Phylogenetic diversity affects the temporal stability of communities by altering species’ functioning and competitive interactions.

Background/Question/Methods

A growing amount of empirical evidence shows that increasing biological diversity tends to reduce the variability (increase stability) of community biomass through time in face of environmental fluctuations. The presumed biological mechanisms by which biodiversity confers stability include overyielding and compensatory dynamics. Their relative contributions to the effect of biodiversity on stability remain a significant source of controversy. We hypothesized that overyielding and compensatory dynamics depend on the evolutionary history of co-occurring species. To assess how each mechanism influences the diversity-stability relationship, we performed a laboratory microcosm experiment using nine widely distributed species of green algae for which we developed a gene-based phylogeny.  We grew monocultures and bicultures in a constant temperature environment and in a fluctuating temperature environment, and measure biovolumes every few days over three weeks. We then partitioned the coefficient of variation of biovolume into the mean temporal biovolume, the sum of individual species variances and the sum of individual species covariances and established how strongly phylogenetic diversity related to each component. Our experimental design also allowed us to determine the relative importance of species interactions and environmental fluctuations in driving the species asynchrony responsible for community stability.   

Results/Conclusions

Contrary to some prior studies, community stability (the inverse of the temporal coefficient of variation) decreased as biological diversity increased.  The primary driver of this trend was 'under-yielding' - bicultures had lower biovolumes than expected from monocultures, which was mainly due to the presence of a strong competitor with low productivity. When the low productivity competitor was excluded from analyses, the relationship between phylogenetic diversity and stability remained negative, but instead of stability being dictated by differences in yield among communities, stability for this subset of data was driven by changes in temporal covariance among species.  Species which were more evolutionarily divergent exhibited weaker competitive interactions, which reduced compensatory dynamics and, in turn, the stability of community biovolumes. Our results suggest that evolutionary history influences diversity-stability relationships via two phenomena.  First, evolution may generate species with singular ecological characteristics that play a major role in the functioning of ecosystems. Second, evolutionary history alters the strength of species interactions, which changes the covariance that stems from compensatory dynamics.