Trophic interactions in open systems: Effects of predators and nutrients on stream food chains

Theory and empirical work on food chains has focused primarily on closed, e quilibrial environments. We tested the combined effects of secondary consum ers (fish) and limiting nutrients (nitrogen) on intermediate trophic levels in an open stream environment, where flow redistributes organisms and mate rials among patches of streambed habitat. Fish reduced the biomass of the d ominant herbivore (baetid mayflies) within habitat patches both by direct p redation and by causing increased emigration from the patches. The resultin g decrease in herbivory caused an increase in the growth and biomass of pri mary producers (algae) in areas containing fish. Independent of the effect of fish, algal growth and biomass was increased by augmenting the nutrient supply to patches. Nutrient-enriched areas (with high algal biomass) also s upported greater populations of herbivores because they either grew faster in these areas or emigrated less frequently from them. Controlling influenc es on trophic structure came from both the top and the bottom of the food c hain, and these influences were not conditional upon one another. Trophic s tructure in this system was determined by a mix of behavioral and trophic i nteractions between the major taxa, most of which were specific to open sys tems and not predicted by conventional theory.