The conservation of arboreal marsupials in the montane ash forests of the central highlands of Victoria, south-eastern Australia. VIII. Landscape analysis of the occurrence of arboreal marsupials

Abstract

We describe the results of landscape analysis of the occurrence of a suite of species of arboreal marsupials in the montane ash forests of the central highlands of Victoria, south-eastern Australia. The species targeted for study were: Yellow-bellied Glider (Petaurus australis), Mountain Brushtail Possum (Trichosurus caninus), Greater Glider (Petauroides volans) and Leadbeater's Possum (Gymnobelideus leadbeateri). Our analyses involved exploring statistical relationships between the presence/absence of these four species and compositional variables used to characterise the landscape within 20 and 80 ha circles surrounding each of 166, 3 ha, sites surveyed for arboreal marsupials. Many potential statistical models were examined for each of the four species targeted for analysis. With the exception of P. australis, the final model chosen was the null model; i.e. none of the landscape variables were statistically significant as predictors of the presence of animals at a site. In the case of P. australis, three measures of the landscape within the 80 ha circles surrounding our survey sites were found to be statistically significant: (1) the presence of old forest, (2) the steepness of the forest terrain, and, (3) the area of forest on different aspects. There was a significantly higher probability of detecting the species on northerly and westerly aspects, on either steep or flat terrain, and within circles supporting old growth montane ash forest. P. australis has a home range of 40–60 ha which is at least an order of magnitude larger than the other species of arboreal marsupials examined. Thus, the size of the 80 ha circle used to capture data on landscape composition matched the home range size of P. australis more closely than other species of arboreal marsupials examined in this study. This may, in part, explain why it was the only species for which landscape composition variables were found to be significant predictors of occurrence. Our findings tentatively suggest that changes in the spatial coverage of old growth forest that have resulted from past wildfires, post-fire salvage logging and more recent intensive and extensive clearfell logging operations, have created landscape patterns unsuitable for P. australis. The implications of our results for the conservation of the species are discussed. In addition, some difficulties were experienced in the use of landscape-scale data in our statistical analyses, such as identifying attributes of landscape composition that are biologically relevant to the taxa targeted for analysis. We briefly discuss the nature of these problems.

Introduction

Many landscapes are undergoing extensive and rapid change as a consequence of human activities (Franklin and Forman, 1987, Hansson and Angelstam, 1991). The spatial structure of landscapes determines important ecological processes such as the flow of water, erosion, and the supply of nutrients (Swanson et al., 1988, Mackey, 1993) and also influences spatial patterns of species distributions (e.g. Wardell-Johnson and Roberts, 1993). For example, the spatial configuration of habitat patches may affect the long-term viability of species susceptible to habitat fragmentation (Lamberson et al., 1992, Akaçakaya et al., 1995). Population Viability Analysis predicts that the presence of arboreal marsupials at a site will be influenced by the amount of suitable habitat in the surrounding landscape (Goldingay and Possingham, 1995, Lindenmayer and Possingham, 1995). Thus, the long-term persistence of populations of some species of wildlife is one of the major concerns associated with human modifications to landscapes (Saunders and Hobbs, 1991).

The central highlands of Victoria, south-eastern Australia, is one of many regions around the world where there have been major and rapid human-induced changes at a landscape spatial scale (Lindenmayer, 1996). The tall, fast-growing, montane ash forests in the region are highly sought after for the production of timber and pulpwood (Lindenmayer, 1996). Logging operations in the central highlands region have changed the size and spatial arrangement of some types of forest such as stands of old growth (Milledge et al., 1991, Lindenmayer and Possingham, 1995). Arboreal marsupials are one group potentially threatened by these changes and they are the focus of this study.

Previous studies in this series have identified the habitat requirements of arboreal marsupials in the montane ash (Eucalyptus regnans, Eucalyptus delegatensis and E. nitens) forests of the central highlands of Victoria (Lindenmayer et al., 1990a, Lindenmayer et al., 1991a, Lindenmayer et al., 1994a). These studies examined statistical relationships between plant species composition and vegetation structure measured at a large number of 3 ha sites and the presence and abundance of species of arboreal marsupials including: the Mountain Brushtail Possum (Trichosurus caninus), Greater Glider (Petauroides volans); and, Leadbeater's Possum (Gymnobelideus leadbeateri) (Lindenmayer et al., 1990b, Lindenmayer et al., 1991a, Lindenmayer et al., 1994a). In this study, we explore factors influencing the occurrence of arboreal marsupials at a spatial scale larger than the 3 ha survey sites. We quantify relationships between the presence/absence of four species of arboreal marsupials (Yellow-bellied Glider [P. australis], T. caninus, P. volans and G. leadbeateri) and attributes of landscapes surrounding each of the field sites surveyed for these taxa. We also discuss some of the implications of our results for forest landscape management and wildlife conservation.

The four species of arboreal marsupials targeted for study vary markedly in life-history attributes (e.g. longevity and fecundity), body size, diet, and home range. T. caninus is a non-volant animal weighing up to 4 kg (Lindenmayer et al., 1995). Its diet comprises the leaves of several understorey trees and shrubs, as well as fungi (Seebeck et al., 1984). Adults live in pairs and occupy a territory of 2–6 ha (How, 1972). They can be long-lived and sedentary and occupy the same area for up to 18 years (Lindenmayer et al., 1991b). G. leadbeateri is a small (110–160 g) species of arboreal marsupial that lives in colonies of 3–12 individuals. The home range of a colony is about 1.5–3 ha in size (Smith, 1984a). The diet of the species includes arthropods, honeydew (a sugar-rich secretion produced by phloem-feeding, lerp-forming insects) and plant exudates; especially saps produced by understorey Acacia spp. trees (Smith, 1984b, Lindenmayer et al., 1994b). Adult P. volans weigh up to 1300 g and feed exclusively on the leaves of Eucalyptus spp. trees (Kavanagh and Lambert, 1990, Comport et al., 1996). The home range of P. volans is approximately 1 ha (Henry, 1984) and the species is usually solitary, except during the breeding season when pairs of animals are often recorded (Henry, 1984). P. australis weigh approximately 650 g (Henry and Craig, 1984). It has highly developed gliding abilities and can volplane distances of up to 100 m (Goldingay and Kavanagh, 1991). The 40–60 ha home range of P. australis (Henry and Craig, 1984, Craig, 1985, Goldingay and Kavanagh, 1993) is substantially larger than any of the other species of arboreal marsupials in the montane ash forests of the central highlands of Victoria. P. australis has a diverse diet of invertebrates, nectar and sap from Eucalyptus trees, pollen, and honeydew (Henry and Craig, 1984, Goldingay, 1986). In the central highlands forests, P. australis appears to be monogamous and colonial, living in groups of 3–5 animals (Craig, 1985).