Translocation of overabundant species: implications for translocated individuals
Whisson, DA, Holland, GJ & Carlyon, K 2012, The Journal of Wildlife Management, vol. 76, no. 8, pp. 1661-1669.
Based on the outcomes of translocation of several koalas from a high-density population in south-eastern Australia to multiple release sites, it is evident that this management technique can have negative impacts on the health and welfare of translocated individuals.
In the 1920s koalas were introduced to Kangaroo Island in an effort to boost their numbers. As with many animal introductions, however, the introduction had negative environmental effects including broad-scale habitat degradation due to rapid population increase on the island. To manage this overabundant koala population, more than 3000 koalas were captured, surgically sterilised and released onto the Australian mainland between 1997 and 2007. The authors of this study examined the fates of translocated koalas from 16 of a total of 35 release sites. Annual monitoring surveys of koala populations at the release sites revealed that population density dropped from an average of 1.0 koala/hectare at the time of release to less than 0.4 koalas/hectare on average. Radiotracking studies revealed that these low densities were due to high mortality and dispersal rates of translocated koalas. Within a year post-release, the mortality rate for translocated koalas was 37.5%. By comparison, no deaths of other koalas in the same populations were reported. Furthermore, unlike non-translocated koalas at the same sites, translocated koalas dispersed from release sites rapidly and over great distances, even though the habitat of the release site was of high quality.
Although the precise cause is unknown, the high mortality rates of translocated individuals may be associated with an unusually cold period post-release. The combined effect of the stress of being captured and released as well as reduced foraging ability during cold weather may have caused koalas to become dehydrated and therefore at greater risk of mortality. The dispersal of translocated koalas may relate to carrying capacity at the release site. In other species, however, translocated animals have demonstrated ‘wandering’ behaviour. Translocated koalas, too, may have ‘wandered’ as a result of being placed in an unfamiliar location and social situation.
While the translocation of koalas from overabundant populations has been justified by ameliorating animal health and welfare issues associated with resource strain and crowding at the source population, it is clear from the results of this study that the welfare of the translocated individuals should also be considered. Incorporating such perspectives into translocation planning and management may be difficult, however, as many of the factors that affect the survival of translocated koalas are difficult or impossible to foresee. The authors of this study suggest that translocated koalas should be routinely monitored post-release, and that monitoring should occur over a long period to account for a potential time lag between translocation and negative health and welfare outcomes.
Summarised by Joanna Horsfall
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