Background/Question/Methods   The direct effects of producer species diversity on primary production are now widely appreciated. However, the food web and community consequences of these effects are less well understood. Plants serve as both food and habitat for mobile animals and thus indirect effects of plant diversity on the composition and functioning of higher trophic levels may be common. For example, plant diversity has been reported to increase the diversity of associated animals. However, the consequences of these changes in the animal community on the plants themselves are rarely investigated. We previously demonstrated in a long-term (6 year) field experiment that seaweed species diversity increases producer biomass and stability in rocky intertidal assemblages. Here we examine the effects of algal diversity on the abundance, diversity and composition of the mobile invertebrate community associated with these algae over the same time period. We also use short-term habitat-choice and feeding experiments in the lab to assess potential mechanisms underlying these effects. We interpret our findings in the context of possible feedbacks between the direct effects of algal diversity on animals and effects of animals on algal diversity.

Results/Conclusions   We found that algal species diversity affects both the diversity of associated invertebrates and the abundance of individual species in complex ways. First, total species richness is greater in more diverse plots because individual species show strong, but contrasting preferences for particular algal species as food and habitat. No species were present in diverse plots that were not already present in at least one individual monoculture, so effects of adding algal species on invertebrate richness were essentially additive. Results of feeding experiments show strong complementarity among grazer species in their preferences for different algal prey, providing an additional mechanism for high species richness in more diverse communities. Second, the abundance of species that were relatively uncommon disproportionately increased in diverse assemblages—this was likely due to the overall higher plant biomass in these treatments and lower rates of desiccation at low tide that facilitated species typical of lower tidal elevations. In contrast, abundance of several common key grazer species (limpets, snails) was reduced in high algal diversity plots. The precise mechanisms underlying this are idiosyncratic with respect to the particular grazer species involved, but the net result is that areas of high algal diversity also have lower abundance of grazers, potentially resulting in lower overall grazing rates and a feedback loop that could help maintain algal diversity.