The intact and the imperilled: contrasting mammal population trajectories between two large adjacent islands

Context Native mammals continue to suffer widespread and severe declines across northern Australia's tropical savannas. There is an increasing body of evidence that the primary driver of these declines is predation by feral cats (Felis catus) and that this is exacerbated by high-severity disturbance regimes (frequent high-intensity fires, and grazing and trampling by exotic megaherbivores) that simplify habitat, thereby increasing hunting efficiency. The large islands off the northern Australian coast - where some threats are either reduced or absent - provide a means of testing the conceptual model's predictions. Aims To compare the trajectory and distribution of native mammal populations on two large, adjacent islands with markedly different disturbance regimes. Methods In 2020 and 2021, we resurveyed 111 historical sites across the two largest of the Tiwi Islands, Bathurst Island (42 sites) and Melville Island (69 sites) that were previously surveyed between 2000 and 2002. The Melville Island sites had also been resurveyed in 2015. We used the same live trapping method used in 2000-2002, supplemented with camera trapping. Key results On Bathurst Island, feral cats are rare, and we found no significant decrease in native mammal trap success or species richness, and the threatened brush-tailed rabbit-rat (Conilurus penicillatus melibius) appears stable. Conversely, cats occurred at relatively high abundance on Melville Island, and there was a 52% decline in trap success, a 47% reduction in species richness, and a 93% decline in trap success for the brush-tailed rabbit-rat over the 20-year period. The highest decreases in native mammal abundance and richness were in areas that were frequently burnt and had higher activity of feral cats. In contrast, in the absence of cats on Bathurst Island, native mammal abundance increased in frequently burnt areas. Conclusions While Bathurst Island remains one of Australia's most important refuges for native mammals, neighbouring Melville Island is experiencing severe and ongoing mammal decline. We contend that this pattern primarily reflects the high abundance of cats on Melville Island compared to Bathurst Island.