An experimental analysis of biological factors contributing to stream community structure

Abstract

Densities of invertebrates were manipulated within stony substrate filled cages in a Wisconsin and a Colorado stream to test the effects of prey densities on colonizaiton of the cages by invertebrate predators and potential competitors. There was no difference between the number of predators colonizing cages with high initial prey densities and that the colonizing cages with zero initial prey density, for any trial, during any season, in either stream. This observed lack of predator response to prey density has been termed a "null numerical response," characteristic of predator populations limited by factors other than prey availability. Food resource, then, was not responsible for the observed distributions of invertebrate predators in these two streams. Physical habitat cues or mutual interference among predators may explain the observed colonization pattern of these predators. Wisconsin stream prey preferentially colonized cages with zero initial densities compared to cages with potential competitors, but only when those cages excluded predators. The presence of spatial refuges from predators enhanced the effect of potential competitors upon prey colonization in this stream. The cage colonization by prey populations in the Colorado stream was reduced by the presence of potential competitors regardless of the availability of spatial refuges. Possible reasons for differences between the responses in the two streams are discussed.