Harsh habitats? Waterfalls and their faunal dynamics in tropical Australia

Abstract

There are few studies of the ecology of waterfalls despite their being prominent landscape features and of ecological interest because of their physical characteristics. We compared invertebrate assemblages of 5 waterfalls in the Australian Wet Tropics over 12–18 mo. Waterfall assemblages were distinct from those of riffles and bedrock, with some taxa particularly abundant on waterfalls (e.g. Simuliidae, Hydropsychidae) and others restricted to them (e.g. Blephariceridae, Pyralidae), and supported more species than bedrock but fewer than riffles. Differences among waterfalls related to differences in discharge, shade and habitat complexity. Waterfalls comprised a complex of microhabitat patches, with high-flow smooth, high-flow rough, vertical and spray zones most prevalent and distinguishable by gradient, roughness, water velocity, depth and invertebrate assemblages. In high-flow microhabitats rheophilic taxa (e.g. Simuliidae) were prevalent, while in the spray zone a range of madicolous taxa (e.g. various Coleoptera) occurred. Within microhabitats, temporal change was moderate, with stable composition over 12 months. Flood disturbance had limited effect on assemblages, as large flows overshot steep surfaces; drought may be more of a threat because recovery is hindered by isolation and lack of hyporheic refugia. Given their isolation, limited extent and distinctive fauna, waterfalls merit special conservation attention.