Interactions between Stream Herbivores and Periphyton: A Quantitative Analysis of past Experiments

This review summarizes the state of knowledge regarding herbivory in stream ecosystems by quantitatively analyzing the results of 89 experimental studies published between 1972 and 1993. Our primary objective was to determine if general patterns exist among stream ecosystems in the type and strength of interactions occurring between herbivores (grazers) and their primary food source, periphyton. We conducted two types of meta-analyses of the published literature: (1) analyses of the proportion of studies showing significant effects for three types of interactions (effects of grazers on periphyton, effects of periphyton on grazers, and effects of grazers on other grazers and benthic animals) and (2) analyses of factors influencing the magnitude of effect that grazers had on periphyton. For effects of grazers on periphyton, we also determined (1) if the likelihood of observing significant effects varied with the spatial and temporal scale at which experiments were done and (2) if the magnitude of effect by grazers on periphyton abundance varied with spatial and temporal scale, grazer taxon, grazer abundance, and periphyton accrual based on the difference in treatments with and without grazers. Grazers held at ambient densities usually reduced periphyton biomass (70% of experiments) and altered algal taxonomic or physiognomic structure (81%) relative to grazer removal treatments, whereas grazers had slightly lower effects on periphyton productivity (usually <70% of experiments, depending on productivity measure). Experiments conducted in laboratory streams and at two spatial scales in the field (few or single habitat units and stream reaches or basins) were equally likely to report significant effects of grazers. Both short-term (≤4 wk) and longterm (>4 wk) experiments also were equally likely to report significant effects of grazers on periphyton. However, the magnitude of effect grazers had on periphyton biomass varied with the amount of periphyton accrual, grazer taxon, and grazer population biomass. Grazer effects also were higher for longer studies conducted under laboratory conditions than for shorter studies conducted in the field. A high proportion of the experiments that manipulated periphyton abundance significantly affected grazer densities and growth. Reduction in periphyton abundance usually reduced grazer density and growth. Experimental manipulations of dominant grazers typically had strong and usually negative effects on densities and growth of other species of benthic animals, either from direct (e.g., interference) or indirect (e.g., resource exploitation) mechanisms. Results of these analyses suggest that stream herbivores regulate their food resources as or more frequently than herbivores in other ecosystems, and strongly contradict the view held by many ecologists that stream communities are regulated primarily by abiotic factors. Although publication bias (i.e., the tendency for journals to publish positive results) appears minimal, we cannot yet generalize from these results to the entire universe of stream ecosystems because (1) most studies were conducted during summer base flow conditions and (2) results do not adequately represent interactions during the more physically stressful conditions that occur during periods of flooding, drought, or extreme cold. If rapid progress in the development of general stream ecosystem theory is to occur, we believe (1) future studies should be explicitly designed within the context of general ecological questions, (2) as much background information as possible describing environmental conditions should be collected, and (3) journals should permit and urge inclusion of tabular data describing both experimental conditions and treatment means and variances.