Felis catus (mammal)
Changes in island fauna after the introduction of cats can provide compelling evidence of their predatory impact. Cats have been introduced to 40 islands off the coast of Australia; seven off the coast of New Zealand and several dozen islands elsewhere in the Pacific (Dickman 1992a, Veitch 1985, King 1973 1984, in Dickman 1996). Feral cats have been implicated in the decline of at least six species of island endemic birds in New Zealand, including the Stephens Island wren, the sooty shearwater (Puffinus griseus) and the kakapo (Strigops habroptilus), as well as 70 local populations of insular birds (King 1984, in Dickman 1996). The elimination of cats often leads to an increase in the population size of prey species. For example, following removal of cats from Little Barrier Island, New Zealand, the stitchbird (Notiomystis cincta) increased from less than 500 individuals to 3000 individuals in just a few years (Griffin et al. 1988, in Dickman 1996).
When a cat is a pet, there are a number of ways in which to help prevent damage caused to wildlife. Brickner (2003) suggests keeping a cat in at night, fitting it with a bell, neutering the animal when it is young and giving it toys. However, the divided results of several investigations shows that the positive outcome of such actions is uncertain. Barrette (1998) found that fitting cats with bells has no significant effect on the amount of prey caught, whereas Ruxton et al. (2002) found that equipping cats with bells reduced prey delivery rates by about 50% (in Brickner 2003). Woods, McDonald and Harris (2003) found that the number of birds and herpetofauna brought home by cats was significantly lower in households that feed birds (but the number of actual different types of bird species killed was greater in households that feed birds). The number of mammals brought home per cat was lower when cats were equipped with bells or kept indoors at night, however, the number of herpetofauna brought home was greater when cats were kept in at night. The outcome of this is that there appears to be a subjective choice to be made as to whether it is more important to protect herpetofauna or mammals. Obviously, if the mammals being caught are introduced species, such as rats and mice, this raises another dilemma.
In the second situation, when a cat is feral and threatening wildlife, a more severe means of controlling cats appears justified. In 1992 the Australian Parliament passed the Endangered Species Protection Act 1992, which obligates the commonwealth to provide a Threat Abatement Plan (TAP) for each listed threatening process, including one for feral cats (Brickner 2003). The key objectives of the feral cat TAP are: eradicate feral cats from islands where they threaten vulnerable native animals; prevent feral cats from occupying new islands where they may be a threat to native communities; promote the recovery of species threatened by feral cats; improve the effectiveness and humaneness of cat control methods and improve the understanding of the impacts of feral cats on native animals. The use of visual lures (such as feathers and cotton wool) and attractants (such as tuna oil) are currently being tested in an effort to attract greater numbers of feral cats to traps and baits. The impact of feral cats on native wildlife is being studied in various parts of Australia in order to have it quantified (Brickner 2003).
Predation by feral cats was listed as a Key Threatening Process under the Federal Endangered Species Protection Act 1992. A Threat Abatement Plan for Predation by Feral Cats was produced in 1999 and amended in 2008 to promote the recovery of vulnerable and endangered native species and threatened ecological communities (Environment Australia 1999 and DEWHA 2008). A recently published review (Denny and Dickman (2010) assesses the efficacy of the methods used to estimate relative abundance of cats; describes currently used cat control methodologies; and discusses possible future directions for the control of cats in Australia. It also includes details of the current legislative framework that exists for cat control in Australia; describes the ecology of feral and stray cats exploiting various habitats. Please follow this link to view Denny E. A & C. R. Dickman 2010. Review of cat ecology and management strategies in Australia
The diet of feral cats on islands may vary significantly to that of feral cats on the mainland, with cats often taking advantage of alternative food sources. On the tiny 28 hectare Herekopare Island, New Zealand, for example, there are no introduced or native species of mammals. Prior to elimination of feral cats there in 1970, fairy prion (see Pachyptila turtur in IUCN Red List of Threatened Species) comprised the bulk of the diet with other sea birds and occasional land birds making up most of the remainder (Fitzgerald and Veitch 1985, in Dickman 1996). The weta (a native insect in the order Orthoptera) also appeared to be important to individual cats; two cats' stomachs were found to contain over 100 insects each. Similarly, in the Galapagos Islands, birds are an important component of the feral cat's diet, with cats sometimes taking birds of similar mass to themselves, such as frigate birds (Fregata spp.), pelicans (Pelecanus spp.) and flightless cormorants (Phalacrocorax spp.) (Konecny 1987, in Dickman 1996). On Aldabra Atoll, Seychelles, hatchlings of the green turtle (see Chelonia mydas in IUCN Red List of Threatened Species) are seasonally predominant in the diet of feral cats (Seabrook, 1989). On Christmas Island, the introduced black rat (Rattus rattus) comprises almost one third of the diet of feral cats by weight, however, 21% of the diet is comprised of the large flying-fox (see Pteropus melanotus in IUCN Red List of Threatened Species) and 28% of the imperial pigeon (see Ducula whartoni in IUCN Red List of Threatened Species) (Tidemann et al. 1994, in Dickman 1996).
Click here to see Major prey of feral cats in Australia (source: Dickman 1996).
Updates with support from the Overseas Territories Environmental Programme (OTEP) project XOT603, a joint project with the Cayman Islands Government - Department of Environment