Evidence for the evolutionary diversity-productivity relationship at several scales

Cadotte, M. W. 2013. Experimental evidence that evolutionarily diverse assemblages result in higher productivity. Proceedings of the National Academy of Sciences.

John J. Stachowicz, Stephanie J. Kamel, A. Randall Hughes, and Richard K. Grosberg. Genetic Relatedness Influences Plant Biomass Accumulation in Eelgrass (Zostera marina). The American Naturalist, Vol. 181, No. 5 (May 2013), pp. 715-724

Ecology is increasingly recognizing the value of non-species based measures of diversity in relation to ecosystem services, community diversity and invasibility, and conservation activities. One result is that we are seeing increasingly strong and interesting experimental evidence for the importance of genetic diversity in understanding populations, species, and communities are structured. Two recent papers are good examples of how our understanding is progressing.

For example, we are now at the point where research has clearly demonstrated the relationship between ecosystem functioning and evolutionary history, and now well-designed experiments can begin to explore the mechanisms that underlie the ecosystem functioning-evolutionary diversity link. The oft-demonstrated correlation between evolutionary diversity and productivity is explained based on the assumption that ecological similarity and evolutionary relatedness are connected. Diverse communities are often thought to have lower niche overlap (i.e. higher complemenarity), but these experiments often rely on highly distinct species (such as a grass and a N-fixer), which could over-emphasize the importance of this relationship. In Cadotte (2013), independent manipulations of phylogenetic diversity and species richness allow the author to explore separately the role of complementarity and selection effects (the increased likelihood that a highly productive species will be present as species richness increases).

The experiment involved old field plots, planted with between 1 and 4 species chosen from a pool of 17 possible species; evolutionary diversity (high, medium, or low) and species richness are manipulated to include all possible combinations. The study found found a much stronger relationship between phylogenetic diversity (PD) and biomass production then between species richness and biomass production, but this isn't especially novel. What is interesting is that it could also identify how selection effects and complementarity were driving this response. High levels of complementarity were associated with high levels of PD: polyculture plots with high complementarity values were much more likely to show transgressive overyielding. Plots with close relatives had a negative or negligible complementarity effect (negative suggesting competitive or other inhibitory interactions). There was also evidence for a selection effect, which was best captured by an abundance-weighted measure of evolutionary diversity (IAC), which measured the abundance of closely related species in a plot. Together, PD and IAC explain 60% of the variation in biomass production.

From Cadotte (2013).

The second study asks the exact same question – what is the relationship between biomass production and genetic diversity - but within populations. Stachowicz et al. (2013) looked at genetic relatedness among individuals in monocultures of the eelgrass Zostera marina and its relationship to productivity. Variation within a species has many of the same implications as variation within a community – high intraspecific variation might increase complementarity and diverse assemblages might also contain more productive genotypes leading to a selection effect. On the other hand, it is possible that closely related, locally adapted genotypes might be most productive despite their low genotypic variation. 

Similar to most community-level experiments, Stachowicz et al. found that looking at past experimental data suggested there was a strong relationship between genetic relatedness and biomass/density in eelgrass beds. Taxa (i.e. the number of genotypes) tended to be a poorer predictor of productivity. However, the relationship was in the opposite direction usually seen – increasing relatedness predicted higher biomass. This is difficult to explain, since it goes against the expected direction of complementarity or selection effects. Possibly cooperative/facilitative relationships are important in eelgrass monocultures. Data obtained from field surveys (rather than experimental data) suggested an alternative: possibly these studies didn’t cover a large enough range of relatedness. This field data covered a much larger range of relatedness values, and showed a unimodal relationship (below), indicating that the productivity-relatedness relationship had an optimum, where highly related or highly diverse assemblages were less productive. Although further work needs to be done, this is an intriguing possibility.

From Stachowicz et al. (2013). Grey dots represent range of relatedness values from experimental data only, compared to range covered by field survey.

At some scales, ecologists are now refining what we know about popular research questions, while at others we are just scratching the surface. Stachowicz et al. suggest that as we scale up or down our expectations should differ -  “the slope and direction of the relationship between genetic differentiation and ecological functioning might depend on the genetic scale under consideration”.

(Disclaimer - obviously Marc Cadotte was my PhD supervisor until very recently. But I think it's a nice paper, regardless, and worth a post :) )