The ecological role of tadpoles in streams of the Australian Wet Tropics

Stream-breeding amphibians are an important link between terrestrial and aquatic habitats, and frogs and tadpoles may be important contributors to ecosystem functioning in their habitats. Amphibian declines due to fungal disease have affected many frog assemblages, especially in upland rainforest sites, leading to reduced abundance and diversities of stream-dwelling tadpoles. This raises the question of what role tadpoles play in streams and what the impact of their declines might be. In the Australian Wet Tropics bioregion, some frog species have declined or disappeared, but others have persisted, and tadpoles can still be seasonally abundant, possibly having important effects on stream ecosystems. The aim of this study was to investigate the role of tadpoles in the ecology of Wet Tropics streams using inferences from natural populations and manipulative experiments. Monthly surveys of streams near Paluma and Tully showed that tadpole and invertebrate assemblages fluctuated seasonally, with abundances highest in spring and summer, reflecting the main reproductive period. Variability of flow was the most important environmental influence on the animals in both locations, explaining up to 25% of the variation in the tadpole assemblages and up to 40% in the invertebrate assemblages. There were no indications of major interactions between tadpoles and invertebrates. The role of tadpoles in stream processes, including leaf litter breakdown and sediment removal, was tested using field experiments in artificial stream channels. Litoria serrata tadpoles and invertebrates interacted during leaf processing, most likely through direct physical activity by invertebrates that facilitated tadpole feeding, but there was no evidence of effects of nutrient regeneration by tadpoles. Mixophyes coggeri tadpoles did not contribute to leaf processing, but they were important in sediment removal, which benefits smaller consumers. Stable isotope analysis was used to determine the trophic position of tadpoles and their place in the food web in Paluma and Tully streams. Tadpoles assimilated mainly biofilm and algae, but they were generalist feeders that used different food sources depending on nutrient quality and availability. The food web structure was simpler in the Paluma riffles, where tadpoles of two species disappeared in the early 1990s, suggesting that tadpoles increase food web complexity. The study showed that the loss of tadpoles from a stream, or from a particular habitat within a stream, affects overall benthic assemblage composition, food web structure and stream ecosystem functioning. Tadpoles of different species can have different functional roles and it is therefore important to consider the identity of species lost, as well as the effects of reduced diversity, during amphibian declines resulting from disease, climate change or land-use changes. Although tadpoles are only abundant during part of the year, their peak occurs in the spring-summer period when invertebrates are also most abundant, so their influence on the ecosystem is likely to be substantial and may well carry over to the cooler season.