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The ecology and management of the agile wallaby, Macropus agilis

Stirrat, Simon C. (2000). The ecology and management of the agile wallaby, Macropus agilis. PhD Thesis, Northern Territory University.

Document type: Thesis
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Author Stirrat, Simon C.
Title The ecology and management of the agile wallaby, Macropus agilis
Institution Northern Territory University
Publication Date 2000
Thesis Type PhD
Subjects 0608 - Zoology
0602 - Ecology
Abstract This thesis describes an investigation of the seasonal influences on several aspects of the behaviour and ecology of the agile wallaby, Macropus agilis, the most common macropod in the wet-dry tropics of Australia. Data were collected between September 1991 and November 1995. The agile wallaby was found to be a generalist herbivore that consumed a wide range of food resources. In the wet season, when food was abundant, the wallabies grazed in cleared areas, preferring grasses and legumes, even though the most abundant foods available were a few species of non-leguminous forbs. In the dry season the quality of preferred foods declined and the wallabies broadened their diet to include a range of alternative food resources, including browse, roots and floral parts of numerous plant species. The shift in diet in the dry season, interpreted as a strategy to compensate for decline in nutrients available in preferred foods, did not prevent the loss of body mass. Analysis of blood chemistry showed that in the dry season, the wallabies exhibited depressed levels of serum protein, albumen and blood urea nitrogen, all indices of recent nitrogen intake. However, the changes recorded in the indices of energy intake did not clearly indicate that energy availability was lower in the dry season. The wallabies may not have been energy limited at the time of dry season blood sampling because they consumed a variety of alternate food resources. A life table analysis showed that pouch young mortality was high but pouch occupancy was high year round, indicating that females reproduced continuously and replaced lost pouch young. Post-juvenile mortality rates in males increased with age while those of females remained relatively constant. The adult sex ratio was female biased, which is consistent with higher male mortality. While predation pressure was high, it is likely that mortality of pouch young regulated the population. The most plausible explanation for high pouch young mortality was the seasonal loss of body condition in females due to the decline in food availability in the dry season. The nutritional environment in the dry season would also have been particularly harsh for small, newly independent wallabies. Declining food availability in the dry season impacted on foraging and ranging behaviour as wallabies sought to maintain adequate nutrient intake. The wallabies were active nocturnally and foraged for a similar amount of time at night in both the wet and dry seasons. However, they foraged longer in the evening and particularly during the day in the dry season. In addition to foraging longer, wallabies moved more while foraging and travelled longer distances while foraging in the dry season. Seasonal variation in foraging behaviour impacted on time available for other activities. Most importantly, the wallabies were less vigilant in the dry season, most likely because of time constraints rather than any real differences in predation threat between seasons. Mean home range size (95% isopleth) increased from 13.9 ha in the wet season to 19.9 ha in the dry season. This change was a reflection of depleting resources in the central area of the study site and the need for wallabies to range further to find food of adequate quality. Core range size (55% isopleth) did not differ between seasons. Habitat use also differed between seasons, the wallabies foraging in forest areas more in the dry season. The density of wallabies in the study site was very high and consequently grazing pressure was high throughout the year. In cleared areas, long-term grazing exclosures showed that total plant species richness was not affected by grazing but both grass abundance and grass species richness were depleted. The wallabies also had an indirect effect on forest habitats in the study site, where they rested in both seasons but also foraged in the dry season. Sapling abundance was higher in fenced areas. It seems likely that the disturbance and consumption of leaf litter by the wallabies may have affected soil moisture, among other things, which in turn influenced seedling survival. The high density of wallabies caused numerous problems for the managers of the study site. Overgrazing caused soil erosion and impacted on the amenity of recreation areas. Control of the wallaby population was required. Culling was not acceptable to the public and would have required regular and frequent wallaby removal to maintain the population at a low level. The only feasible option for management of the population was habitat modification to control availability of resources. Access to water was greatly restricted in 1995 and a significant decline in wallaby numbers ensued. The extent of the reduction in population size may have been a result of the interaction between water restriction and an inevitable, natural decline after more than 5 years at very high density.

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