Effect of species and genotypic diversity on eelgrass detrital systems
Declines in biodiversity around the world have spurred numerous studies investigating the role of diversity in ecosystems, most commonly focusing on species-level consequences. However, intraspecific genetic diversity can also have important effects, including increased and ecosystem functioning and resistance. Eelgrass (Zostera marina) is found in temperate coastal waters, creating habitats for many organisms. Increased genotypic diversity in eelgrass beds is associated with higher densities of invertebrate grazers. However, most eelgrass production is not consumed, and instead enters the detrital food web. We examined how eelgrass detrital processing is affected by diversity at both the consumer species level and the eelgrass genotypic level, using three distinct eelgrass genotypes and three species of invertebrates (the polychaete worm Platynereis bicaniculata, the isopod Idotea resecata, and the amphipod Ampithoe lacertosa) commonly found in seagrass beds in Bodega Bay, California.
When we manipulated both the number of consumer species and number of eelgrass genotypes, high genotypic diversity resulted in an increased rate of detritus consumption. In contrast, increasing the number of consumer species did not affect the amount consumed. Invertebrate species varied in their rates of detritus consumption as well as their preferences to these three genotypes. Amphipods preferentially consumed one of the three genotypes in this experiment, whereas polychaetes and isopods did not show clear preferences to any of the genotypes. Variation in physical structure, nutritional value, or chemical defenses among eelgrass genotypes likely underlie the differences in consumption rates at the higher diversity level. These results suggest that detrital processing may be faster in areas of higher genotypic diversity, highlighting the importance of diversity in detrital food webs.