Historically, most ecologists have assumed that strong competitors will evolve to be increasingly different from one another (niche divergence), thereby reducing interactions. However, niche convergence has also been proposed as an evolutionary outcome of competition and may be important for explaining the origin of neutral species. Distinguishing between these competing hypotheses has been hampered by the lack of direct tests of the simultaneous evolution of multiple species in natural communities. We followed the evolution of four species of bacterivorous protozoa found in the leaves of the carnivorous pitcher plant, Sarracenia purpurea. Over a sequence of community ages (2, 7, 12, and 17 weeks, representing approximately 40, 150, 250, and 350 generations), we isolated the species from natural communities and created monocultures and two-species mixtures to quantify species interactions. We also grew each species at each age against a common competitor and with their natural predator, the mosquito Wyeomyia smithii. Using this design, we quantified the evolution of competitive interactions to determine whether species niche use diverges or converges through time and if there are simultaneous trade-offs with the evolution of responses to predators.
Results/Conclusions
The four species exhibited a relatively consistent hierarchy of competitive effect negatively correlated with species response, with the poorest competitor being Poterioochromonas sp. (small competitive effect on other species, large response to other species), followed by Bodo sp., Colpoda henneguyi, and Colpoda steinii. As the species evolved, the initially weak competitors increased in competitive ability, while the more dominant species increasingly suffered more from competition, resulting in a general trend of convergence. However, the competitively subordinate species evolved to be less affected by predation, while the evolution of the dominant species (Colpoda), caused it to be more vulnerable to predation. This may indicate an actually divergence in response to predation, as the small, subordinate species such as Bodo sp. already are less affected by predation than larger dominate species such as the Colpoda. While the resulting patterns of species evolution are complex, they generally support the idea of convergence of competitors. This finding of evolutionary convergence in competitors is remarkable and suggests a mechanism by which ecologically equivalent species might evolve. Community-wide evolutionary patterns in response to competition and predation have important implications for our understanding of natural assemblages.
