Invited review: Direct and indirect effects in herbivore periphyton interactions

In this article we review the indirect interactions prevailing in littoral communities and compare their importance to direct trophic interactions. We focus on the interaction between benthic algae and their herbivorous consumers, i. e. invertebrate grazers and herbivorous fish. In addition to the effects of grazers on periphyton biomass and production, we categorise and describe the types of indirect interactions observed and we present a quantitative meta-analysis to compare the effect magnitude of indirect and direct effects. Our review shows that herbivore-plant interactions comprise considerable complexity beyond consumption. We identify and review three different indirect interactions, which play important roles for periphyton-grazer interactions in benthic food webs. These are keystone predation, trophic cascading and habitat facilitation. Direct negative effects were stronger than indirect positive effect, because grazing leads in almost all studies to a decrease in algal biomass. We found positive effects of grazing on algal nutrient content, biomass-specific productivity and evenness. We found negative overall effects on species richness and area-specific productivity. Moreover, spatial heterogeneity was on average increased by grazing. The magnitudes of direct and indirect effects were often correlated.