Cascading diversity effects in an eelgrass-epiphyte system

The dramatic loss in biodiversity and its consequences for ecosystem processes has raised considerable interest in ecological studies recently. The complex and interacting processes influencing diversity effects in multitrophic systems are, as yet poorly, understood. Conceptual models predict that cascading effects from consumer diversity to prey diversity are to be expected in multitrophic systems. Consumers are known to mediate coexistence of their prey by feeding on the competitive dominant prey species, thus, increasing diversity and confining competitive exclusion at the prey level. Therefore, consumer extinction may also reduce prey species richness and diversity. We used an experimental eelgrass-epiphyte system and three common mesograzers to study the effects of changing consumer diversity on diversity and taxonomic composition of epiphytes. The isopod Idotea baltica, the amphipod Gammarus salinus, and the periwinkle Littorina littorea were established in a substitutive design, whereby the biomass of grazers was kept constant in each treatment. We found a cascading diversity effect from the consumer level to the prey level. Epiphyte diversity was highest in the three-grazer treatment. Epiphyte species richness, H’ (based on the Shannon-Wiener function), and evenness (J) significantly increased with increasing consumer species richness. Additionally, strong effects of consumer species identity on epiphyte diversity and on epiphyte taxonomic composition were found.