Cats are generalist predators that once introduced to the wild (or simply allowed to roam outdoor), can prey on a variety of native species, which may suffer severe population declines and even face extinction. On this regard, it makes no difference whether the cats are owned (in which case their impact might be even more subtle, because often unnoticed), stray or feral: as stated by George F. Will (American journalist, and author) The phrase "domestic cat" is an oxymoron. The result is that at the global level, cats are considered responsible for at least 14% of bird, mammal and reptile extinctions and are the principal threat to almost 8% of critically endangered birds, mammals and reptiles. The figures are impressive. In continental Australia and its offshore islands there are some 15-23 million feral cats which are estimated to eat about 75 million native mammals, reptiles, birds and even insects a night, more than 20 billion every year (see here for details). These astonishing figures are very similar to the results of a study made in the US. Also in Britain, estimates derived from scaling up local studies to the national level show that cats kill 25–29 million birds per year. It is easy to imagine how detrimental this species can be, considering that cats have been introduced to about 179,000 islands worldwide. According to another recent study, the impacts of feral cats is known from at least 120 different islands on at least 175 different species of vertebrates (25 reptiles, 123 birds, and 27 mammals), many of which are listed on the IUCN Red List. For example, in the Canary Islands, four species (one endemic bird — the Fuerteventura stonechat — and three endemic giant lizards) out of a total of 68 species (including invertebrates) identified as preys are considered threatened (for a review on the impact of cats and other invasive alien species, see the EEA technical report No 16/2012 discussed here).
|Stray cat in Rome © Photo: Riccardo Scalera|
Cats are predators and are not to be blamed for this, but people and particularly cat owners could do more to prevent all this to happen. Prevention would be the most effective and easy option to ensure a reduced predation of cats on small mammals and birds (not to consider reptiles and amphibians, and a number of invertebrates) and raising awareness should be a fundamental step in any nature conservation campaign. Otherwise, there is no doubt that the more effective way to prevent exacting such a heavy toll on native wildlife would be the implementation of policies to prevent the establishment of feral cats and their colonies in (semi)natural environments (see for example the Australian 2008 threat abatement plan predation by feral cats). This could save millions, if not billions, of birds and other animals, yet for many people might inevitably sound inconceivable. For those pet lovers who consider cats as family members, it may be difficult to believe that their companion pets may turn into such harmful threat to biodiversity. In fact, cat owners should make a special effort to acknowledge the problem and ensure keeping their pets indoors. Whenever this is not feasible, an alternative partial solution would be to fit cats with quick-release collars equipped with a bell or other deterrents (like bibs), which may significantly reduce predation rates on small mammals and birds (although cats can learn to silently stalk their prey anyway).
The implementation of effective control measures on feral cats can be a challenging task. First of all, to maintain the necessary political/public support and funding, it is pivotal to consider humane, socially acceptable options, including ways to avoid or minimize methods that cause animal suffering or affect domestic cats, particularly for the inherent problems associated with the opposition of citizens and animal welfare groups. Disregarding the importance of these aspects might lead to the failure of the operations. In addition, some drawbacks have been reported in situations where cats have been removed without taking into proper account the presence of other introduced species (such as rabbits, rats or mice). The risk is that some problems linked to hyperpredation and predator release effect may create trophic cascades leading to rapid, landscape-wide ecosystem changes. It was the case of the removal of cats on the sub-Antarctic Macquarie Island, which resulted in a significant increase in rabbit abundance (formerly reduced by cat predation), which in turn led to substantial local- and landscape-scale changes in vegetation. Although this trophic cascade was predictable given the history of rabbit impacts via grazing on both this and other islands and was not entirely unexpected, its extent was not fully anticipated. This episode (see here for details) shows the importance of carefully assessing the risks of management interventions and planning for their indirect effects.