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Abstract
This thesis investigates the microclimate of Australian alpine boulder fields inhabited by the endangered mountain pygmy-possum Burramys parvus. This endemic specialist is highly sensitive to extreme temperatures and therefore threatened by global warming. In the Australian alpine region, B. parvus survives by living deep within boulder fields. Extreme high and low temperatures within boulder fields are known to be critical factors that determine its survival. Whether alpine boulder fields can offer a stable, long-term microhabitat that effectively buffers extreme temperatures is an open question.
This study made detailed investigations of the thermal properties of boulder fields to indentify where refuges for B. parvus may occur in the future. By deploying 273 temperature data loggers in 38 boulder fields for more than 12 months and logging hourly temperatures, it was demonstrated that some boulder fields are better than others at buffering extreme temperatures. Specifically, temperatures become more stable at increasing depths within boulder fields, so the number of available rock layers is important. Vegetation cover, rock cavity size and topographic position of the boulder fields are also significant factors in buffering extreme temperatures. Light intensity meters deployed concurrently with the temperature sensors demonstrated that boulder fields at high elevation, on gentle slopes with south facing aspect accumulated snow for the longest duration. It was also found that the snow depth threshold for achieving effective insulation was 70 cm, therefore hibernating B. parvus are likely to be influenced by extremely cold temperatures when a snow depth <70 cm exists. Moreover, the most dangerous temperatures for B. parvus do not occur during mid-winter when persistent snow covers the boulder fields, but happen before and after the persistent snow cover is established.
Significant relationships existed between environmental variables and temperatures in the boulder fields. Statistical modelling showed that elevation was a powerful predictor of several crucial temperatures for B. parvus, such as night-time and minimum temperatures during all seasons at all three depths measured at the point level. Vegetation cover, insolar, rock layer and rock cavity size were also important in predicting extreme temperatures at the point level. Topographic environmental variables such as slope and elevation influenced minimum temperatures at the cluster level. Importantly, this study reported diminished maximum and minimum temperatures in boulder fields covered with vegetation and possessing small cavities. Deep boulder fields were found to provide a more reliable microclimate for B. parvus than shallow ones.
A major outcome of this study was the definition and calculation of both the internal and absolute thermal buffering capacities of alpine boulder fields. The internal thermal buffering capacity of alpine boulder field habitat was defined as the differences in temperature ranges between different depths, which express the ability of a boulder field to buffer against outside air temperature changes. The results revealed why certain boulder fields provide potential refuge from extreme temperatures and others do not. Burramys parvus naturally selects boulder fields with the most favourable available microclimate. Deep boulder fields provide the best microclimate in all seasons and, therefore, were identified as the prime potential thermal refuge for B. parvus. Shallow boulder fields at high elevation or deep boulder fields at low elevation might provide thermal refuge for B. parvus if covered by reasonable vegetation and possessing good rock structure. During winter, snow cover moderated the microclimate, so boulder fields with better conditions for snow preservation such as high elevation, on gentle slopes with a south facing aspect were identified as important thermal refuges for B. parvus.
Understanding how the buffering capacity of alpine boulder fields varies at a landscape scale and identifying potential refuge conditions crucial to the survival of B. parvus will provide important information for improving assessment of the vulnerability of the species, identifying refuge habitat, identifying suitable translocation sites and constructing artificial habitat in future conservation planning. This is globally relevant for oligostenothermic alpine species in the face of habitat loss due to climate change.