A field study of Pseudocheirus occidentalis (Marsupialia : Petauridae) I. Distribution and Habitat

Abstract

Surveys aimed at determining the distribution and habitat of the rare and endangered western ringtail possum (Pseudocheirus occidentalis) were undertaken in south-western Australia during 1990-92. Surveys relied on sightings of animals, or their characteristic faecal pellets or dreys. Habitat descriptions were collected in areas occupied by P. occidentalis to describe the vegetation and topography. Additional information about habitat was collected at subsets of sites to reflect leaf nutrient levels (nitrogen, phosphorus and potassium) of the major food plants, and to reflect the relative senescence of vegetation communities, and their fire and fox-baiting history. The species has a patchy occurrence from the Collie River to Two Peoples Bay, occurring most commonly in coastal or near-coastal forest that includes Agonis flexuosa as a major component. The most inland population occurs at Perup and this is the only known population living in forest without A. flexuosa. Local extinction has been extensive in the inland and northern parts of the original range (as it was known about 1900) and local decline has been patchy and occurred in most decades (1900-89) in different parts of the original range. Analysis of a matrix of relative abundance and habitat variables identified two habitat parameters associated with higher abundance of P. occidentalis: higher levels of nitrogen in foliage of the major food plant and a higher degree of canopy continuity. Hollow abundance was also implicated as a covariate of relative abundance. Assessment of levels of nitrogen, potassium and phosphorus in A. flexuosa leaves indicated that occupied sites tended to have relatively high nutrient levels. The pattern of decline and persistence of populations in different habitats is discussed with respect to environmental factors considered to have changed over the last century.