“Variety is the spice of life” takes on a whole new meaning: variation between individual plants can have population and community level effects

Human activities impact all aspects of the environment – from climate change and habitat destruction, to nutrient loading and pollution. Understanding how these changes impact natural populations will allow us to find the most effective ways of reducing our impacts and developing management and conservation strategies. Habitat-forming species, like seagrasses, are of particular concern because they provide the foundation of entire communities. Seagrass studies have found that populations with higher genotypic diversity, (i.e., with greater genetically based variation among individuals) per area, are more resilient to environmental stress. Although, the relationship between diversity and resilience suggests that genotypes differ in ecologically important ways, few studies have explored how traits vary among individual genotypes. Moreover, little is known about the role of trait differences in influencing ecological processes, and how variation among individuals might compensate for environmental stress. We explored these questions by studying differences between genotypes of seagrass in the presence and absence of nitrogen addition (chosen to imitate stress from eutrophication). We examined physical (growth rate, size, etc.) and chemical (C:N, phenols) plant traits, as well as vulnerability of genotypes to a common marine isopod herbivore. We found significant differences between genotypes and between nitrogen enriched and unenriched plants. Plant traits (growth rate, size etc.) differed among genotypes, and isopods showed strong feeding preferences for particular genotypes. Nitrogen addition had negative impacts on seagrass performance (reduced size and growth rate), and isopods preferred eelgrass that was not enriched with nitrogen. Interestingly, for plant traits, differences among genotypes were similar in magnitude to differences between nitrogen enriched and unenriched seagrass. Furthermore, differences among genotypes affected isopod preference more than nitrogen enrichment. Our results highlight that differences among genotypes can have important consequences for habitat-forming species and the communities that depend on them and that these consequences are strong enough to not be overwhelmed by environmental stress. Given that many human activities, like habitat destruction and fragmentation, cause a reduction in genetic variation within species, our results suggest that restoring and conserving genetic variation within species may be a critically important component to maintaining ecosystem resilience to human-caused stressors. This insight can be used to make informed management decisions about conserving and restoring important populations of habitat-forming species.