Zoology - Theses
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ItemCompetition and coexistence: the ecology of sympatric common and mountain brushtail possums (Trichosurus spp.)( 2014)Closely related species often possess similar ecology due to their shared phylogenetic history, and consequently they usually require similar resources. As a result, potential for competition between closely related species is high. Without the rapid renewal of shared resources, coexistence between such species can usually only occur in the presence of resource partitioning. I studied the ecology of sympatric common and mountain brushtail possums (Trichosurus vulpecula and T. cunninghami) within a network of linear forest remnants in Boho South, north-east Victoria, with the aim of uncovering the processes facilitating coexistence between these congeneric species. Both common and mountain brushtails are primarily folivorous, arboreal/semi-arboreal marsupials, and occur sympatrically through some parts of their distributions. I examined three-dimensional resource partitioning by the foraging guild of arboreal marsupial folivores to which the two brushtail species belong. At Boho South, this guild included the koala, the greater glider and the common ringtail possum, in addition to the two brushtail species. I found that significant resource partitioning occurred in both the horizontal and vertical plane, with each species adopting a particular foraging niche with regard to tree species use and the available vegetation strata. Vertical stratification of such a diverse marsupial guild has not previously been demonstrated in temperate forest systems, and this result highlights the role of habitat heterogeneity in maintaining diverse faunal assemblages. I radio-tracked pairs of adult female common and mountain brushtails occurring in close sympatry in the linear forest remnants. Female mountain brushtails maintained home range areas more than twice the size of those of common brushtails, and the home ranges of mountain brushtails overlapped those of sympatric common brushtails more significantly than the reverse. Although the degree of total (95 %) home range overlap was high, the core (50 %) home ranges of study pairs overlapped significantly less. Patterns of den use between the two species were similar, with common and mountain brushtails using a mean of 6.0 ± 1.1 and 5.7 ± 1.4 dens per individual, respectively, over a mean of 15.5 months monitored per individual. The majority of den fixes were in tree hollows (75.7 % and 89.5 % of common and mountain brushtail den fixes, respectively). Vertical vegetation strata were used differently by each species, with common brushtail using the canopy and midstorey significantly more, and the understorey and ground layer significantly less, than mountain brushtails. The maintenance of relatively discrete core home range areas combined with fine-scale differences in use of tree species and vegetation strata appeared to facilitate the high degree of home range overlap between adult females. I conducted an extensive analysis of the diet of each species, examining faecal samples for both plant and fungal content. Dietary resources were strongly partitioned between the two brushtail species: common brushtails foraged primarily on Eucalyptus, while mountain brushtails foraged primarily on silver wattle. Common brushtails also consumed significantly more mistletoe foliage. Both species used fungal and floral food resources, however, these were also strongly partitioned between the two species: common brushtails supplemented their diet more often with flowers, while mountain brushtails consumed a greater amount and diversity of epigeal and sequesterate fungal taxa. Given the tripartite relationship occurring between sequesterate ectomycorrhizal fungi, the forest tree and shrub taxa with which the fungi form symbiotic relationships, and the mycophagous mammals that disperse fungal spores in their faeces, the consumption of sequesterate fungi by mountain brushtails is likely to play an important role in ecosystem health at Boho South, particularly given the local extinction of specialist fungivores, the potoroids, in this area. I propose that the strong partitioning of foliage between the two brushtail species, combined with an abundance of shelter resources in the form of suitable tree hollows, facilitates the stable coexistence of common and mountain brushtails at Boho South. By significantly reducing competition for food resources, dietary partitioning appears to allow each species to maintain home ranges that overlap strongly with those of their congeners. As the foraging niches of each brushtail species are consistent with other research on their digestive tolerances, I suggest that the divergent diets of common and mountain brushtails are a result of ecological character displacement having evolved in the two species to minimise competition for food resources. Current fine-scale partitioning of habitat components and supplementary food sources by common and mountain brushtails at Boho South are a possible further means by which these two species avoid competition for resources.