Dispersal patterns in a regional koala population in south-east Queensland
Dique, DS, Thompson, J, Preece, HJ, de Villiers, DL & Carrick, FN 2003, Wildlife Research, vol. 30, pp. 281-290.
Between 1996 and 2000, koala dispersal was monitored in a population within the Koala Coast. A total of 195 koalas were radio-tracked, of which 40 dispersed, five made exploratory movements and 150 remained within the same natal home range. Significantly more males than females dispersed from their natal range.
At three sites in the Koala Coast, south-east Queensland, koalas were captured and fitted with radio-collars, with individuals tracked at least once per month over the course of the study. If, in a short period (weeks to a few months), an individual moved from its natal home range to a breeding home range and did not return, it was deemed to have dispersed. All males (23) and 14 females dispersed prior to three years of age, with three females dispersing as adults. The vast majority of dispersal (93%) took place between 20 and 36 months of age and was limited to the months between July and December. Ten individuals died during dispersal, eight of which were killed by either vehicle collisions or dog attacks. Male koalas aged 12-24 months were less likely to survive than those aged 24-36 months, while female koalas aged 12-24 months were most likely to survive. Individuals were more successful in their dispersal if they travelled south or west of the study sites, away from urban areas.
There exist a number of mechanisms that drive the movement of young koalas away from their natal area. This study presents clear male-biased dispersal, a phenomenon that can be explained through consideration of social behaviour relating to mating systems. As koalas possess a polygynous mating system, intra-sexual competition is extreme amongst young males and, thus, dispersal takes place as a means of avoiding dominant males and enhancing breeding opportunities. This assertion is further supported by findings that show a peak in male dispersal just before the breeding season. A large proportion of females also disperse; however, this does not appear to be wholly driven by mating systems. As females are required to invest a significant amount of energy in their offspring, competition for resources amongst young females is a further mechanism for dispersal. In contrast to urbanised parts of the region where high mortality was observed, areas of remnant eucalypt vegetation in the south and west successfully facilitated dispersal. These areas exhibit lower koala densities and, thus, resource competition and mating pressures are minimised with mate accessibility maximised.
From a regional perspective, the dispersal of koalas from high-density, natal ‘source’ areas to urban areas is restricted by high mortality associated with dogs and roads, with dispersal to bushland areas encouraged by large, connected patches of remnant vegetation. Accordingly, urban population recruitment is limited and Koala Coast management programmes should focus on expanding habitat and reducing mortality in urban areas, while securing existing bushland areas.
Summarised by Julian Radford-Smith
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