Can evolutionary history help explain coexistence and the mechanisms behind it?

Background/Question/Methods

Explaining the diversity of life has been a quest in ecology since Darwin. In the 1920s,Volterra proposed that species coexist despite competition because they exploit unique niches. The concept of limiting similarity also suggests that species should evolve in such a way as to limit interspecific competition and promote niche partitioning. This idea became dogma for several decades. In a seminal paper, Chesson unified niche and neutral mechanisms of coexistence, and showed that species will display stable, long-term coexistence if their niche differences (or stabilizing forces) are larger than their fitness differences.

Using a laboratory microcosm experiment, we tested whether species' evolutionary histories can affect their coexistence and the balance of niche differences and fitness equalities among them. We hypothesized that distantly related species are more likely to coexist overall, because they are more likely to display niche partitioning. In contrast however, we expected that closely related species may also coexist due to the small size of their fitness differences.  To test these hypotheses, we performed a laboratory experiment using nine species of green freshwater algae. We selected these species so that their pairwise combinations would span a wide range of phylogenetic distances based on our molecular phylogeny. After growing each species to a steady-state biomass, we separately introduced each of the eight other species to an established population, measured 'growth rates when rare' (invasion rates), and calculated 'mutual invasibility' (a criteria for coexistence). By comparing demographic parameters in monoculture and in mixture, we were able to determine how niche differences and fitness inequalities vary with phylogenetic distance.

Results/Conclusions

Upon examination of 27 of the 36 total pairwise species combinations, coexistence occurred in 30% of the combinations. The probability of coexistence (mutual invasion) was not significantly related to phylogenetic distance among phytoplankton species (logistic regression, P=0.187). In spite of there being no general relationship between coexistence and phylogenetic distance, there was evidence that evolutionary history alters the mechanisms of coexistence. For example, Staurastrum punctulatum coexisted with one closely- and one distantly-related species (phylogenetic distance = 0.028 for Cosmarium botrytis, 0.1082 for Elakatothrix viridis).  Contrary to our expectations, however, the average niche difference between the distantly related pair was  less than 20% the size of that for the closely related pair, while the fitness difference was less than half the size. This suggests that more closely related species undergo stronger competition and partition niches more strongly.