When pets go wild: Integrating DNA metabarcoding and morphological analyses to investigate the impacts of free-ranging cats (Felis catus) on oceanic islands

Abstract

Oceanic island ecosystems are particularly vulnerable to the impacts of introduced mammalian predators. Among these, free-ranging cats (Felis catus) are especially detrimental, and have been linked to the extinction and population decline of numerous insular taxa. Yet, a more holistic understanding of the impacts of free-ranging cats has been hampered by an incomplete understanding of their diet and population densities. Using a peri-urban protected area in the subtropical Madeira Island as a case study, we combined DNA metabarcoding, morphological scat analyses, and camera trapping to investigate the magnitude of native and introduced prey consumption by cats. Based on 58 scats collected over the summer and autumn of 2021, we found that cats consume over 20 distinct taxa from ten orders, including native and non-native prey, as well as taxa associated with anthropogenic food. Cat diet was seasonally modulated, with a higher incidence of reptiles in the summer months. Although DNA metabarcoding revealed a higher diversity of prey taxa compared to traditional morphological scat analyses, both methods provided complementary insights, highlighting the benefits of their combined use. Combining our diet results with a free-ranging cat density of 1.4 cats/km², as revealed by the protected area-wide camera trap survey, we estimate an annual prey intake of over 18,000 vertebrates by cats in a protected area of 7.5 km², of which over one-third corresponds to native birds and reptiles. These findings underscore the complex interactions between cats and native wildlife, emphasising the need for targeted conservation strategies in ecologically sensitive areas.