The ecological importance of diversity within species

Background/Question/Methods

Ecologists have long explored the implications of differences among species or groups of functionally similar species, but the ecological importance of variation within species has received less attention. There is rapidly growing evidence that genetic and phenotypic variation within species can have important effects on community and ecosystem processes. In some cases, variation within a single keystone species may have effects as large as those from the incidence of that keystone species. The general importance of variation within species, compared to variation among species, is not yet clear.  Here we conduct a meta-analysis to ask: how important is variation within species relative to variation among species for community and ecosystem function? Our meta-analysis takes advantage of two complementary methods by which ecologists have quantified the importance of species and genotype: removal and replacement studies, and biodiversity studies. From the removal and replacement studies, we quantified a species effect and an intraspecific effect, and then calculated the log ratio of those two effects as a measure of the relative strength of the two forms of diversity. From the biodiversity studies we calculated the CV for ANPP and arthropod responses among monocultures (of species or genotype), and the effect size of increasing diversity from the monoculture to the full polyculture for ANPP and arthropod responses.

Results/Conclusions

We found evidence for very strong intraspecific effects across all of the removal and replacement studies.  For animal studies, the log ratio of intraspecific to species effect size was not significantly different than zero, indicating that intraspecific effects were of similar magnitude to species effects. For plant studies, the log ratio of intraspecific to species effects was significantly greater than zero, indicating that intraspecific effects were much stronger than species effects.  For diversity studies, we found no significant difference in the CV among monocultures of genotypes or species for ANPP and arthropod community structure, indicating that there was a similar amount of phenotypic variation among and within species. In contrast, increasing species richness had much greater effects on ANPP and arthropod community structure than increasing genetic richness, indicating that increasing genetic diversity has much less effect on communities and ecosystems than increasing species diversity. Overall, we found that effects of variation within species can be as large or larger than species effect. Our results suggest variation within species should receive greater attention among in community and ecosystem ecology, and for conservation and restoration.