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Threatening processes

Failure to respond to food resource decline has catastrophic consequences for koalas in a high-density population in southern Australia

Whisson, DA, Dixon, V, Taylor, ML & Melzer, A 2016, PLoS One, vol. 11, no. 1, pp. 1-12.

Between September 2011 and November 2013, the growth and consequent collapse of a koala population resulting from a decline in key food trees was documented. During the study, population density grew from 10.1 to 18.4 koalas/ha, with a significant decline in manna gum (Eucalyptus viminalis) canopy condition taking place as a consequence of high browsing pressure. Individuals were shown to remain in small home ranges (0.4-1.2ha), regardless of the severity of food tree defoliation.

The study area consisted of a mosaic of manna gum woodland, pasture and shrubland at Cape Otway, Victoria. Six sites were surveyed for koala abundance and 20-40 manna gums at each location were examined for canopy condition. Thirty koalas were captured and fitted with radio-collars, to be tracked over three breeding and two non-breeding seasons. Findings demonstrate considerable population increase, whereby 80% and 76% of females gave birth to joeys in the 2011 and 2012 breeding season respectively. Consequently, by September 2013, 71% of manna gums exhibited high or complete defoliation. In 93.5% (72/77) of sampling periods, within-sample site fidelity was observed, with temporary movement up to 140m outside of existing ranges occurring in 12.5% of sampling periods (9/72). In males, range size was impacted by season, increasing during breeding periods. Female range size remained constant over the first two years, but increased by more than double in the 2013 breeding season. With extensive defoliation having occurred, 71% of koalas were euthanised or died during the 2013 breeding season, in most cases resulting from starvation.

Revealing a female-biased sex ratio, insignificant dispersal and high rates of fecundity and survival, the koala population at Cape Otway exhibited unsustainable growth. While high-density populations are often observed in areas of high quality habitat, decline in canopy condition is unsurprising given the significant growth and browsing pressure displayed. However, individuals rarely responded to this defoliation, despite the high connectivity of surrounding habitat and the close proximity of adjacent forest, containing favourable food trees. An inability to disperse over long distances was not observed in this study and as such, it is indicated that failure to respond to resource loss stemmed from the existence of small ranges, which previously allowed for significant reproductive success. Furthermore, as many females carried young at the time of foliar depletion, it is likely that the energetic requirements for dispersal were too demanding. As mates are considered a resource priority for males, instead of locating alternate food resources, these koalas are likely to have stayed with existing mates.

It is clearly apparent that, where koalas display high site fidelity, their ability to track and respond to habitat change is compromised, with significant consequences for individual fitness. Accordingly, this has severe implications for the continuance of populations in a developing landscape, where urban expansion and forestry are likely to increase habitat fragmentation.

 

Summarised by Julian Radford-Smith

 

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