Invertebrate drift — A review

Abstract

This paper reviews the literature on invertebrate drift in running waters, emphasising papers published during the last 10–15 years. The terms constant drift, catastrophic drift, behavioural drift, active drift and distributional drift are defined, but their use should be limited as much confusion has arisen. Sampling methods are briefly reviewed.

The composition of drift in streams and rivers is assessed, especially with respect to temporal variation, drift densities and drift distances. This body of descriptive literature is subsequently analysed in relation to both abiotic and biotic variables, such as current/discharge, photoperiod, temperature, benthic densities, predators and life cycle stage.

The ecosystem significance of drift in terms of colonization and distribution, population dynamics and its importance as a food resource are then reviewed and discussed. Drift enables organisms to escape unfavourable conditions and gives them the potential to colonize new habitats. However, mortality poses a constant threat. The drift community is composed of components whose presence in the drift may be due to widely differing reasons. This renders unsuccessful most attempts to explain drift in terms of one or even a few factors, except in extreme cases, such as floods or pollution. The question whether drifting organisms are alive, dead, or “ecologically dead” is seldom addressed, as is variation at the level of the individual.

The drift literature is dominated by large numbers of discriptive papers and there is a need for laboratory and field studies aimed at testing specific hypotheses.