The effects of Daphnia genotypic richness and phenotypic dissimilarity on consumer-resource dynamics and ecosystem function

Background/Question/Methods

Intraspecific diversity, particularly genotypic richness of dominant or keystone species, impacts species interactions, consumer-resource dynamics, and ecosystem function. The effects of intraspecific diversity depend not just on number of genotypes, but also on interactions among genotypes such as facilitation or inhibition. Consequently, it is important that studies looking at the effects of intraspecific diversity consider phenotypic dissimilarity and functional differences among genotypes in addition to genotype number. Daphnia, a keystone species in freshwater ecosystems, is an intermediary consumer that plays a critical role in trophic cascades. In lakes, a single species often dominates the water column, making it ideal and relevant to test the effects of intraspecific diversity on ecosystem function using Daphnia. Here, we relate phenotype to genotype in a series of experiments measuring Daphnia i) life history and ii) grazing rate, and then apply this information to the results of a mesocosm experiment manipulating genotypic richness of Daphnia (1, 2, 4, and 8 clones). We monitored consumer-resource dynamics (algae and Daphnia), ecosystem function (net primary productivity), and genotypic richness (microsatellites and intergenic spacer length) during the mesocosm experiment to determine the functional effects of phenotypic differences in addition to the effects of genotypic richness.

Results/Conclusions

The life history experiment identified clear phenotypic differences in growth and reproduction of Daphnia clones. Variation in Daphnia grazing rate matched functional differences of clones in monocultures of the mesocosm experiment – clones with higher grazing rates limited algae growth, resulting in earlier and lower peak algae density. Both genotypic richness and phenotypic dissimilarity affected consumer-resource dynamics. In addition, genotype interactions resulted in differences between predicted (based on mean monoculture values) and observed heights of algae and Daphnia peaks in polyculture. The molecular data identified certain clones as superior, consistently dominating the population at the end of the mesocosm experiment, regardless of diversity treatment. However, the rank order of these genotypes differed across life history traits, making it difficult to identify one characteristic that consistently led to the success of certain clones. It is likely a combination of life history, grazing rate, tendency to switch from asexual to sexual reproduction, and possibly intergenic spacer length contributed to clonal success. The results of this study clearly indicate that i) a population of Daphnia harbors enough genotypic diversity and phenotypic variation to affect trophic interactions and ecosystem function, and ii) genotypic richness and phenotypic dissimilarity influence consumer-resource dynamics in important and unique ways.