Crayfish process leaf litter in tropical streams even when shredding insects are common

Comparisons of leaf-litter processing in streams suggest that tropical streams have fewer leaf shredders than temperate streams and that insect shredders might be replaced by other taxa such as Crustacea in tropical systems. Australian wet-tropical streams have abundant insect shredders, and also abundant crayfish, which may contribute to litter processing. We monitored litter input and retention in a Queensland rainforest stream to determine availability of litter in different seasons, and we conducted experiments to test the hypothesis that crayfish were important contributors to litter processing. Litter fall peaked in the late dry season and litter accumulated steadily in pools, whereas in riffles, the standing crop was maintained at a threshold level. All accumulated litter was washed from the stream during a flood. The crayfish, Cherax cairnsensis, readily fed on leaves. Its processing rate was related negatively to leaf toughness and positively to leaf nitrogen content. The crayfish assimilated up to 28.5% of the material processed at 24°C, and none at 11°C. These results confirm that there are alternatives to insect shredders in tropical streams, even when insect shredders are abundant