Competing species often coexist, but the mechanisms allowing long-term coexistence are rarely tested via direct experimental manipulation. We experimentally tested the mechanisms of coexistence in a classic model system, laboratory microcosms in which two species of ciliate protists competed for bacteria. Previous work shows that the species used here compete for bacteria, but can coexist despite large differences in grazing ability. We tested three hypotheses that might explain this surprising coexistence: resource partitioning, chemically-mediated interference competition, and differential use of space. To test for resource partitioning, we conducted an experiment testing the effects of bacterial species richness and composition on the long-term outcome of competition. Manipulating bacterial diversity and composition alters the scope for resource partitioning. Despite strong evidence for differential resource use (e.g., the two ciliates shifted bacterial species composition in different ways), initial bacterial richness and composition did not affect the long-term outcome of competition. Remarkably, the competitive outcome was unchanged even when ciliates competed for a single bacterial species, indicating that the observed resource partitioning is irrelevant to the competitive outcome. In further experiments, we ruled out differential space use and chemically-mediated interference competition as explanations for this surprising coexistence. Coexistence of ciliates on a single bacterial species might reflect partitioning of intraspecific bacterial diversity, and/or osmotrophy or consumption of particulate detritus by the weaker competitor. The results show that this classic model system is not as well-understood as had been previously thought. More broadly, the results dramatically illustrate that merely observing “niche differences” between coexisting species is no evidence that those differences are either necessary or sufficient for long-term coexistence.
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