Field Experiments on the Drifting, Colonization and Continuous Redistribution of Stream Benthos

Abstract

The role that invertebrate drift plays in the colonization of new areas of stream bed, exposed both experimentally and naturally, was studied. Concurrent experiments were performed on the spatial and temporal variability of drift and estimates were made of the rate and extent of drift mediated losses and gains by all the principal taxa of the benthic community. The work was done in midsummer, when densities and population movements are at their highest level. Of movements involved in the colonization of experimentally introduced substrate, 82% were by drifting. The average naturally-occurring density of the stream benthos would be achieved on new substrate after approximately 38 days of colonization. These relatively high rates of movement occurred even though flow rates were often less than 5 cm/s. There was a significant correlation between colonization rate and local water velocity. Colonization of a newly diverted section of stream was more rapid than on introduced trays of substrate, but this occurred because the flow rates were much higher in the former case. The vast majority of animals in the drift were probably derived locally from within 2 m of the stream bed. Species of colonization was rapid, with representatives of most species arriving within 3 days of the start of the experiment. The patterns of colonization of the major taxa showed discontinuities. These can be explained in terms of variations in arrival and departure rates which are influenced by the changing nature of the introduced substrate and particularly by the build-up of detritus and microflora. Not all species exhibited the same rates of drift. On average 3.6% of benthic individuals shift their position daily, but Plectrocnemia conspersa, Pentaneurini and Nemurella picteti had much higher than average daily rates of redistribution of 20, 5 and 43%, respectively. All 3 of these mobile taxa had patchily distributed food resources. In addition, the net-spinning predatory caddis larva P. conspersa occurred in very high densities, which in the laboratory would have resulted in intraspecific conflict and cannibalism. The results are discussed in terms of the likely adaptive significance of entering the drift. The continuous process of benthic redistribution serves a dispersive role for young stages to all habitats in the stream benthic community.