Consequences of genotype identity on ecological processes and response to disturbance in a marine foundation species

Background/Question/Methods Decreases in global biodiversity have stimulated work on the role of species diversity and identity in ecosystem functioning.  However, functioning of some ecosystems is strongly affected by a single dominant species (e.g. habitat-forming species). In these systems, variation among genotypes may play an analogous role to species diversity in regulating ecosystem processes. While previous studies have observed this effect, the mechanisms underlying consequences of genotypic diversity are poorly understood. A key step in understanding genetic diversity – ecosystem functioning relationships is to clarify which traits vary among genotypes, how they influence ecological processes, and assess their relative contribution in comparison to other factors to ecosystem functioning. This is particularly relevant on clonal plants such as seagrasses, for which one particular genotype can dominate large areas in space (kilometres) and time (thousands of years). Here we used a mescosom experiment to analyze plant performance traits in response to eutrophication of distinct genotypes of eelgrass, a marine angiosperm that forms monospecific meadows throughout coastal areas in the northern hemisphere. In addition, we performed feeding experiments to assess the effects of genotype variation on the feeding preference and performance of crustacean herbivores and examined whether the interactions between herbivores and eelgrass genotypes were altered by eutrophication.

Results/Conclusions We found that genotypes differed in many ecologically important traits (e.g. aboveground biomass, recruitment, detritus production), and all suffered negative impacts of eutrophication, which decreased leaf production and carbon reserves. Genotypes strongly differed in susceptibility to herbivorous isopods. Nutrient addition did decrease plant palatability, but did not alter the relative preferences of herbivores among genotypes, indicating that genotype effects are strong. Herbivores preferences for specific genotypes are driven by chemical, and not mechanical, differences among genotypes. Highest survivorship and growth of herbivores occurred on the most preferred eelgrass genotype, while those fed the least preferred genotype exhibited high mortality and virtually no growth. Our results highlight that differences in key traits among genotypes of habitat-forming species can have important consequences for community structure and processes by affecting crucial ecological properties (e.g. primary production) and interactions with higher trophic levels.  These effects are strong enough that they are not modified or overwhelmed by the addition of known stressors such as eutrophication, and thus need to be accounted for in models of ecosystems based on foundation species such as seagrass beds.