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Population dynamics

A dangerous idea: that Koala densities can be low without the populations being in danger

Close, R, Ward, S, & Phalen, D 2017, Australian Zoologist, vol. 38, no. 3, pp. 272-280.

In New South Wales, many local koala populations are considered threatened under the state’s legislation, either due to their declining population statuses or their low densities.  Through assessing historical evidence from pre-European settlement and present data from previous tracking and translocation studies, it was observed that koala populations in the Sydney basin area existed in low densities prior to European settlement, and that koalas possess great dispersal abilities that allow them to persist in healthy, small, widely-distributed populations, allowing them to remain functional and sustainable.

  A low-density population does not necessarily suggest that a population is in danger, as indicated by the good health condition and increasing population status of koalas in a low-density population near Campbelltown.  Most tagged individuals had dispersed long distances, some as far as 20 km.  Similarly, a 20-year radio-tracking study of the same population showed that the females within that population not only lived long lives, but also had high reproductive success with a majority of them weaning one young each year.  The offspring of these radio-tracked females were then found either in areas adjacent to their mothers, or several kilometres away establishing their own new home-ranges.  The authors of the study further investigated the viability of a very low-density population through translocation of koalas from Campbelltown to Tarlo River National Park where koalas had not been sighted since the 1970s.  Translocated females established home-ranges of 50-100 ha soon after translocation while the males travelled large distances independently and were occasionally found near the females.  A translocated female was also found breeding with a local male and produced offspring.

  These findings all led  the authors to the conclusion that koalas have the ability to maintain viable and functional populations even in low density.  One of the reasons for this is their large home-ranges and ability to disperse great distances, which allows them to occupy larger areas with more resources to meet their demands, as well as more opportunities to encounter mates.  Furthermore, lower density populations may benefit from the lower risk of highly transmissible pathogens, such as Chlamydia, and over-browsing of food trees, which are often associated with high-density populations.  This is supported by the observation that none of the animals in the study were observed to have chlamydial disease, nor were there signs of over-browsing.

  The findings of this review indicate that, if provided sufficient connected bushland remnants, koalas from small populations can be sustained in the absence of heavy pressures.  The authors pointed out that this conclusion may potentially be disadvantageous to koala conservation, as the perception of their decreased vulnerability may lead to down-grading of the species’ status and political power.

 

Summarised by Cherie Chan

 

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