Exact compensation of stream drift as an evolutionarily stable strategy

The colonization cycle hypothesis predicts that adults of stream-dwelling i nsects preferentially disperse in the upstream direction in order to compen sate for larval drift. Upstream biased dispersal has indeed been shown in m any, albeit not all, natural populations. Based on a recently published ana lysis, we develop a simple stochastic model for the competition of genotype s with different dispersal strategies in a stream habitat. By means of an i nvasion analysis, we show that exact compensation of larval drift by upstre am biased adult dispersal is an evolutionarily stable strategy. Exact compe nsation means that, on average, the net movement of individuals from birth to the time of reproduction is zero. At the population level, we show that, in general, upstream biased dispersal is not necessary for persistence, un less the reproductive rate is very low. Under all conditions, however, popu lations of exact compensators attain highest sizes or persistence times, re spectively. Although selection pressure towards exact compensation is argua bly very general in populations subject to stream drift, trade-offs or cons traints might change the outcome of selection. Therefore. the analysis pres ented in this paper has to be viewed as a null model for optimal dispersal behavior in stream habitats.