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 invertebrates. To compensate for the loss of individuals it has been proposed that adults of aquatic insects preferentially fly upstream to oviposit and thereby maintain populations, The observation of adult flight biased in the upstream direction has been advanced as confirmation of the hypothesis. I argue here that upstream-biased dispersal is not sufficient for population persistence nor is it necessary. Population persistence can only be explained on the basis of density dependence at some point in the life cycle. Computer simulations showed that density dependence by itself is not sufficient for population persistence. Infrequent dispersal by randomly flying adults coupled with density dependence does allow population persistence. Upstream-biased dispersal can be explained by increased individual fitness. If upstream reaches are depopulated by drift, these reaches provide more rapid growth to successful colonists. Computer simulations of competition for space by two genotypes that differ only in the directionality of dispersal showed that genotypes with upstream-biased dispersal always drove random dispersers to extinction.