Nestbox use indicates declining arboreal mammals in an Australian savanna may be limited by tree hollow availability

In the tropical savannas of northern Australia, many native mammal species are experiencing rapid and ongoing declines due to multiple interacting threats. These mostly arboreal and semi-arboreal mammals are reliant on tree hollows for shelter, which may be contributing to their decline. We monitored 198 nestboxes over three years across a 100-km transect on Cobourg Peninsula, where a diverse arboreal mammal community still occurs. Most nestboxes (67%) were used by at least one of five mammal species, all of which are declining and four are threatened. Hollow density (proxied by eucalypt stand density) varied across sites, with the number of eucalypts (>20 cm DBH) ranging between 20 and 140 per hectare and averaging 74 ± 28 (SD) per hectare. Correspondingly, the likelihood and frequency of nestbox use by arboreal mammals increased with declining local tree hollow density, indicating that hollows are a critical limiting resource at the local scale for arboreal mammals in this region. Frequent, local disturbances e.g., fire and introduced species, contribute to threatening processes. Therefore, reversing the decline of arboreal mammals in Australia's tropical savannas requires multiple management interventions, including conserving hollow-bearing trees as ecosystem assets.