Weak evidence of a halo effect for bats at fenced conservation reserves during La Niña conditions

Fenced conservation reserves are a key strategy for protecting biodiversity around the world. The benefits of fenced reserves are well-documented for non-volant terrestrial species, and halo effects have been observed, whereby increased abundance within reserves creates spill over into the surrounding landscape. Our study investigated whether fenced conservation reserves benefit bat species and create halo effects through reduced predation pressure, increased food resources or recruitment of roost trees. Using large-scale bioacoustic monitoring at various distances inside and outside two arid-zone fenced reserves in Australia, established for 27 years and 7 years respectively, we assessed spatial and temporal patterns of bat activity using genaralised linear mixed models and generalised additive models following a La Niña event. Our results revealed no difference in overall bat activity inside versus outside the reserves. However, all species showed spatiotemporal interactions, and in some instances appeared to largely roost inside the reserves. This behaviour may be driven by increased protection within the fenced reserves, or a legacy of pre-existing roost sites. As found in other habitats, site-specific habitat features, rather than the fence itself, predominantly drive bat activity which fluctuates temporally. While our study provides some weak evidence of a halo effect in some species, several factors may influence bat halo effects in the arid zone. Essential habitat components like roost trees may take decades to re-establish; halo effects might diminish during periods of abundant resources (La Niña events); and fenced reserves may not benefit bats if predation is minimal or if reintroduced mammals exploit increased resources.