THE RESPONSE OF ARBOREAL MARSUPIALS TO LANDSCAPE CONTEXT: A LARGE-SCALE FRAGMENTATION STUDY

Abstract

We describe a landscape-scale study of fragmentation effects on arboreal marsupials at Tumut, southeastern Australia. Embedded within a 55000-ha plantation of exotic Monterey pine (Pinus radiata) are 192 patches of remnant Eucalyptus forest, with large contiguous areas of eucalypt forest beyond the plantation boundary. We compared presence and estimated abundance of arboreal marsupials in three broad groups of sites: remnants of native Eucalyptus forest in the P. radiata plantation; P. radiata stands in the plantation; and large areas of contiguous Eucalyptus forest surrounding the plantation. The study animals were Trichosurus vulpecula, T. caninus, Petaurus breviceps, P. norfolcensis, P. australis, Acrobates pygmaeus, Pseudocheirus peregrinus, and Petauroides volans. We used randomized, replicated statistical procedures to sample 86 eucalypt remnants varying in size, shape, and other features. We matched 40 sites in large contiguous Eucalyptus forest to those remnants, based on environment, climate, terrain, and vegetation cover. In addition, 40 sites in P. radiata stands were matched to sites in the remnants and the large eucalypt forest on the basis of geology, climate, and terrain. We also surveyed 41 micropatches (remnants < 1 ha), where almost no animals were detected. These micropatches, smaller than the home ranges of most arboreal marsupials in the Tumut region, probably represent a minimum threshold patch size below which animals cannot survive. Highly significant landscape context effects were identified. P. radiata sites were excluded from analysis because few animals were detected there. P. australis and P. norfolcensis were not recorded in remnants. P. norfolcensis was recorded only once in contiguous eucalypt forest; P. australis was absent. Four species (P. volans, P. peregrinus, T. vulpecula, and T. caninus) occurred in sufficient numbers for statistical analyses of presence and abundance on remnants vs. large contiguous forest. P. volans was less likely to occur in remnants, whereas P. peregrinus was less likely to occur in contiguous eucalypt forest. Site context had no significant effect on presence of T. caninus, and arboreal marsupials per se, or on abundance of P. volans, T. vulpecula, T. caninus, and arboreal marsupials overall. P. peregrinus was significantly more abundant in remnants than in contiguous eucalypt forest. Statistical adjustment for environmental, climatic, terrain, and vegetation variables by regression modeling did not change animal abundance results. Similar outcomes were recorded for presence of T. vulpecula and T. caninus. However, context effects for presence of T. vulpecula, P. volans, and P. peregrinus could be explained by some of the vegetation, terrain, and landscape covariates. For P. volans, probability of occurrence was significantly greater in large remnants, sites on flat terrain, and sites dominated by particular eucalypt forest types. T. vulpecula was significantly more likely to occur in large remnants and sites with open understories. There was no evidence that P. peregrinus responded to remnant area, but it was significantly more likely to occur on sites with limited understories and trees with few cavities. Life history attributes of the eight marsupial species (e.g., home range, body size, diet, and social organization) provided no insight into the landscape context and fragmentation effects observed. However, the only two species detected in P. radiata stands surrounding the remnants displayed either a complete absence of context or fragmentation effects (T. caninus) or were more likely to occur in remnants than in large contiguous eucalypt forest (P. peregrinus). Thus, fragmentation effects may be related, in part, to how animals use the exotic softwood “matrix” around the remnants. Our investigation indicated that eucalypt forest remnants are valuable refugia for some arboreal marsupials. Future expansions of P. radiata plantations planned for degraded and partially cleared agricultural landscapes in many parts of southeastern Australia should ensure that existing Eucalyptus remnants are not cleared.