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Threatening processes

Individuals matter: predicting koala road crossing behaviour in south-east Queensland

Dexter, CE, Appleby, RG, Scott, J, Edgar, JP & Jones, DN 2018, Australian Mammalogy, vol. 40, no. 1, pp. 67-75.

An investigation into the factors influencing the movements of koalas has revealed that, within a population, not all koalas are equally likely to cross a road. Road mitigation strategies that target those individuals most at risk of vehicle strike are likely to deliver greater benefits for koalas than strategies that do not account for these inter-individual differences.

  The authors of this study aimed to determine whether particular characteristics, such as age, sex and distance from the road, influenced a koala’s likelihood of crossing a road. Of 51 koalas studied across six subpopulations in south-east Queensland, only 18 crossed a road or wildlife crossing structure at least once within a median observation period of 232 days. Male koalas were approximately three times more likely to cross roads than female koalas, and females without joeys were far more likely to cross roads than females with joeys. Koalas less than five years of age were four times more likely to cross roads than those aged five years or more. Interestingly, koalas that crossed roads often did so frequently, with 143 crossings completed by only 18 koalas. Koalas were significantly more likely to cross a road if they were situated within 100 metres of the road, whereas those found at least 500 metres away only rarely interacted with roads.

  Vehicle-related mortality has been implicated as a cause of koala population decline in urbanised areas, with an average of 395 koalas being struck by cars each year in south-east Queensland between 1997 and 2011. The majority of previous research into wildlife-vehicle collision mitigation strategies has focused on population-level data; however, the present authors suggest that studying within-population variation in road crossing behaviours will lead to more efficient interventions. The increased likelihood of males and young koalas crossing roads can be expected due to the greater net movement of these animals. Males are known to travel widely during the breeding season while young koalas may traverse great distances in the search for unoccupied territory. Although it may seem obvious that a koala within 100 metres of a road should be more likely to cross that road than a koala at a further distance, this finding is notable regarding management implications.

  Given the immense cost of installing wildlife crossing structures, it is important that efforts are targeted where they are likely to deliver the greatest benefits. One way to improve the efficiency of mitigation measures may be to develop strategies that target those subsets or individual members of a population most at risk of vehicle strike, such as in this study. In particular, strategies that target koalas situated within 100 metres of high-risk roads, and especially young males, should be prioritised.


Summarised by Joanna Horsfall


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