Modelling distributions of arboreal and ground-dwelling mammals in relation to climate, nutrients, plant chemical defences and vegetation structure in the eucalypt forests of southeastern Australia

Abstract

Numerous studies over the past 15 years have investigated relationships between the distributions of arboreal and ground-dwelling mammals and environmental, structural and leaf compositional variables in the temperate Eucalyptus forests of southeastern and northeastern New South Wales, Australia. This paper draws together the general trends emerging from these studies and identifies some clear messages for the future modelling of regional biodiversity in these forests. The studies on arboreal mammals (all are marsupials in these forests) reviewed here generally fall into two broad categories: those that conclude that the nutrient status of forests is the prime determinant of habitat quality for arboreal marsupials and those that put equal or greater emphasis on variables related to structural characteristics of forests. Recent studies suggest a hierarchical model that is consistent with both of these emphases. They postulate that a proportion of temperature eucalypt forests, regardless of their climatic or structural characteristics, cannot support permanent populations of arboreal marsupials, especially leaf-eating species, due to low food quality and/or high phytochemical toxicity. Above a postulated nutritional or phytotoxicological ‘threshold’, food quality is adequate and other variables, including climatic and structural ones, apparently interact to determine habitat quality. Hence, differences in the extent to which different studies sample regional environmental variability, the range of nutrient status and forest structure, are likely to greatly affect which variables appear most significant in models of habitat requirements. Structural characteristics (measured as habitat complexity) of the forest have emerged as explanatory variables for the ground-living mammals also. Variables such as nutrients, lithology, terrain and climate exhibit a different trend to that seen for arboreal marsupials. Relative abundance of small ground-dwelling mammals is negatively correlated with site nutrient status as indicated by nutrient concentrations in tree foliage. Small mammals are present at all measured nutrient levels, but their abundance falls substantially as habitat complexity decreases. The influence of nutrients is masked in habitats of high complexity, there being no relationship with nutrient status. Many ground-living mammals occur across the gradients of lithology, terrain and climate, although there is wide variation in relative abundance for some species. The importance of structural variables for explaining distributions of both arboreal and ground-living fauna in eucalypt forests indicates that adequate modelling of habitat requirements for these fauna can only be achieved if surveys obtain adequate data on forest structure to encompass gradients in seral stage and disturbance history. The present-day mammalian fauna of the southeastern Australian forests has been influenced strongly by the effects of urbanisation, clearing for farming, forestry activities and fire on forest structural complexity and nutrient dynamics as well as by predation by introduced camivores. While modelling with respect to broadly defined climatic and terrain variables might be useful for broad-scale spatial prediction of faunal distributions, such models are unlikely to provide descriptions of habitat requirements or predictions of impacts of forest management at a scale necessary for sustainable management of faunal biodiversity.