Food resources and the decline of woylies Bettongia penicillata ogilbyi in southwestern Australia

Since 2001, woylie Bettongia penicillata ogilbyi populations in southwestern Australia have declined by at least 95%. The scale of this decline is larger and more rapid than that observed for the Tasmanian devil Sarcophilus harrisii, African elephant Loxodonta africana and lion Panthera leo. This rapid decrease in population size has prompted investigations to identify the putative causes of the decline. It is well established that changes in diet, food availability or foraging patterns can influence population trends, body condition and breeding and reproductive patterns, which in turn can lead to population declines. On the basis of this observation, the present study was designed to evaluate the dietary ecology of the woylie. Knowledge of the dietary ecology of the woylie is also important for conservation, recovery and ongoing management of existing populations. This study aimed to 1) quantify the diet of the woylie, by examining temporal and spatial variation of dietary components (primarily from faecal material), 2) investigate fungal food availability, the primary dietary component of the woylie and, 3) examine diet as a putative cause of recent population decline. Faecal material collected between July 2006 and May 2007, from several populations within southwestern Australia was used to quantify the woylie’s diet. The results from the dietary analysis confirmed that woylies are predominantly mycophagous, with fungi (particularly sequestrate fungi) the major dietary component throughout the year at all study locations. Plant, invertebrate and seed material were also found but formed only minor components of the diet. Spatial variation in woylie diet was examined on two scales, regional level variation (southwestern Australia) and subregional variation (within the Upper Warren region). Very little spatial variation in the composition of dietary items was recorded between the subregional or regional study locations with the exception of Karakamia Wildlife Sanctuary, where significantly less fungi and more plant material were recorded. Seasonal variation was observed in the diet, whereby more fungi were consumed in winter and less fungi and more plant, fruits and invertebrates were consumed in the drier months of summer and spring. In addition, greater species richness and diversity of fungi were recorded in the diet during winter and spring compared to summer and autumn. Sequestrate fungal sporocarp surveys were conducted in addition to the dietary analysis to investigate seasonal and spatial availability of fungi for woylies. Sequestrate sporocarp diversity and availability was shown to vary seasonally, but not spatially within the Upper Warren sites. During the winter months species richness and species diversity were high, sporocarps were more abundant and dry weight was higher. During spring, autumn and summer, sporocarps were less abundant and species richness and diversity were lower. Overall, when sporocarp availability was high (usually in winter) so too was the contribution of fungi in woylie faecal material and when sporocarp abundance was lower (spring and summer), woylies demonstrated a dietary shift and more plant, seed and invertebrate material were consumed. These results indicate a strong relationship between fungi availability and diet composition for the woylie. When dietary trends were compared to demographic parameters which are often associated with population declines (e.g. body condition and reproductive success), it was found that the contribution of fungi in the diet was not related to woylie body condition, however it was a predictor for woylie reproductive success. Yet, reproductive success is unlikely to be associated with the recent decline of woylie populations as the overall proportion of females encountered with pouch young during the project was consistently high in both declining and declined populations and the proportions are consistent with results from earlier studies. The results from the dietary data comparing pre-decline diet (1999) with post decline diet (2006/7) did not reveal any significant association between decline and the amount of fungi within woylie diets. Furthermore, the decline status of the site showed no relationship to the amount of fungi in the diet. These results indicate that the dietary ecology of the woylie is not the primary causative agent of and is unlikely to have a significant role in the recent decline of woylie populations. This study represented the first detailed investigation of woylie diet and, in conjunction with food availability data, provided important information on the dietary ecology of the woylie. While the results from this study indicate that diet and/or food resources are unlikely to be the cause of the recent decline in woylie populations, they form an important foundation to actively manage existing woylie populations, and also in the planning and establishment of populations in reintroduction programs