North Stradbroke Island: An island ark for Queensland’s koala population?

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South East Queensland (SEQ) is experiencing the fastest human population growth in Australia, with attendant challenges for wildlife conservation due to expanding urbanisation. The documented dramatic decline of koalas Phascolarctos cinereus on mainland SEQ has provoked popular suggestions that North Stradbroke Island (NSI) should become an “island ark” for koalas.

A multidisciplinary study was undertaken to determine the status of koalas on NSI. Aboriginal and European references to koalas on NSI were collected and analysed. To study koala distribution, direct and indirect visual surveys were conducted, whilst habitat use and home ranges were determined by fitting 33 koalas with VHF collars and radio tracking them. Population characteristics, including health status, proximate causes of mortality and genetic profile, were gathered from radio-tracked koalas and from hospital databases of the Department of Environment and Resource Management.

Historical and Aboriginal records of koalas on NSI are scarce, but in concert with molecular genetic analyses, they indicate that on the balance of probabilities a koala population appears to have occupied NSI from before European occupation. There is a consensus based on radio-tracking and visual surveys that koalas predominantly occupy the northern two-thirds of the western side of the island, including some rehabilitated mining areas and the three urban areas of NSI. The ecological characteristics of NSI koalas, including body size, breeding season, reproductive output, home ranges and movements, are consistent with those of other Queensland koalas. Compared to mainland SEQ koalas, diseases were found to play less of a role in NSI koala mortality, diet appeared to be more reliant on Eucalyptus robusta than typical on adjacent mainland areas and NSI koalas exhibited the lowest genetic diversity. Koalas on NSI face signifcant anthropogenic threats now, whilst future threats may include habitat destruction and climate change.

It is concluded that due to several characteristics including their low genetic diversity, NSI koalas are unique and the location and population should not be considered an island ark for the rest of SEQ, but be conserved and managed as a separate entity.

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