Fire severity and its local extent are key to assessing impacts of Australian mega-fires on koala (Phascolarctos cinereus) density

Aim Mega-fires are predicted to increase with climate change. Australia experienced the largest ever documented forest fires in 2019-2020, but the response of most taxa remains poorly known. We used acoustic arrays to estimate impact of the mega-fires on the density of an iconic marsupial. Location North-east New South Wales, Australia. Time period Pre-fire density estimated in 2018-2019 and post-fire density in 2019-2020. Major taxa studied Koala, Phascolarctos cinereus. Methods We estimated male density before and after fires using large acoustic arrays and spatial count models. Acoustic arrays sampled three timber production forests with a gradient in fire severity and three unburnt controls in national parks. Results Koalas were temporarily extirpated where high fire severity dominated the landscape, but some localized recovery was evident after 1 year. Where moderate severity fire dominated, density was reduced by about 50% within 1 year, but koalas were widespread throughout the burnt area. In our third area dominated by low severity fire, no impact was detected as pre- and post-fire uncertainty intervals overlapped. Control sites surveyed at similar times showed little change in density between years. There was no relationship between pyrodiversity and koala density. Within arrays broadly dominated by moderate or high severity fire, density 1 year after fire was lower in burnt patches of both low and high severity fire. Regionally, 9.8% of landscape samples (2 km x 2 km) in koala habitat were dominated by high fire severity, while a further 6.1% were dominated by moderate fire severity, suggesting c. 13% decline in koala density. Main conclusions A substantial impact of high severity fire was confirmed. Severe impacts were localized across the landscape, recovery had begun within a year and resilience was evident where low severity fire dominated. However, more frequent fires in the future will compound koala losses.