A Hollow Argument: Understory Vegetation and Disturbance Determine Abundance of Hollow-Dependent Mammals in an Australian Tropical Savanna

Abstract

Native mammals are suffering widespread and ongoing population declines across northern Australia. These declines are likely driven by multiple, interacting factors including altered fire regimes, predation by feral cats, and grazing by feral herbivores. In addition, the loss of tree hollows due to frequent, intense fires may also be contributing to the decline of hollow-dependent mammals. We currently have little understanding of how the availability of tree hollows influences populations of hollow-dependent mammals in northern Australian savannas. Here, we test the hypothesis that the abundance of hollow-dependent mammals is higher in areas with a greater availability of tree hollows. We used camera-trap data from 82 sites across the savannas of Melville Island, the largest island in monsoonal northern Australia. Royle–Nichols abundance-induced heterogeneity models were used to investigate the biophysical correlates of the abundance of three threatened mammals: northern brushtail possum (Trichosurus vulpecula arnhemensis), black-footed tree-rat (Mesembriomys gouldii), and brush-tailed rabbit-rat (Conilurus penicillatus). Our analyses included two variables that reflect the availability of tree hollows: the density of tree hollows, estimated from the ground, and the density of large eucalypt trees (Eucalyptus and Corymbia spp.). We found no evidence that the abundance of the three hollow-dependent mammals is positively associated with the availability of tree hollows on Melville Island. Despite their reliance on hollow-bearing trees for denning, the abundance of these mammals appears to be more strongly associated with other factors, such as the characteristics of the understory (i.e., shrub density), which affords protection from predators (including feral cats) and access to food resources. Future conservation management should aim to maintain a dense, diverse understory by managing fire and feral herbivores to facilitate the persistence of hollow-dependent mammals across northern Australia.

Landscape-Scale Effects of Fire, Cats, and Feral Livestock on Threatened Savanna Mammals: Unburnt Habitat Matters More Than Pyrodiversity

“…Gray circles indicate observed data values and are darker when repeated observations occur at the same point. Davies et al, 2017;Penton et al, 2021), cat occupancy (Davies et al, 2017;Stobo-Wilson et al, 2020a), fire 'activity' or extent (Lawes et al, 2015;Stobo-Wilson et al, 2020a) and the presence of feral livestock (Legge et al, 2019). Our study also supported the hypothesis that site-scale disturbance and habitat features are more directly important to local mammal populations than broader landscapescale (see above) or regional-scale (as per Radford et al, 2020a;Stobo-Wilson et al, 2020a) fire mosaic attributes.…”

“…This corroborates Stobo-Wilson et al (2020a) in finding that vegetation productivity was positively associated with mammal richness at broad scales except for when fires removed the cover. The importance of shrub cover as a model attribute for volcanic habitats also support previous findings for the threatened mammal Conilurus penicillatus in Melville Island savannas (Davies et al, 2017;Penton et al, 2021). The dynamic nature of ground layer vegetation highlights a key implied threat to savanna mammals in these open habitats.…”

“…Stobo-Wilson et al (2020b) found a negative relationship between vegetation productivity and cat occupancy, and these results are mirrored in our study with cat activity levels higher at low productivity (lower rainfall) sites (e.g., Mount Elizabeth, Solea Falls, Orchid Creek; Figure 1). High feral cat activity in savannas has also been linked with disturbance of ground layer vegetation either through frequent fire or extensive grazing (Davies et al, 2020;Penton et al, 2021). This is also reflected in this study, with feral cats more active at sites on one pastoral station (Mount Elizabeth) and sites subject to extensive wildfires during the study period (Solea Falls, Orchid Creek).…”

“…Studies have now linked regional patterns of mammal abundance and richness with cat occupancy, disturbance regimes and vegetation cover, providing some support for these conceptual models (Davies et al, 2017(Davies et al, , 2020Radford et al, 2020a;Stobo-Wilson et al, 2020a;Penton et al, 2021). However from a management perspective, it is still unclear what sitescale habitat or disturbance features support or threaten mammal assemblages.…”

Tropical savanna small mammals respond to loss of cover following disturbance: A global review of field studies

“…Of 17 studies of fire effects conducted in Northern Australia, 12 of which were experimental, six concluded that loss of cover explained significant small-mammal responses (Kutt and Woinarski, 2007;Legge et al, 2008;Kutt and Gordon, 2012;Leahy et al, 2015;Davies et al, 2017;Radford et al, 2021), five concluded that both cover and food resources were partly important as limiting factors (Lawes et al, 2015;McDonald et al, 2016;Ondei et al, 2020;Radford et al, 2020;Penton et al, 2021), and six studies were inconclusive (Pardon et al, 2003;Woinarski et al, 2004;Griffiths and Brook, 2014;Abom et al, 2016;Davies et al, 2018;Legge et al, 2019; Supplementary Table S1, Figure 2). One study found 90% direct mortality of small mammals from intense fires, with the individuals surviving being in unburned refuges (Legge et al, 2008).…”