Movement and dispersion of insects in stream channels: what role does flow play?

Abstract

Abstract The interplay between landscape and movement can have far-reaching consequences for populations, so an understanding of movement (or lack of) is fundamental. Flow forces create heterogeneity within stream channels - the movement and spatial distribution of insects is related to this flow and topographic heterogeneity. No stream insects are sessile, all are mobile. Researchers often describe large-scale distribution patterns (e.g. abundance in riffles versus pools), but these may arise from small-scale movements and the insect's perception of the environment is only ever small-scale. The crawling behaviour of at least some insects (e.g. cased caddisflies) is influenced by microscale hydraulics and microtopography, and these movements are context-sensitive as they change with largerscale flows. Thus, it is critical that movement studies are carried out on realistic topographies and near-bed flows. Drift is frequently assumed to dominate insect movement in streams, but this view may be biased by colonization studies that are ill-suited to this purpose and because other forms of movement are under-studied. Movement is multi-phasic and a full understanding requires integrating all forms. Various methods and approaches to studying movement are reviewed, with particular reference to their application to stream insects. Movement may play a key role in producing common distribution patterns and tests are required to discriminate between alternative mechanisms.