Unexpected overlapping use of tree hollows by birds, reptiles and declining mammals in an Australian tropical savanna

Tree hollows are a critical resource for many arboreal vertebrates, including many threatened species. In northern Australia’s vast tropical savannas, arboreal mammals are of particular conservation concern, as many have exhibited rapid population declines in recent decades. To understand the extent to which arboreal vertebrates compete for tree hollows in these savannas, we used motion-activated cameras to estimate rates of hollow visitation (i.e. individuals seen at the hollow entrance) and hollow use (i.e. individuals seen entering or exiting a hollow) by all vertebrate species at 29 hollows over a nine-month wet season period in the tropical savanna of Melville Island, Australia. We assessed the frequency of interactions between vertebrate species at the tree hollows. We recorded 21 vertebrate species visiting hollows, with larger hollows most frequently visited: 84% with entrance diameter > 10 cm. Larger-bodied mammal species were more likely to use larger hollows, in larger trees, at greater heights. However, smaller-bodies species also visited and used large hollows. The brush-tailed rabbit-rat (Conilurus penicillatus) used relatively shallow hollows close to the ground, and these hollows were most frequently visited by monitors (Varanus spp.) and snakes. This arboreal rodent has declined severely in recent decades, most likely due to predation by feral cats, and it is noteworthy that their hollow selection behaviour may be associated with increased predation risk. Our study highlights the importance of large tree hollows for a range of taxa in the tropical savannas of northern Australia and the potential impact of reductions in the density of large hollows on hollow-dependent species. Frequent, intense fires are a threat to large hollow-bearing trees, with appropriate fire management needed to conserve this resource and help to stem mammal declines.