An endangered cave-roosting insectivorous bat supports strongly distinct macro-invertebrate communities in caves of an oceanic island

Cave ecosystems are often underrepresented in global conservation management, despite frequently holding diverse biota. As key providers of allochthonous nutrient, cave-roosting bats are important in sustaining cave food webs through guano deposits. However, bat populations globally are declining, thereby threatening cavernicolous biodiversity. Oceanic island caves, in particular, face elevated risks due to their additional isolation which may be expected to increase vulnerability of their associated biodiversity to extinction. Understanding the influence of cave-roosting bat on eventual cave-dependent biodiversity, especially where a single cave-roosting bat species exists, matters for predicting possible cascading effects of loss or gain of cave-roosting bat colonies. We compared macro-invertebrate diversity between lava tube caves with and without the endemic insectivorous bat, Mormopterus acetabulosus (Molossidae) across Mauritius, sampling from cave entrance to deep sections and across guano piles. Four caves with permanent bat colonies and four unoccupied caves were surveyed. Macro-invertebrates were collected using pitfall traps and direct collection, and identified to the lowest taxonomic level possible. Species richness, community composition, and beta diversity were compared between bat-occupied and unoccupied caves. Macro-invertebrate richness, abundance, and diversity increased significantly with bat colony size. The three metrics dropped further from guano piles, while light intensity had a positive effect. Community composition differed significantly between bat-occupied caves and unoccupied caves. Allochthonous nutrients from bats substantially boost cave biodiversity, which include possible endemic cave specialists. Bat guano acts as a keystone resource, shaping invertebrate communities, fostering higher species richness and abundance. The drop in diversity away from guano piles reflects nutrient limitation and spatial heterogeneity, while the near absence of invertebrates in unoccupied caves shows that resource availability drives community structure. Our results highlight M. acetabulosus as plausible ecosystem engineers that provide guano “islands” whose size and numbers influence cave invertebrate diversity, reflecting island biogeography theory. Conservation measures should prioritise bat-occupied caves, particularly those with large or multiple guano piles. This is important as caves in Mauritius are currently neglected in conservation management strategies. Legal protection of bat roosts, and restricting access to bat colonies and guano piles could be considered. Integrating these measures into conservation policies can prevent biodiversity declines and provides a model for other islands approaching Mauritius’ levels of rapid urbanisation and high population density.