Cave-roosting specialisation of an oceanic island endemic microbat elevates extinction risks as caves face multiple threats

Bats (Chiroptera) constitute one-fifth of the global mammalian diversity, with over 1,450 species. Yet, 16.5 % of them are threatened with extinction, a situation exacerbated on islands, underscoring an urgent need for conservation efforts there. However, effective conservation action can only be as sound as the understanding of the basic ecology of such insular species. Mauritius harbours one endemic Endangered cave-roosting insectivorous bat, in recent decline, whose ecology is poorly known, hampering its conservation. We used Mormopterus acetabulosus as a model species to inform insectivorous island bat conservation in highly transformed habitats. We compared the species’ historical and current known roost distribution and sizes and estimated any change in its Area of Occupancy (AOO) and Extent of Occurrence (EOO) using GeoCAT. We also compiled the physical characteristics, environmental and anthropogenic variables of caves that are occupied and unoccupied by the bats to identify potential factors influencing roost occupancy. We found roosts in 17 of the recorded 130 lava cave tunnels, up from the nine previously recorded, including 11 permanent and six seasonal roosts, mostly (∼71 %) on privately owned land. Compared to the last surveys in 2017, the EOO remained stable at 818.9 km², while the AOO increased by 33.3 %. This expansion may reflect previously undetected roosts or recent colonisation events or both. Cave dimensions in terms of entrance size and height as well as the number of chambers, relative humidity and distance to water best explained roost occupancy by M. acetabulosus. Caves with larger entrances may be preferred for roosting as this characteristic eases manoeuvrability and predator avoidance, and larger caves may be preferred for enhanced microclimatic stability. Occupied and unoccupied caves were found to differ in terms of physical characteristics, although cave microclimate did not vary notably. Based on their characteristics and their potential contributions to increasing the species’ EOO and AOO, six currently unoccupied caves were identified as priority sites for future conservation efforts. Legal protection of all caves, regardless of current bat occupancy is necessary to mitigate extinction risks and preserve their associated ecological functions.