Effects of genetic diversity on eelgrass community structure and ecosystem functioning

Some communities, such as coral reefs, kelp forests, and seagrass beds may be comprised of a single dominant species, and genetic diversity within these species may play an analogous role to species diversity in more speciose systems. Genetic diversity is high in many Zostera populations (as many as 15 genotypes per square meter) and small scale genetic structure can be strong and patterns are consistent on decadal time scales.  Through manipulative field experiments and laboratory mesocosms, we have been examining the effects of genetic diversity in the eelgrass Zostera marina on community function and stability. Field experiments show that genetic diversity enhances community resistance to natural disturbances by grazing geese and algal blooms as well as experimentally imposed disturbances. These differences in disturbance response affect the abundance of epifaunal grazers such as amphipods and other small crustaceans on seagrasses.  In addition, longer term experiments show that as grass density reaches natural levels, we find consistent positive effects of genotypic richness on diversity (Hughes and Stachowicz 2011).

In laboratory experiments we have been measuring the physiological performance of individual seagrass clones grown in common gardens. We find strong physiological differences in nutrient uptake, grazer resistance, and growth rate as well as allocation to shoot vs. root biomass in which no single clone maximizes all aspects of clonal performance. This suggests there are phenotypic differences among clones that may underlie observed diversity effects in the field (Hughes et al. 2009; Tomas et al. 2011).

Our current work in this area focuses on understanding the influence of genetic relatedness, trait differences, and genotypic identity on seagrass ecosystem functioning, in collaboration with Susan Williams and Rick Grosberg. Our previous work used only the number of genotypes as a measure of genetic diversity. Recent analyses of species diversity experiments show that phylogenetic diversity may be a more reliable predictor of ecosystem functioning than simply the number of species. However, such approaches have not yet been applied to understanding the effects of genetic on ecosystem functioning. Our initial re-evaluation of our previous experiments finds that both genetic and phenotypic relatedness contribute to shoot production to a greater extent than simply the number of genotypes (Stachowicz et al 2013).  We are now experimentally testing these links using a larger collection of over 40 genotypes grown in the field in various mixtures of genetic richness, genetic relatedness, and trait diversity.  More broadly we are interested whether genetic relatedness can predict ecological relatedness and serve as a consistent proxy for trait diversity that might be useful in designing restoration activities.

Further experiments test the extent to which genotypes vary in their response to experimental warming treatments and interface with our eelgrass microbiome work to examine the role of variation in plant microbial associates in driving phenotypic variation in eelgrass clones.  At broader scales this work interfaces with our Zostera Experimental Network partnership to scale up lessons from small scale experimental to global scales.

Relevant Papers

Abbott, J.M. and J.J. Stachowicz. 2015. The relative importance of trait vs. genetic differentiation for the outcome of interactions among plant genotypes. Ecology, in press. [-preprint-]

Stachowicz, J.J., S.J. Kamel, A.R. Hughes, and R.K. Grosberg. 2013. Genetic relatedness influences plant biomass accumulation in eelgrass (Zostera marina). American Naturalist. In Press.[-pdf-]

Kamel, S.J., A.R. Hughes, R.K. Grosberg, and J.J. Stachowicz. 2012. Fine-scale genetic structure and relatedness in the eelgrass Zostera marina. Marine Ecology Progress Series 447:127-137[-pdf-]

Tomas F., J.M. Abbott, M. Balk, C. Steinberg, S.L. Williams, and J.J. Stachowicz. 2011. Plant genotype and nitrogen loading influence seagrass productivity, biochemistry, and plant-herbivore interactions. Ecology 92:1807-1817[-pdf-]

Hughes, A.R. and J.J. Stachowicz. 2011. Seagrass genotypic diversity increases disturbance response via complementarity and dominance. Journal of Ecology 99:445-453.[-pdf-]

Hughes, A.R., R.J. Best, and J.J. Stachowicz. 2010. Genotypic diversity and grazer identity interactively influence seagrass and grazer biomass. Marine Ecology Progress Series 403: 43-51. [-pdf-]

Hughes, A.R., J.J. Stachowicz, and S.L. Williams. 2009. Morphological and physiological variation among seagrass (Zostera marina) genotypes. Oecologia 159:725-733.[-pdf-]

Hughes, A.R. and J.J. Stachowicz. 2009. Ecological impacts of genotypic diversity in the clonal seagrass, Zostera marina. Ecology 90: 1412-1419.[-pdf-]

Hughes, A. R., and J. J. Stachowicz. 2004. Genetic diversity enhances the resistance of a seagrass ecosystem to disturbance. Proceedings of the National Academy of Sciences of the United States of America 101:8998-9002. [-pdf-]