Phylogenetic distance does not predict competitive interactions in protists
Competition for resources is an important process structuring communities, but competitive outcomes can be very difficult to quantify in natural systems. Many have proposed that phylogenetic distance between species may be a good proxy for the strength of competitive interactions, as more closely related species should have similar resource use. Here we tested whether phylogenetic distance predicts competitive interaction strengths between protists. We selected 8 protists at different taxonomic distances to compete in an additive competition design; the competitive effects and responses of each species were determined. We then sequenced the SSU rRNA gene of each species to create a phylogeny using maximum likelihood methods in order to estimate phylogenetic distance.
We found no linear relationship between phylogenetic distance and competitive interactions. Instead, closely related and distantly related species have no significant competitive effect on each other, whereas protists of intermediate relatedness have high variance in competitive effects and responses. These results are notably different from those obtained by other researchers who found a relationship between phylogeny and coexistence in ciliated protists, as well as other taxonomic groups. Our results suggest that processes like rapid sympatric evolution of closely related species may result in a break down of the hypothesized relationship between phylogeny and resource use.