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, equilibrial environments. We tested the combined effects of secondary consumers (fish) and limiting nutrients (nitrogen) on intermediate trophic levels in an open stream environment, where flow redistributes organisms and materials among patches of streambed habitat. Fish reduced the biomass of the dominant herbivore (baetid mayflies) within habitat patches both by direct predation and by causing increased emigration from the patches. The resulting decrease in herbivory caused an increase in the growth and biomass of primary 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 supported greater populations of herbivores because they either grew faster in these areas or emigrated less frequently from them. Controlling influences on trophic structure came from both the top and the bottom of the food chain, and these influences were not conditional upon one another. Trophic structure in this system was determined by a mix of behavioral and trophic interactions between the major taxa, most of which were specific to open systems and not predicted by conventional theory.