Pre-eradication assessment of feral cat density and population size across Kangaroo Island, South Australia.
Context: Feral cats (Felis catus) are a significant threat to wildlife in Australia and globally. In Australia, densities of feral cats vary across the continent and also between the mainland and offshore islands. Densities on small islands may be at least an order of magnitude higher than those in adjacent mainland areas. To provide cat-free havens for biodiversity, cat-control and eradication programs are increasingly occurring on Australian offshore islands. However, planning such eradications is difficult, particularly on large islands where cat densities could vary considerably. Aims: In the present study, we examined how feral cat densities vary among three habitats on Kangaroo Island, a large Australian offshore island for which feral cat eradication is planned. Methods: Densities were compared among the following three broad habitat types: forest, forest-farmland boundaries and farmland. To detect cats, three remote-camera arrays were deployed in each habitat type, and density around each array was calculated using a spatially explicit capture-recapture framework. Key results: The average feral cat density on Kangaroo Island (0.37 cats km-2) was slightly higher than that on the Australian mainland. Densities varied from 0.06 to 3.27 cats km-2 and were inconsistent within broad habitat types. Densities were highest on farms that had a high availability of macropod and sheep carcasses. The relationship between cat density and the proportion of cleared land in the surrounding area was weak. The total feral cat population of Kangaroo Island was estimated at 1629 ± 661 (mean ± s.e.) individuals. Conclusions: Cat densities on Kangaroo Island are highly variable and may be locally affected by factors such as prey and carrion availability. Implications: For cat eradication to be successful, resources must be sufficient to control at least the average cat density (0.37 cats km-2), with additional effort around areas of high carcass availability (where cats are likely to be at a higher density) potentially also being required.