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

Abstract

Invertebrate drift is a phenomenon of prime importance to the benthic community of streams (see review papers by Elliott 1967; Hynes 1970a, b; Muller 1966, 1974; Ulfstrand 1968; Waters 1969, 1972). The downstream movement in stream currents of large numbers of invertebrates profoundly influences benthic community dynamics in two ways. Firstly, the continuous loss of animals into the water column reduces benthic density, and since some species and size classes are more prone to drift than others, the composition of the local community is affected. Secondly, the continuous settling out of animals from the drift plays an important colonizing role. Thus, the twin influences of drift contribute to a continuous redistribution of benthos. The extent and rate of these two dynamic processes are among the most basic of ecological parameters in the stream ecosystem, and yet very little study has been specifically directed to their measurement. The phenomenon of drift is one which lends itself to experimentation in the field, and it is surprising that so little has been made of this opportunity. Studies involving field experiments have, however, been carried out into the question of the distance of drift (Waters 1965; McLay 1970; Elliott 1971a), the relationship between drift and density of the benthos (Waters 1965; Dimond 1967), the influence of light on drift rates (e.g. Elliott 1965a; Holt & Waters 1967), and the influence of temperature on drift rates (Wojtalik & Waters 1970). In the present study, we distinguish between movements of invertebrates brought about by crawling over the substrate, as opposed to drifting, and evaluate the role that drift plays in the colonization of new areas of stream bed exposed both experimentally and naturally. Colonization of unpopulated areas of stream bed, whether exposed by erosion, excavation of new channels, or denuded by pollution, has often been reported to be initiated by the drifting of invertebrates from upstream (Moffett 1936; Surber 1937; Laurie & Ericksen Jones 1938; Leonard 1942; Muller 1954; Patrick 1959; Chutter 1968; Crisp & Gledhill 1970). Colonization of experimentally exposed substrates has been studied by Moon (1940), who never mentioned drift, and more quantitatively by Waters (1964), Cairns & Ruthven (1970), Dickson & Cairns (1972), Glime & Clemons (1972), Nilsen & Larimore (1973) and Ulfstrand, Nilsson & Stergar (1974). However, none of these workers specifically attempted to measure the role of drift in colonization as opposed to movements across the substrate. We also performed concurrent experiments on the spatial and temporal variability of drift in the stream, and estimated the rate and extent of drift mediated losses and gains by all the principal taxa of the benthic community.