Sharpe, D 2009, ' Ecology of the squirrel glider (Petaurus norfolcenis) in subtropical Australia', PhD thesis, Southern Cross University, Lismore, NSW.
Copyright D Sharpe 2009
Human activities have resulted in increasing levels of habitat loss and fragmentation, threatening many species with extinction. Contemporary wildlife management seeks to provide an understanding of the ecology and behaviour of animal species that can assist their management and conservation. Describing the population ecology of wildlife species can assist conservation planning, for example, by providing input data for population viability assessments. Similarly, understanding animal behaviour provides insights into additional aspects of a species’ biology that ultimately affect population processes. The squirrel glider (Petaurus norfolcensis), which is the subject of this thesis, provides a good example of a species where conservation planning is required. It has an extensive distribution in eastern Australia, covering a geographic distance of approximately 3000 km. However, its habitat continues to be threatened by habitat loss and fragmentation throughout its range. The squirrel glider is listed as threatened in the southern half of its range, while its status needs to be reviewed in the northern part. Prior to the late 1990s, when research for this thesis commenced, few detailed studies had been conducted on the squirrel glider. This hindered the ability of land managers to provide better management of the squirrel glider or its habitat to prevent a further decline in its conservation status. This thesis aimed to target gaps in knowledge of the squirrel glider and to provide information that will assist its conservation and management.
The influence of flowering patterns on the population ecology of the squirrel glider was described at a site in north-east NSW over a 5-year period (1997 – 2001). The population on a 38 ha trapping grid was relatively stable over the first four years of study and contained at least 12 adult gliders. Following reproduction, their associated offspring created population peaks that rarely exceeded 20 individuals. During the winter of 2000, there were more than 30 gliders present, including juveniles, which was a 5-year high. Eucalyptus robusta and Banksia integrifolia flowered heavily during that winter and were important food resources. Eucalyptus robusta had not flowered in the previous four years, while B. integrifolia was a reliable winter nectar resource. Despite the availability of nectar, reproductive success appeared to be low due to the loss of pouch young. Gliders rapidly lost weight between July and September 2000, which coincided with extremely dry conditions and a lack of flowering in Eucalyptus siderophloia, an important nectar source. The number of gliders on the grid fell by almost 80% between September and November 2000. However, the total population declined by 55% and the adult population by 42% when compared to their numbers averaged across the previous four years. Between September 2000 and March 2001, only seven squirrel gliders were known to be resident. Glider numbers remained low during 2001, indicating that recovery was slow. The observed decline appeared to be widespread throughout the region. Therefore, there was little opportunity for migration to assist population recovery. These observations suggest the squirrel glider may be sensitive to flower failure in key winter/spring flowering species.
The population ecology of the squirrel glider was also examined in a forest remnant in Brisbane, where nectar was a dominant food item. A total of 201 gliders (adults and juveniles) were captured 705 times in 3,729 trap-nights (19% trap success) over a 4-year period (2002 – 2006). The population peaked in the first year at a density of ~1.6 individuals ha-1, but then declined to ~0.5 individuals ha-1 by the final year. Such a fluctuation has been observed in another population of this species where nectar was dominant in the diet. In both cases the fluctuation appeared to be mediated by variation in flowering intensity. Births occurred from March to November, peaking between April and July. All females >1 year bred in each year of the study, with a mean litter size of 1.7 (n=122). The overall natality rate was 1.9, indicating that females occasionally bred twice per year. The sex ratio was at parity in the pouch and in the trappable population. Gliders first entered the trappable population at four months of age, and persisted for a mean of 32 months. The maximum longevity was seven years and eight months. The demographic characteristics of this squirrel glider population within a forest remnant surrounded by urban development were similar to that reported elsewhere.
The management of threatened species requires detailed knowledge of key population parameters to be effective. The trapping of adult squirrel gliders in Brisbane was the subject of a mark-recapture analysis. A total of 187 adult gliders (96 females, 91 males) were captured 620 times, in 19 tree-trapping sessions. A Cormack-Jolly-Seber model was employed in program MARK to quantitatively estimate the population parameters of adult survival and abundance. A variety of factors (e.g. gender, year, season) that may affect survival were examined. Survival is simply defined as the disappearance of gliders from the trappable population. Density was highest early in the study and gradually declined during the following three years. The overall annual survival probability was 0.49 ± 0.08 (females 0.51 ± 0.12, males 0.48 ± 0.12). The estimated adult population density ranged between 0.6-1.9 ha-1. This value was overestimated to some degree by the dispersal of gliders early in their second year despite having attained breeding age. However, because the study was conducted in an urban remnant mostly surrounded by relatively hostile matrix habitat, the effect of dispersal may have been relatively minor. Despite this constraint, the estimated probability of adult survival in this study was considerably lower than the main value (0.65) used in a population viability analysis previously conducted in the locality of our study site. This suggests that population viability had been substantially over-estimated and now requires re-evaluation. Further studies that assess the survival of squirrel gliders are needed to assess the extent to which this parameter varies among localities.
The vocal behaviour of the squirrel glider was described from 465 h of observation across five sites in north-east New South Wales and south-east Queensland. A monosyllabic or polysyllabic nasal grunt was the most frequent call (56% of 208 calls). It ranged from single calls to sequences up to 20 min duration (mean 2.1 min ± 0.6 s.e.) and was heard on 34% of nights (n=83) at two sites. The rate of the nasal grunt showed a positive relationship with population density at one site. The nasal grunt was typically made when conspecifics were near the caller, but responses were infrequent (7% of observations). Call playback produced no discernable change in call response. The nasal grunt appears to regulate individual spacing by facilitating mutual avoidance, a function hypothesised to be an evolutionary precursor to the use of calls in territorial defence. Threatening calls were the next most common vocalisation (17% of calls) and were accompanied by scuffles and/or chases. They were also used when gliders were preyed upon and during animal handling. The calling behaviour of the squirrel glider confirms the importance of vocal communication among petaurid gliders.
Exudivorous mammals exploit food items of high quality and rates of renewal, offset by wide dispersion, low abundance and temporally variable availability. How this influences foraging effort and size-related foraging efficiency remains poorly described. The time budget of the squirrel glider was examined in an urban forest remnant in Brisbane. During each of three seasons that were stratified by moon phase, six males and six females were observed during a series of 1 h focal observations that extended from dusk until dawn. Feeding dominated the time budget, accounting for 78% of observation time, or 85% of time when combined with behaviours associated with foraging. Females appeared to maximise feeding rates prior to entering the energetically demanding phase of late lactation. Little time was spent resting and all other behaviours combined were a relatively small component of the time budget. Longer nights and the full moon were associated with later emergence and earlier retirement times. Animals re-entered their dens in tree hollows during the night, representing 2% of activity in spring, 18% in winter and 9% in autumn. The reasons for this are unclear but may be related to predation risk and reproductive activity. The high proportion of the time budget devoted to foraging suggests that the squirrel glider is likely to be adversely affected by all forms of habitat disturbance.
Both studies of the population ecology of the squirrel glider found that the populations varied substantially in size over time. While the squirrel glider is able to maintain population processes in fragmented habitats, its survival rate was found to be lower than assumed during an earlier population viability analysis (PVA) study. Both these factors (population variability, survival) will impact on population viability and suggest that viable populations will need to be larger than previously recognised. Further studies of population viability that incorporate this new information are now needed to guide the conservation and management of the squirrel glider throughout its range.