Evolutionary relatedness does not predict ecological similarity of freshwater green algae

Background/Question/Methods

Darwin proposed >150-yrs ago that closely related species are more ecologically similar to each other than distantly related species.  In turn, he argued closely related species should compete more strongly and be less likely to coexist.  For much of the last century, Darwin's hypothesis was taken at face value, and sometimes rose to the status of dogma among those who proposed that conservation and restoration of evolutionary diversity could help maximize the resilience, stability, and ecological functioning of ecosystems.  Despite its intuitive appeal, the competition-relatedness hypothesis has received surprisingly little testing.  Here we present results from a suite of laboratory and field studies that used freshwater green algae to determine if evolutionary relatedness impacts species interactions, trait similarity, and patterns of co-occurrence.  We cultured and sequenced ~60 of the most common species of green algae in lakes across North America to generate a robust molecular phylogeny describing evolutionary relationships.  We then (a) performed laboratory competition experiments in which we manipulated the genetic distance among species and measured interaction strengths and niche overlap, (b) examined the evolutionary conservatism of biological traits that underlie interactions, and (c) explored whether patterns of species co-occurrence in real lakes are predicted by relatedness.

Results/Conclusions

Using lab experiments in which we grew 8 common algal species in mono- and biculture, we fit time series of population dynamics to Lotka-Volterra competition equations to estimate interaction coefficients (alphas).  Even though alphas clearly predicted the winners/losers of competition, interaction strengths were independent of phylogenetic distance (F_1,23 = 0.56, P = 0.46).  We then expanded the species pool and ran experiments using 216 pair-wise combinations of algae, but still found no evidence that phylogenetic distance impacts species relative yields (biomass in mono- vs. biculture, P = 0.19).  Following this, we experimentally quantified key biological traits thought to dictate competition for >50 algal species and found the minimum resource levels needed to sustain positive growth (R* values for N, P, light) were not evolutionarily conserved (P > 0.10 for all estimates of Bloomberg's K).  Lastly, we examined patterns of species co-occurrence across >1,200 North American lakes and found that the probabilities of co-occurrence were not significantly different than the random expectation of a null model.  These diverse empirical studies are all congruent, and suggest the mechanisms of competition may not always be evolutionarily conserved. Therefore, biologists may need to re-evaluate the often assumed generality of Darwin's competition-relatedness hypothesis.