PREDATION AS AN AGENT OF POPULATION FRAGMENTATION LN A TROPICAL WATERSHED

Stream fish sometimes show multimodal distributions, with high densities in the tributaries of a river but rarity or absence in the river itself. To assess if predation can produce such a fragmented distributional pattern on a large geographic scale, we determined the density and habitat use of a prey fish in two tropical stream watersheds, each with a barrier waterfall that split the drainage into a region with a strongly piscivorous fish and a region lacking a strong piscivore. In contrast to sites with the strong piscivore, the prey fish in areas above barriers showed a dramatic expansion into the main river, thereby spatially consolidating an otherwise fragmented distribution. Manipulation of piscivores in a third-order stream flanked by a series of first-order experimental streams also showed that the prey distribution expanded into the third-order stream when piscivores were excluded, and that the presence of the piscivores in the third-order stream reduced prey densities both by killing the prey and by inducing the prey to ascend cascades to enter the tributaries. Because the predator created spatial fragmentation of the prey population, we examined prey-fish dispersal in the experimental stream facility to ask whether the predator could reduce prey movement into and beyond the predator-occupied sites. As hypothesized, the experiment revealed that predators could block prey dispersal by killing prey. However, the experiment also suggested that predators may increase prey movement between tributaries by inducing shifts out of river sites. The results suggest that realistic models of dispersal by prey would need to account for effects of both predator consumption and prey behavioral responses to the predator.