SCALING OF COLONIZATION PROCESSES IN STREAMS - PARALLELS AND LESSONS FROM MARINE HARD SUBSTRATA

Understanding colonization processes specifically requires a comprehen sive understanding of the frequency and scales over which dispersal oc curs, which in turn determines the degree of linkage within and betwee n populations and possibly results in metapopulation structure. What i s our level of such knowledge for stream systems! We examine colonizat ion processes in streams using a framework produced by analogous liter ature on sessile, marine species, which are comparatively well-known a nd may provide insights for stream biota. For simplicity, we separate dispersal into two, somewhat artificial scales: microscale (between ha bitat patches - cm to m) and mesoscale (between groups of patches - te ns to thousands of metres), and consider dispersal links between them. Traditional views on dispersal developed similarly in both systems, w ith a strong initial focus on mesoscale dispersal followed by an aware ness that some species do not disperse usually over these scales and t hat there are a wide range of dispersal profiles. In both habitats, th ere are few data on actual dispersal distances, and longevities are so metimes used to infer them instead. Organisms can be transported by wa ter currents (marine larvae, stream larvae) or wind (adult insects), t he directions and strengths of which are relatively predictable at mes oscales. However, behavioural choices of organisms during dispersal ca n change their potential dispersal distances and directions markedly. Additionally, predictability of transport processes at microscales is very poor. As a consequence, simple, lone biological measures (like lo ngevity) or simple, lone physical measures (like discharge) are useles s for predicting dispersal frequencies and scales in most cases. Morta lity during dispersal is also extremely important but there are few da ta; this represents a major information gap in both sets of literature . Finally, if organisms end dispersal by searching for and being able to respond to specific cues, then we may be able to predict colonizati on by looking at the distribution of such cues; different cues are lik ely though to vary greatly in space and time. There is potential for d ispersal 'structure' to develop in rivers of different hydrology, and for sets of correlated life-history characters to result in dispersal at particular scales, but there are very few stream studies that bear on these issues unambiguously. Progress in understanding the scales of dispersal, in both habitats, will require a lot more studies designed formally to test clear hypotheses about the scales of dispersal, rath er than continuing a status quo of generating essentially anecdotal in formation.