No room to move: bat response to rainforest expansion into long-unburnt eucalypt forest

Abstract

The expansion of rainforest trees into long-unburnt open forests is widespread globally, including in high-rainfall regions of eastern Australia. Increased tree density can reduce insectivorous bat activity and species richness by constraining echolocation and foraging success. Yet it is unknown whether sclerophyll and rainforest trees differ in their effects on open forest bat communities. We sampled insectivorous bats and nocturnal flying insects at two heights (understorey, canopy) in dry sclerophyll forest of eastern Australia with contrasting fire histories and levels of rainforest pioneer invasion. We found that both time since fire and functional identity of midstorey trees influenced the local bat community, whereas insect biomass had little effect. Long-unburnt forests with a rainforest pioneer midstorey had lower bat activity (63% lower) and species richness (35% lower) than recently burnt forests with a more open midstorey. Bat species richness also declined beneath the dense sclerophyll midstorey in long-unburnt forests, although was unaffected in the canopy above. Strong negative correlations between rainforest tree density and bat community activity and richness indicate that rainforest trees, in contrast with sclerophyll trees, exert additional negative control over open forest bat communities. Our results show that habitat suitability for clutter-intolerant bats declined well before the often-recommended maximum fire-intervals for dry open forest, providing evidence of an upper threshold for fauna conservation in rainforest-invaded open forests. To conserve bat communities in dry open forests vulnerable to rainforest invasion, fire should occur with sufficient frequency to prevent rainforest pioneers developing a dense midstorey that displaces clutter-intolerant bats.