Effects of hunting and rainfall on Bennett's wallaby and Tasmanian pademelon populations

This study investigated the effects of hunting and rainfall on populations of Bennett's wallabies, Macropus rufogriseus, and Tasmanian pademelons, Thylogale billardierii, and represents the first major field based study on the biology of these species in their native environment. Three aspects of their biology were considered: body condition, breeding and population structure. Neither body condition nor population structure have been previously studied in either species. This study was based on animals shot by commercial and non-commercial hunters. Study areas were located throughout Tasmania and represented a wide range of hunting and rainfall levels. The start of the study coincided with the end of one of Tasmania's longest droughts. This drought provided an opportunity to investigate the effects of low rainfall on macropod populations in Tasmania. Hunting had a direct effect on the age structure of the two wallaby species. High levels of hunting reduced the average age in populations of both wallaby species as a result of selective shooting of larger and hence, older individuals. In Bennett's wallabies, the effect of selective shooting of large animals was greater on males than on females and this caused a reduction in the proportion of males in the population. By comparison, there was no evidence that hunting caused a difference in the sex ratios of Tasmanian pademelons. This suggests that discrimination between male and female pademelons by hunters was minimal. It may also reflect the fact that Tasmanian pademelons are hunted less intensively than Bennett's wallabies. The loss of reproductively mature animals as a result of hunting was largely compensated for by increases in breeding performance. In areas subjected to high hunting pressure, wallabies of both species reproduced at a younger age than wallabies in areas subjected to low hunting pressure. Moreover, wallabies in areas subjected to high hunting pressure showed little decline in breeding success during the drought. The higher breeding perlormance in areas subjected to high hunting pressure was attributed to a decrease in density and subsequently an increase in the quantity of resources available for the surviving individuals. The results of this study indicate that hunting had a greater impact on Bennett's wallabies than on Tasmanian pademelons. At present, the two species of wallaby are managed by the Department of Parks, Wildlife and Heritage as one species in relation to hunting. This greater vulnerability of Bennett's wallabies to hunting should be taken into consideration in its management, especially given that the reproductive rate of Bennett's wallabies is lower than that of Tasmanian pademelons. The 1987/88 drought was shown to reduce the body condition (as measured by kidney fat index) of both wallaby species which, in tum, reduced breeding performance. The drought had less effect on Tasmanian pademelons than on Bennett's wallabies and this was attributed to differences in diet and breeding patterns. Survival of dependent young was lower during the year of the drought than during the year of normal rainfall. Fewer male Bennett's wallaby pouch young survived than females during the year of the drought. The drought also delayed the onset of maturity with males being more affected than females. For Tasmanian pademelons, this resulted in first-time breeders giving birth late in the breeding season. The effects of drought on the biology of the two wallaby species were, in general, similar to those reported for other macropods on mainland Australia. Although the 1987/88 drought was severe by Tasmania's standards, it did not produce the same level of response seen in macropods experiencing severe drought in arid and semi-arid areas of Australia. This reflects the fact that the rainfall of Tasmania is characterised by low variability. Nevertheless, the drought did significantly reduce the breeding performance of both species and this should be taken into account in their management.