteristics of the prey species, the suite of predators present in the environment, and the population and management of the resident cats. These issues will be discussed with reference to the scientific literature with emphasis on the situation in New Zealand and Australia.
The owned domestic cat lives indoors or indoor/outdoors and depends on humans for food and shelter. Cats may be stray, living on the periphery of human society, or feral living independent of humans. All cats may predate but those living indoors have little opportunity to do so. Young owned cats are more likely to hunt than older animals. Feral cat populations are controlled by food availability and population densities are usually low. These cats depend on wildlife for their survival. Stray cats live around human habitations and eat food supplied by humans either deliberately or as rubbish but they may also predate. The % of owned cats allowed to roam varies considerably. Rural cat owners are more likely to allow their cat to roam. In some countries more than 50% of urban cat owners do not allow their cat to roam. Owned cats do not need to predate for food but do so. Owned and stray cat population density can greatly exceed those seen in feral populations and by sheer force of numbers (229 cats /km2 in one study, Baker et al., 2005) impact even on species which have evolved to cope with cat predation, e.g., English songbirds. Cats may bring killed prey home but many do not, and surveys in which owners are asked to count prey delivered home always underestimate the kill. Domestic cat numbers have doubled in the USA since 1970 to about 100 million in 2002. Cats display individual differences in activity periods and this is influenced by season. Cats may be diurnal in winter, crepuscular in spring and autumn and nocturnal in winter.
The Target Species
On continental land masses cats will predate on mammals, and birds may be only a small part of their diet. Nevertheless in America cats may play an important role in bird population fluctuations. In Australia many marsupials are susceptible to cat predation and cats have a significant impact on small marsupials. In New Zealand cats predate on rabbits and rodents but also on birds, bats, lizards particularly skinks, insects and frogs. The common prey size for cats is 1% of their body weight. On islands where cats are recent immigrants they predate on immigrant mammals, rabbits, rats and mice but may also predate on birds. Cats have been removed from many islands for conservation reasons.
Conservation problems occur when the prey species are rare, have evolved in an environment without mammalian predators (e.g., New Zealand) or have poor reproductive capacity (e.g., the kakapo). The bird species which have become extinct due to cat predation (e.g., Stephens Island wren) or whose numbers have been reduced greatly due to cat predation (e.g., kakapo on Stewart Island) were few initially and had no anti-cat behaviour and poor reproductive rates. The size of the prey species may also be important (e.g., skinks) in that cats will eat a large number of them. Ground living species are probably more likely to be targeted by cats.
The suite of predators targeting vulnerable species will influence the impact of cats. If there is a large suite of predators then the cat may or may not be particularly problematic. In New Zealand mammalian predators include weasels, stoats, ferrets, hedgehogs, rats, mice, pigs, dogs and cats. The impact of these on vulnerable species and on one another influences whether any one predator is particularly important. Removing cats may allow rat populations to increase and target vulnerable species.
The availability of suitable cat prey (e.g., rabbits, rats or mice) may protect vulnerable species by providing food for cats, but increases in cat numbers or reduction in rabbit (e.g., following rabbit calicivirus disease introduction), or rat numbers may force cats to prey on a vulnerable species. Cats also predate on stoats. Thus the removal of cats may not provide much benefit to endemic fauna if rodents or other predators remain. However, on some islands cat removal impacted positively on seabird populations (e.g., black vented shearwater) In Australia the absence of dingoes may allow cat numbers to increase and to predate on ground-dwelling mammals.
It is easier to identify the damage done by cats on islands and most of the conservation problems and extinctions blamed on cats have happened on islands where cats are an invader species. The fauna of these environments have often evolved without pressure from cats and are vulnerable to them. In New Zealand a few bird species have become extinct due to cat predation (Stephens Island wren, Little Barrier snipe) but many local island populations have been severely reduced by cats. On Herekopare Island cats eliminated yellow-crowned parakeets, robins, fernbirds, brown creeper, New Zealand snipe, banded rails and broad billed prions. However even on the main islands of New Zealand cats have had significant impact on black stilts, and black fronted terns and in some places skinks are an important part of their diet.
Keeping cats indoors is probably the only sure way to stop them predating. Anti-predator collars and bibs may reduce predation but are not guaranteed to stop it. Maintaining colonies of stray cats may impact on wildlife by keeping cat populations high although if colony cats are de-sexed colonies eventually disappear. Cat curfews, when cats are kept indoors at night, may protect nocturnal mammalian species including rats and mice, but will not save diurnal species. Feral cats are controlled by hunting, trapping and poisoning. Poisoning programmes which also target rodents are probably better than those which only kill cats. In cities and towns where cat numbers are greatest other factors, especially loss of habitat, impact on endemic fauna. Cat free developments may have little impact on endemic fauna unless other activities, such as rodent control, are integrated into a protection programme. Manipulating prey species may ensure cats focus on them rather than endangered or protected species. 'Trap neuter and release' programs allow cats to continue to predate on endangered species and 'trap and remove' may be a better alternative. Generalisations are dangerous but surveys suggest that the public may or may not be willing to accept increased control of cats in order to protect wildlife.
Cats are only one element in the complex of environmental factors which impact negatively on wildlife populations. Habitat destruction and other predators may be more important than cats. In any one situation with endangered species the importance of cats has to be determined by local research perhaps involving the removal of cats or other species. This type of research is difficult and expensive because of its local nature.
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