When dry floodplain systems flood, terrestrial predators (e.g. birds, lizards and spiders) can focus their diet on aquatic taxa reflecting a subsidy from the aquatic to the terrestrial ecosystem. Despite the abundance of bats on floodplains, little is known of how this high-level predator balances terrestrial and aquatic prey, particularly during periods of high energy demand (pregnancy and lactation). We used stable isotope analysis (δ13C & δ15N) to compare short and medium term diet of four functional groups of bats (aquatic trawler, mesic edge-space, ubiquitous gleaner and arid edge-space) with available terrestrial and aquatic prey during flooding of Barmah-Millewa Forest. We predicted that diet of bats in the medium term would reflect expected functional roles, while short-term diets would reflect a shift to aquatic prey during flooding, with a stronger shift for lactating females.
We found that aquatic trawlers and mesic edge-space bats foraged predominantly within aquatic habitats while ubiquitous gleaner and arid edge-space bats fed on terrestrial prey in both short and medium term diets. This may be due to size and echolocation call frequency constraints on the type of prey bats can forage on. Lactating females did not show a consistent preference for aquatic habitats but foraged at a higher trophic level than males for all groups, except the aquatic trawlers. This may reflect a more generalist strategy in lactating females, or in the case of aquatic specialists, potentially a constraint on foraging for higher trophic level prey, such as fish.