Of droughts and flooding rains: koala population occupancy in a north-east New South Wales conservation reserve, and implications for range-wide surveys

Population monitoring is fundamental to the conservation of threatened species such as the koala (Phascolarctos cinereus). This study extends by 2 years, a previous 8-year annual monitoring project of a koala population in north-east New South Wales. This study relied on audio-recorders and occupancy modelling to investigate whether the population has remained unaffected by the driest period on record followed by the wettest period on record. Audio-surveys were conducted at 42 sites over a 2-week period in each of 2022 and 2023. The probability of detection varied among types of audio-recorders, but averaged 0.45–0.88 per night, which equates to requiring 2–5 survey nights to have 95% confidence that koalas were detected. The probability of occupancy increased from 0.80 (95% CI, 0.66–0.94) in 2022 to 0.95 (95% CI, 0.89–1.0) in 2023. Local abundance of calling males detected by the more sensitive recorders increased from 1.6 ± 0.2 in 2022 to 2.2 ± 0.1 in 2023. The probability of occupancy when viewed across 10 years showed little change despite dramatic fluctuations in rainfall. This may reflect high local habitat suitability as well as occupancy being insensitive to variation in abundance because koalas are long-lived. Changes over time in the probability of detection may be more reflective of changes in abundance. This study highlights that the paucity of multi-year population studies on the koala means that our current understanding of koala population dynamics is inadequate for effective conservation.