DENSITY-DEPENDENCE RESOLVES THE STREAM DRIFT PARADOX

The downstream displacement of individuals by drifting in the current is a prominent feature in the population biology of stream invertebrat es. To compensate for the loss of individuals it has been proposed tha t adults of aquatic insects preferentially fly upstream to oviposit an d thereby maintain populations, The observation of adult flight biased in the upstream direction has been advanced as confirmation of the hy pothesis. I argue here that upstream-biased dispersal is not sufficien t for population persistence nor is it necessary. Population persisten ce can only be explained on the basis of density dependence at some po int in the life cycle. Computer simulations showed that density depend ence by itself is not sufficient for population persistence. Infrequen t dispersal by randomly flying adults coupled with density dependence does allow population persistence. Upstream-biased dispersal can be ex plained by increased individual fitness. If upstream reaches are depop ulated by drift, these reaches provide more rapid growth to successful colonists. Computer simulations of competition for space by two genot ypes that differ only in the directionality of dispersal showed that g enotypes with upstream-biased dispersal always drove random dispersers to extinction.