Effects of Algal Richness on Community Biomass and Stability Depend on Herbivory an Aquatic Microcosm Experiment

Abstract

Hundreds of studies exploring how changing biodiversity alters ecosystem functioning have led to a consensus that higher diversity tends to both increase and stabilize community biomass through time. However, the majority of this work has focused on a single trophic level, even though trophic interactions also influence community biomass and stability. The relatively few studies investigating the effects of changing diversity on trophic interactions such as herbivory have produced mixed results; whether such effects are important for community biomass and stability is unclear. We performed an experiment using freshwater laboratory microcosms to test for effects of algal diversity (one or four species) on community biomass and temporal variability in the presence of two different herbivores (cladocerans Ceriodaphnia dubia and Daphnia pulex). We measured algal biomass, herbivore density, and the variability of both measurements over four weeks. We also tested for effects of algal diversity on the strength of herbivory by comparing to a control treatment with no herbivores. The effects of algal diversity differed qualitatively between herbivore treatments. With no herbivores, algal biomass was greater and less variable in the high diversity treatment. Total herbivory by C. dubia did not differ between diversity treatments, preserving the qualitative effects of diversity on algal biomass and variability, and leading to only weak effects of diversity on the herbivore population. In contrast, total herbivory by D. pulex was twice as great in polycultures, leading to a larger and less variable population of D. pulex but lower and more variable algal biomass in polycultures versus monocultures. Thus a differential effect of algal diversity on herbivory led to opposite effects of diversity on algal biomass and variability. Our results suggest that trophic interactions lead to a richer array of diversity-function relationships than previously documented by studies that have focused on a single trophic level.