Hollow futures? Declining arboreal mammals and tree hollow availability in tropical northern Australia

    Student thesis: Doctor of Philosophy (PhD) - CDU


    The ongoing, rapid decline of arboreal mammals in northern Australia is of critical conservation concern. This decline has been variously attributed to the impacts of feral species, namely cats and large herbivores, and inappropriate fire regimes which have depleted key resources, such as shrubs and large trees. The broad aim of my thesis was to determine whether tree hollows could be a limiting resource for arboreal mammals in the tropical savannas of Melville Island, northern Australia. My study represents the first targeted research on tree hollow availability and the arboreal community dynamics of the northern Australian mammals the brush-tailed rabbit-rat (Conilurus penicillatus), black-footed tree-rat (Mesembriomys gouldii), and northern brushtail possum (Trichosurus vulpecula arnhemensis), and their use of tree hollows. 

    My research reveals that although arboreal mammals most frequently use large hollows, which can be a limiting resource in tropical savannas, populations of arboreal mammals in regions supporting higher densities of hollows are still significantly impacted by other threats (e.g. feral predators and habitat simplification). Firstly, I found that hollows were commonly blocked by hard termitaria (i.e. termite nesting material, typically clay-rich and set hard). Secondly, I found that arboreal mammals most commonly selected large eucalypt (Eucalyptus and Corymbia spp.) trees (diameter at breast height >30 cm) and dead trees as denning sites, and these trees are relatively uncommon. Interestingly, I found arboreal mammals overlapped in their den use with all taxa that frequently utilise hollows >10 cm entrance diameter and commonly share dens intra- and interspecifically. Finally, I found increases in hollow abundance did not significantly influence the relative abundance of arboreal mammals on Melville Island. Rather, terrestrial factors relating to understorey quality (i.e. shrub density, fire frequency, large herbivore presence) and predator assemblages (i.e. predicted cat activity, dingo presence) were more important contributors to arboreal mammal abundance. Due to the open canopy structure of tropical savannas, arboreal mammals spend varying amounts of time foraging and travelling across the savanna floor, thereby exposing themselves to ground-based threats (e.g. predation by feral animals). Furthermore, arboreal mammals have been particularly vulnerable to habitat simplification due to their reliance on a complex, multi-dimensional habitat structure (e.g. large hollows, fruiting trees, grasses). In regions where hollow densities are lower, threats such as hollow competition and predation pressure are likely amplified.

    My thesis contributes to our understanding of the tree hollow ecology of northern Australian tropical savannas. Though the presence of a complex understorey is essential for the persistence of arboreal mammals, my findings highlight the overall importance of large hollows and eucalypts (Eucalyptus and Corymbia spp.) for arboreal mammals and other hollowdependent fauna. The high degree of overlap in both tree hollow characteristics, the scarcity of large hollows in the tropical savanna, coupled with changes in resource availability due to altered fire regimes and invasive species potentially help explain why arboreal mammals have been susceptible to declines.

    Date of Award2021
    Original languageEnglish
    SupervisorBrett Murphy (Supervisor), Leigh-Ann Woolley (Supervisor) & Ian Radford (Supervisor)

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