The overall aim of this thesis was to investigate the ecological, physiological and genetic responses of a forest-dependent mammal to island life, using the squirrel glider (Petaurus norfolcensis) in southeast Queensland, Australia, as a case study. Human-induced habitat loss and fragmentation, and associated anthropogenic pressures are key contributors to the decline of mainland wildlife worldwide. Conversely, off-shore islands have been largely protected from these threatening processes, and as a result may play an important role in the long-term protection of many wildlife species. Although islands offer many protective advantages, historically, wildlife populations on islands have been more prone to extinction than mainland populations. The vulnerability of island populations to extinction is believed to be a result of several processes that include random stochastic events (e.g. disease, fire), limited diversity of defences, population-level changes resulting from isolation termed the “island syndrome”, or to the genetic costs of being small and isolated.
As a group, forest-dependent mammals are particularly sensitive to habitat loss and fragmentation, due to their specific habitat requirements. As a result the number of forest-dependent mammals declining on the mainland is increasing. Despite this, few studies to date have investigated the conservation potential of island populations for these vulnerable species. Consequently, much of the information gained from this research is novel for the squirrel glider, and indeed for forest-dependent mammals. By using a multidisciplinary approach, including genetics, physiology and ecology, information from this research may be employed by land managers to develop effective and scientifically informed management strategies to ensure the long-term persistence of declining mainland species that are concurrently persisting on islands.
The research was conducted using both island and mainland locations throughout southeast Queensland, Australia. Initially, targeted surveys were conducted on seven islands to confirm the presence/absence of the squirrel glider (Chapter 2). Following initial surveys, a mark and recapture program was established in four island and three mainland sites. Trapping surveys were conducted for four consecutive nights every six to eight weeks over a minimum duration of twelve months to account for seasonal variability. Habitat surveys were incorporated within each site to provide information on the availability of habitat resources and structure. Biological samples (i.e. fur and ear notches) were collected for genetic and physiological stress hormone analyses.
Density estimates were calculated for each island and mainland population, and compared using REML linear mixed models (Chapter 2). An ear notch was collected from each squirrel glider captured to enable calculation and comparison of genetic diversity, inbreeding, effective population sizes and genetic differentiation and structure in island and mainland populations (Chapter 3). Analysis of variance was used to identify any differences in habitat resources and structure between island and mainland habitats, and how this relates to squirrel glider density (Chapter 4). Data recorded throughout the mark and recapture study was used in order to investigate whether island populations displayed characteristics of the island syndrome, including; (1) higher densities; (2) increased survival likelihood; (3) larger body size; (4) reduced aggressiveness, and (5) decreased reproductive output (Chapter 5). A REML linear mixed model was used to determine whether there were any differences in physiological stress between island and mainland individuals (Chapter 6).
This study provided up-to-date records for the squirrel glider on islands in southeast Queensland, including the first record of this species on Woogoompah Island. While island habitats were found to support similar densities of squirrel gliders to those on the mainland, results indicate the importance of retaining large areas (> 700 ha) of bushland. Island populations were found to have significantly lower levels of microsatellite diversity in comparison to mainland populations. The inbreeding coefficient in island populations was equal to, or less than that recorded in other species on islands, and is currently not at levels associated with elevated extinction risks. There was no difference in effective population size estimates between island and mainland populations. There was a high level of genetic differentiation between all populations of the squirrel glider, particularly those on islands. Although island populations have retained lower levels of genetic diversity they still remain important for conservation because they are genetically distinct from core parts of the range, which can have implications for the evolutionary potential of the species as a whole. This study identified three key habitat components important for supporting high squirrel glider densities; including large trees, trees containing nest hollows and Banksia serrata. These key habitat features were found to be more abundant on islands in comparison to mainland sites. The reduction of these features in mainland sites is most likely the result of habitat loss and fragmentation, which is further exacerbated by habitat degradation. Island populations showed general trends of larger body sizes, higher body mass, sex ratios favouring females, lower social tolerance and longer longevity. There was a general trend for island squirrel gliders to have lower physiological stress levels in comparison to mainland populations and this is likely to be a consequence of reduced predation and interspecific competition. The different characteristics identified in island populations, further support the notion that island populations warrant separate conservation and management efforts to those optimally suited for the management of mainland populations.
This thesis makes an important contribution to the current understanding of the ecological, physiological and genetic responses of a forest-dependent mammal to island life. Overall there are a variety of costs and benefits to island life for the squirrel glider. Island populations of the squirrel glider do hold conservation potential, especially with the continuing depletion of mainland habitats, and should not be overlooked for the future persistence of the squirrel glider. To optimise the effectiveness of management strategies for the long-term persistence of the squirrel glider a balance needs to be found in order to secure both island and mainland populations for the future.