Effects of grazer diversity on marine microphytobenthic biofilm: a ‘tug of war’ between complementarity and competition

Species loss is one of the most striking problems related to human-driven environmental changes. Nevertheless, biodiversity and ecosystem functioning experiments have mainly focused on primary producers, paying less attention to the consequences of changing diversity at higher trophic levels. We performed a field experiment using cage enclosures to test the effects of species richness, identity and density of gastropod grazers on the photosynthetic efficiency and biomass of intertidal biofilm on an exposed rocky shore in the northwest Mediterranean. The diversity and composition of intertidal grazers affected the photosynthetic efficiency of biofilm with only negligible effects on biomass. Individual species showed strong identity effects. In assemblages of 2 or more species, positive or negative complementarity effects occurred. The magnitude of the ecosystem response is expected to depend on the particular species assemblage and its density, which will determine whether niche partitioning or competition is the prevailing process. Grazer preference in specific components of biofilm, characterized by different photosynthetic efficiency and competitive abilities, might explain concomitant changes in photosynthetic efficiency and comparable levels in biomass among treatments. The effects of grazers declined following the natural trend of decreasing biomass of biofilm during the study period, highlighting the importance of considering temporal variability in the effects of biodiversity on ecosystem functioning. This work emphasizes the key role of species identity to predict effects on their resources and ecosystem functioning.