Human Subsidies Facilitate Hyperpredation of Mediterranean Island Wildlife by Outdoor Cats

Domestic cats (Felis catus) are avid wildlife predators and one of the most harmful invasive species, having been a causal factor in over 60 species extinctions worldwide (Doherty et al., 2016). The lethal impacts of an introduced predator such as cats on wildlife can be further exacerbated by the introduction of an additional abundant non-native prey species capable of supporting an exceptionally dense predator population, a phenomenon known as hyperpredation. A special case of hyperpredation involves human food subsidies, when an invasive predator impacting native wildlife is supported not by another invasive taxon, but by human-derived food sources. To test whether access to anthropogenic food subsidies by cats is causing hyperpredation, mark-recapture methodology was used to measure twelve cat populations experiencing a gradient of human subsidies on the Mediterranean island of Naxos, Greece. Line-transect surveys were conducted at each site to measure reptile population abundance across this gradient, and other factors including human density and distance of reptiles from villages were considered as well. Strong evidence was found that the population size of cats is a direct result of the density of anthropogenic food available to them, and a corresponding decline in the size of reptile populations as cat density increased was observed. It was also shown that as cat populations exceed the available supply of human food, the negative effects of cat density on reptile populations are exacerbated. These results demonstrate that access to anthropogenic subsidies has allowed cat populations to expand to exceptional levels, driving hyperpredation on island wildlife.