Interpreting the decline of a koala population at Marys Mount, Liverpool Plains, north-west New South Wales (2013–2016)

Context The Liverpool Plains of inland NSW carry a priority population of koalas in a State in which the species is listed as Endangered under both New South Wales and Commonwealth legislation. Aims Our primary questions were: had the koala population declined between 2013 and 2016? If so, what were the likely causes of the decline? Do our findings reflect koala population trends in the region? Methods Our study, conducted in a remnant patch of vegetation at Marys Mount Blue Metal Quarry, used direct counts rather than relying on indirect evidence. A 2013 count was part of an environmental assessment and we repeated the count in 2016. Key results The count in 2013 observed 18 koalas in a single night. In 2016, no koalas were sighted within the study area, however, three were observed on adjacent properties. Conclusions The most likely explanation for the decline is a combination of threats acting simultaneously. As our study was not designed to distinguish among these, we cannot determine their relative contributions. However, threats including habitat loss due to mining, over-browsing, disease and extreme weather likely had concurrent impacts on koalas. The potential impact of extreme heat and lack of rain is consistent with similar declines observed in the region. Implications Our study considered several possible causes for the decline. Future research is needed to determine the specific impacts of each threat on koala populations in this region.