Identifying important environmental variables in the niche partitioning of two keystone ecosystem engineers (Bettongia gaimardi and Potorous tridactylus) in Tasmania

Abstract

Context. The eastern bettong (Bettongia gaimardi) and the long-nosed potoroo (Potorous tridactylus) are mycophagous marsupials regarded as both keystone species and ecosystem engineers. Despite Tasmania being a refuge for these declining species, their niche partitioning is poorly understood. Aims. Our aim was to identify factors that distinguish the distributions of B. gaimardi and P. tridactylus, and to develop a better explanation of their individual niches. Methods. The Department of Primary Industries, Parks, Water and Environment conducted mammal surveys between 1975 and 2019. We used GIS to analyse these data, and geospatial information to identify relationships between B. gaimardi and P. tridactylus presence/absence and environmental variables. We then developed a model describing the distributions of these species in Tasmania. Key results. Temperature seasonality (s.d. × 100), precipitation of wettest month (mm), precipitation of the driest month (mm), precipitation seasonality (coefficient of variation), the presence of vegetation dominated by Eucalyptus amygdalina and the presence of lowland grassy woodland/forest were the components in the best model for B. gaimardi. Our model broadly predicts that the distribution of B. gaimardi is restricted to the more fertile eastern half of Tasmania. P. tridactylus was associated with very few variables, with the presence of E. amygdalina-Eucalyptus obliqua damp forest being the only component in a very weak model. Transects with P. tridactylus and not B. gaimardi were more associated with rainforest and wet forest communities and areas of higher annual and wettest-month precipitation than were those with B. gaimardi and not P. tridactylus. Conclusions. The importance of infertile sites to B. gaimardi may have been overstated in the literature, with moderate to high fertility being more characteristic of its range. B. gaimardi is adapted to persist in environments of low truffle (food) density, typical of the eastern half of Tasmania, through its ability to adopt a larger home range than for P. tridactylus, which requires dense ground vegetation. Implications. Sites of high fertility in fragmented landscapes should be considered to be potential habitat for B. gaimardi. This challenges previous assumptions that infertile sites are the primary habitat of the species, with fertile sites offering poorer-quality habitat.