Species niche and fitness differences influence community assembly and ecosystem functioning
Modern ecological theory emphasizes the importance of niche and fitness differences for species coexistence. Several empirical studies supported the prediction that niche difference between competing species must overcome fitness difference to permit coexistence. However, this coexistence theory has rarely been linked to community assembly and ecosystem functioning. Increasing niche and fitness differences may result in more deterministic competitive outcomes, leading to weaker historical contingency of community assembly. On the other hand, increasing niche and fitness differences may promote ecosystem functioning, as they may contribute to niche complementarity between species and the positive selection effect, respectively. We tested these hypotheses with a laboratory microcosm experiment involving six bacterivorous protist species. We estimated the niche difference between each pair of species based on the differences in their impacts on bacterial prey community structure, and quantified the fitness difference based on their relative competitive abilities. We manipulated the assembly history of communities of each possible pair of competing species, and measured the total biovolume of protists and the total abundance of bacteria as two focal ecosystem functions.
We found that the strength of priority effects, measured as dissimilarity in the structure of communities experiencing different assembly histories, decreased with increasing fitness difference but was unaffected by niche difference. When two competing species had similar fitness, we observed alternative community states subjected to different assembly histories, with earlier colonizers often being more abundant. By contrast, when large fitness difference existed between species, we found that communities were generally dominated by high-fitness species and often unaffected by introduction orders. Furthermore, we found that protist community biovolume were positively and bacterial abundance negatively correlated with protist niche difference but not fitness difference, indicating the more important role of niche complementarity in regulating ecosystem functioning. Our results illustrate the value of elucidating the mechanism of species coexistence for better understanding community structure and ecosystem functioning.