Feral Cats and Biodiversity Conservation: The Urgent Prioritization of Island Management

804 BioScience • October 2013 / Vol. 63 No. 10 www.biosciencemag.org Islands harbor a disproportionate amount of Earth’s biodiversity and are characterized by the presence of a great number of endemic plant and animal species (MacArthur and Wilson 1967, Carlquist 1974, Myers et al. 2000, Kier et al. 2009). Invasive predator species, parti­ cularly mammals, are one of the primary extinction drivers on islands (Groombridge and Jenkins 2000, Courchamp et al. 2003, Blackburn et al. 2004). Reviews of the impact of mongooses (Herpestes spp.; Hays and Conant 2007), rats (Rattus spp.; Towns et al. 2006, Jones et al. 2008), and feral cats (Felis silvestris catus; Medina et al. 2011) on islands all note significant impacts on native mammals, birds, and reptiles. Since domestication from the African wildcat (Felis silvestris lybica) some 9000 years ago (Driscoll et al. 2007), the domestic cat (figure 1) has established feral populations on many of the world’s islands, even in the most remote oceanic archipelagoes (Ebenhard 1988, Courchamp et al. 2003, Hilton and Cuthbert 2010). Feral cats are usually a superpredator in the trophic network of islands (Fitzgerald 1988, Courchamp et al. 1999). This generalist and oppor­ tunistic predator has a strong and direct effect on a great variety of native prey, including birds, mammals, reptiles, and invertebrates (for a review, see Bonnaud et al. 2011 and the references therein). Native mammalian carnivores are usually rare on islands because of their low dispersal ability over sea (except bats). Because island vertebrates are often not adapted to coexist with mammalian carnivores (Stone et al. 1994), introduced mammals on islands can have severe impacts on native populations. Introduced mammals (rodents and lagomorphs) are often the most common prey on the islands where feral cats are present; however, when they are available, other native vertebrates (mostly birds and reptiles) are important components of feral cats’ diet on islands (Bonnaud et al. 2011). The presence of alterna­ tive, abundant, year­round prey can facilitate the survival of and sustain large feral cat populations that can have an exacerbated impact on native species through a super­ predator effect (Courchamp et al. 2000). Therefore, even if native species are a lesser component of feral cat diet on islands, presumably because of the lower relative or absolute abundance of native species, and when introduced rodents and lagomorphs are present, feral cats still represent a threat to native island species (Nogales et al. 2004). A meta­analysis of 72 diet studies (based on scat, gut, and stomach contents) revealed that at least 248 species were preyed on by feral cats on 40 worldwide islands (27 mammals, 113 birds, 34 reptiles, 3 amphibians, 2 fish, and 69 invertebrates; for more detail, see Bonnaud et al. 2011). Impacts of feral cats on endangered species have pri­ marily been inferred from dietary studies (Fitzgerald 1988, Fitzgerald and Turner 2000). These studies are useful in Feral Cats and Biodiversity Conservation: The Urgent Prioritization of Island Management