Drivers of bat activity and community structure within floodplains of the Murray-Darling Basin, Australia

Abstract

Floodplain wetlands are threatened worldwide by climate change, river regulation, pollution and vegetation clearing. Insectivorous bats are high trophic level terrestrial predators that potentially rely on availability of fresh water and aquatic prey subsidies of floodplains. In this thesis, I led a meta-analysis into continental scale relationships between Australian bats and wetlands. Next, I examined relationships between bat species, their prey and structural and flooding variables, in the Murray-Darling Basin Australia, using acoustic bat surveys and terrestrial laser scanning. Finally, I investigated the role of aquatic subsidies in floodplain bat diets using stable isotopes. The meta-analysis revealed that floodplain wetlands supported comparatively higher activity of bats than surrounding dry habitats and compared to wetlands in other ecoregions in Australia. In field surveys, bats were most active in rivers and lakes with open water and riparian trees where total activity, foraging activity and richness were 5, 12 and 1.5 times greater than dry vegetation respectively. Bat preference for open structure was apparent in managed flooded forests, with total activity in unthinned regrowth 60% lower than mature open forest and thinned regrowth. Insect biomass, however, was greater in regrowth compared to mature sites, indicating that forest structure was more important for bat habitat use than prey availability. Detailed surveys of vegetation structure revealed that bat activity was negatively associated with stem density, with total activity halving (from 380 to 190 calls per night) as stem densities increased from 60 to 1350 stems per ha, while foraging activity declined from 8 to < 1 feeding buzzes over the same range. Foraging strategy, rather than call or morphological traits, was the most important trait driving this relationship with forest structure, with edge-space bats negatively correlated with stem density, while closed-space bats were positively correlated. In inundated open habitats, where I recorded the greatest bat activity, stable isotope analyses revealed that in the medium term (< 1 year), two out of four species (water-dependent and mesic-adapted bats) relied predominantly on aquatic prey, while widespread generalist and arid-adapted bats foraged predominantly on terrestrial prey. In the short-term (< 1 day), bat diets converged towards terrestrial sources, potentially due to terrestrial productivity booms which follow flooding. Using multiple lines of evidence I demonstrated the importance of floodplain wetlands for insectivorous bats, highlighting the value of structurally open forest, water availability and aquatic prey subsidies. This reinforces the urgency to conserve floodplains, given the current and future threats affecting these declining ecosystems including river regulation, agricultural clearing and climate change.