Phylogenetic diversity affects biomass production through two opposing forces

Background/Question/Methods

Over the past few decades, >400 experiments have shown that species rich ecosystems have greater biomass production than less diverse systems.  Recent meta-analyses of these experiments have shown that phylogenetic diversity (PD) - a measure of the amount of genetic divergence among species that has occurred through evolutionary time - explains more variation in biomass among experiments than species richness per se.  The more genetically dissimilar species are, the greater the impacts on biomass production for any given level of richness.

It has been assumed that the relatively strong link between PD and biomass production is the result of niche differences among species that grow increasingly strong through evolutionary time.  But an alternative possibility is that PD is simply proportional to the evolutionary time it takes to produce a 'super-species' - a superior competitor that is unusually productive.  To examine the contribution of these two alternatives to PD-biomass relationships, we developed a molecular phylogeny for 22 common species of freshwater phytoplankton, and grew them in mono- and bicultures in laboratory well-plates.  After 45 days of growth, we measured species’ biovolumes at steady state, and used Fox’s tripartite partition to parse the net effect of diversity on biovolume into three additive components: trait independent complementarity, trait dependent complementarity, and competitive dominance. 

Results/Conclusions

Contrary to our a priori expectation, results from 42 pairwise species combinations suggest that biomass production did not vary as a function of PD.  But interestingly, this lack of a relationship was caused by an opposing influence of PD on two distinct factors that contributed to biovolume of the bicultures.  First, trait independent complementarity grew more positive with PD (p = 0.02), suggesting that combinations of species that are evolutionarily distinct are more likely to exhibit some form of ecological differentiation that enhances biomass production.  But second, the dominance effect also increased with PD (p = 0.04).  Because the competitively superior species had low biomass, species bicultures that were evolutionarily distinct had reduced biomass.  Results suggest the effects of species composition and effects of diversity per se on production change with evolutionary history, but they do so in opposing ways that cancel each other out.