An ecophysiological investigation of the effects of anthropogenic habitat loss, fragmentation and degradation on the agile antechinus

Anthropogenic habitat fragmentation has well-established deleterious effects on populations and communities of native vertebrates, but the mechanisms underlying population decline under fragmentation remain poorly understood. Most studies of vertebrates in anthropogenically-fragmented habitats have focused on population density, demographics or fecundity. Relatively little attention has been given to indices of health status, body condition or physiological stress. In this study, 30 populations of a small marsupial, the agile antechinus (Antechinus agilis), living in anthropogenically-fragmented forest patches were sampled in two years (2007 and 2008). Immediately after sampling in each fragment, a population in a matched control site in similar, but unfragmented forest (a 'pseudofragment') was sampled. Indices of population density (relative abundance), estimated fat reserves (mass-size residuals), health status (erythrocyte variables), parasite load (simplified ectoparasite counts and eosinophil percentages) and chronic physiological stress (total and differential immune cell counts) were examined. Relative abundances were lower and parasite load indices higher in fragmented than continuous forest. Fragment populations displayed indications of regenerative anaemia, which is related to poor health status and potentially caused by chronic stress, frequent blood loss or heavy parasite loads. Estimated fat reserves were higher in fragment than continuous forest populations. Nonetheless, differential leukocyte counts suggested that chronic physiological stress was greater (i.e. greater neutrophil-to-lymphocyte ratios) (N:L) in populations in fragments. Anthropogenic fragmentation effects are not often distinguished in the ecological literature from those of co-occurring processes, such as habitat loss and degradation. To investigate the effects of these processes, environmental factors were examined that were thought to have a potential influence on agile antechinus (e.g. fragment patch core area, proportion of edge habitat, isolation, woody debris abundance, shrub density etc). Relative abundance of agile antechinus was positively correlated with forest patch core area and native tree-cover within a 0.5 km radius of a study site. Estimated fat reserves, particularly in males, were greater in populations in fragments with a smaller core area, but statistical modelling indicated that the effect was an indirect one: males had greater estimated fat reserves where the abundance of conspecifics was lower, suggesting that this metric was responding to intraspecific competition and per capita food availability. Health status, indexed by erythrocyte indicators of regenerative anaemia,was positively associated with greater microhabitat heterogeneity, and abundance of shrubs, logs and native trees other than Eucalyptus species. Female abundances were lower in edge habitat (< 60 m from edge) than in fragment interiors (> 80 m from edge), and females had higher chronic stress indicators (N:L) where fragments were more highly dissected by edge habitat. Although parasite load indices and male N:L were higher in fragment than continuous forest sites, the environmental factors responsible were not identified. The study has demonstrated that anthropogenic habitat fragmentation, loss and degradation can have broadly negative effects on a native vertebrate, not only on its population density, but also in terms of health status and chronic physiological stress. This is a serious concern from a conservation management perspective, because chronic stress has pronounced fitness-reducing effects in vertebrates, including reduced reproductive investment, fecundity and survivorship.