Top-down versus bottom-up control of autotrophic biomass—a meta-analysis on experiments with periphyton

Abstract

Nutrient supply and herbivore presence can regulate plant biomass. The relative impact of both of these factors, and their interactions, on periphyton biomass were examined in a quantitative meta-analysis. A literature survey revealed 85 experiments with factorial and replicated manipulation of grazer access and nutrient supply. Two measures of effect sizes were calculated for each experiment, one based on final biomass yields (Hedges’ d) and the other on rates (Δr). Grazers significantly reduced periphyton biomass across all experiments, whereas nutrient addition significantly increased periphyton biomass. Effect sizes were very large for both factors. Thus, periphyton can be viewed as highly controlled by top-down and bottom-up mechanisms. Grazer effects were greater than nutrient effects. This result would be expected because nutrient enrichment represents a relative relief from nutrient limitation, whereas grazer exclusion represents a categorical removal of grazing pressure. Moreover, nutrient uptake and growth response are time-lagged, whereas grazer effects are more immediate. The assembled data set was used to test for significant impacts of the habitat and of the experimental design on the importance of grazers and nutrients. Both factors had consistent effects in streams, lakes, and coastal environments, although some variation in effect strength was found. Background productivity levels also did not affect the main effects, but increases in periphyton biomass led to increasing grazer effects and decreasing nutrient effects. The interaction between grazing and nutrients differed between habitats from different aquatic realms and with different productivity. The interaction metric for Hedges’ d was significantly positive for lakes and coasts, indicating a greater impact of nutrients in the absence of herbivores than in their presence. No such interaction was found for streams or for mesotrophic habitats. Similar trends were evident for Δr. The experimental design profoundly affected the effects of grazers and nutrients on periphyton biomass, with regard to type of grazer manipulation, type of nutrient enrichment, and duration of the experiment. This outcome indicated the crucial role of a careful experimental design.