Decline of the urban koala (Phascolarctos cinereus) population in Warringah Shire, Sydney
Smith, P & Smith J 1990, Australian Zoologist, vol. 26, nos. 3-4, pp. 109-129.
Koala populations within Warringah Shire, Sydney have rapidly declined over the last 50 years. Initial declines can be attributed to habitat loss, as primary koala habitat was converted to urban area from the 1970s onwards. Surviving populations are now restricted to two main areas: Barrenjoey Peninsula and Ku-ring-gai Chase National Park, with only rare sightings in other areas of the Shire.
Barrenjoey Peninsula, which once held the largest koala colony in Sydney, experienced a drop in forested area from 47% in 1946 to 8% currently. This remaining forested area has very few of the favoured food sources for koalas; namely, Grey gum (Eucalyptus punctata), Scribbly Gum (E. haemastoma) and Swamp Mahogany (E. robusta), and many of its eucalypt species are experiencing dieback and heavy competition with rainforest species as a consequence of urban runoff. Koalas were found to occur in plant communities that contained at least one of their three most-preferred food sources and disappeared from areas where these trees became absent. Other than habitat loss, dog attacks have accounted for 24-40% of koala deaths in the last 50 years; higher than in other studies which attributed only 6-7% of mortality to dog attacks. Vehicle collisions accounted for 8-24% of deaths, which was comparatively lower than in other studies where it accounted for up to 60% of deaths. Incidences of chlamydiosis appeared to be low in the area and did not contribute significantly to mortality rates.
This study showed that koalas in Warringah Shire are threatened with local extinction from further habitat loss, dog attacks, vehicle strikes and possible chlamydiosis outbreak. Recovery of the population to previous numbers is made difficult by the lack of urban planning and reserve management from previous decades which has resulted in koalas existing in suboptimal habitat. These findings highlight the need for future urban management plans to preserve areas which contain the most favoured food trees for koalas, and minimise their chance of encountering dogs and vehicles.
Summarised by Miranda Rew-Duffy
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