Niche partitioning and the evolution of character displacement continue to be driving concepts in ecology, yet remain controversial after over 50 years of study. Observational support for character displacement continues to accumulate, although conflicting patterns have also been documented. Meanwhile, theory on the evolution of competitors is ahead of empirical work, predicting a broad set of possible evolutionary outcomes other than simple divergence. This theory remains largely untested by experiments, which require following the evolution of individual competing species, preferably while in multispecies systems. We are using the well-characterized protozoan community found in pitcher-plant leaves to follow the evolutionary trajectories of species both in monoculture and in different competitive mixtures to determine how “community context” affects the evolution of competitive ability. Simultaneously, we are determining how the magnitude of competitive interactions correlates with resource-use by using “tag” sequencing methods to characterize the bacterial communities in each of our mixtures. We can ask not just if species converge or diverge in resource use, but perhaps when they converge or diverge.
Results/Conclusions
The experiments measured the competitive abilities and resource-use of each protozoa species, then allowed species to undergo selection in monocultures or different mixtures, then re-measured the competitive abilities and resource-use. The five species of protozoa used exhibited a range of competitive abilities, where a high competitive effect on other species is generally correlated with low competitive response to other species. The bacterial data suggests there are different ways to be a “good” competitor. Some species simply consume lots of bacteria, reducing availability for others, while other species specialize on a unique suite of bacteria and so dominate a particular resource “niche”. Monod experiments show that dominant competitors do not survive as well as subordinate competitors at low resource levels, which contradicts the idea that competition only is occurring through resource consumption. Selection for over 150 generations does result in evolved differences in competitive performance among protozoa species. However, not all species respond to selection and no clear patterns appear between subordinate and dominant species or between selection in monocultures and mixtures. The “take-home” message is that (1) competitive performance does evolve, but that (2) these protozoa may have complex feeding behavior, resulting in complicated evolutionary patterns in multispecies communities.