Occupancy of urban roosts by spectacled flying‐foxes (Pteropus conspicillatus) is not affected by diurnal microclimate

Abstract

AbstractOne of the most significant changes to Earth's climate in recent decades has been an increase in the frequency, intensity and duration of heatwaves. During heatwaves, animal's thermal window can be exceeded, and in extreme cases, mass mortality events have been observed. In 2018, a heatwave in north‐eastern Australia resulted in the death of approximately one‐third of the spectacled flying‐fox (Pteropus conspicillatus) population at urban roosts in Cairns. The species has now been listed as endangered with future heatwaves considered the greatest threat to its survival. In this study, we investigated long‐term climatic trends for Cairns, paying particular attention to the frequency of extreme heat events from 1943 to 2022. We then characterized the microclimate of urban flying‐fox roosts during the Austral summers of 2021/2022 and 2022/2023 across Cairns to assess the long‐term feasibility of urban spectacled flying‐fox roosts. From the long‐term climate records, we observed an overall increase in Cairns' average annual temperature of 1.3°C from 1943 to 2022 and an increase in the number of excessively hot days per decade, from 16 in the first decade (1943–1952) to 67 in the last (2013–2022). We regularly detected maximum roost temperatures of 30–35°C during our study, with excessively hot days (>35°C) recorded more frequently than expected compared to Cairns's maximum temperatures from the last decade (2013–2023). We detected only 1 day where roost temperatures exceeded 40°C and no period that replicated the 2018 heatwave conditions. Furthermore, we found a significant negative relationship between roost ambient temperature and humidity, where the hottest days also coincided with those with the lowest humidity. Importantly, we found no difference in microclimate between roosts that were occupied and unoccupied by flying‐foxes during our study, suggesting that other environmental or behavioural factors are more influential for roost selection than the roosting microclimate. Ensuring the long‐term conservation of spectacled flying‐foxes under a changing climate will require the management of urban roosts to increase their thermal resistance to heatwaves, and more research is needed to identify the variables modulating this aspect.