How do niche and fitness differences affect the coexistence among species?

Background/Question/Methods

A major goal of community ecology is to understand mechanisms underlying species coexistence. Modern theory of species coexistence suggests that species fitness and niche difference combine to influence their coexistence, such that large fitness difference favors competitive exclusion and large niche difference favors coexistence. For coexistence to take place between species, niche difference must overcome fitness difference.  We tested this theory using freshwater bacterivorous ciliated protist communities established in laboratory microcosms. Species niche differences were quantified via measuring impacts on their bacterial prey communities, and fitness differences were quantified via measuring population growth in pairwise competition experiments.  

Results/Conclusions

Our results showed that fitness difference, not niche difference, was significantly related to species coexistence. Increasing species niche difference, however, was associated with increased community biovolume, suggesting increased complementarity effects among communities with more diverse niches. These results indicate that species niche and fitness differences can strongly influence community assembly and ecosystem functions.