An empirical test of stable species coexistence in an amphipod species complex.

The ability of phenotypically similar species to coexist at local scales is paradoxical given that species that closely resemble each other should compete strongly for resources and thus be subject to competitive exclusion. Although theory has identified the key requirements for species to stably coexist, empirical tests of coexistence have rarely been conducted. We explored a key requirement for species to stably coexist: a species can invade a community when it is initially rare. We also assessed whether primary productivity (manipulated using phosphorus availability) affected invasion success by increasing the amount of resources available. Using two mesocosm experiments and an assemblage of phenotypically similar amphipod species in the genus Hyalella, we found no evidence for invasion success among the three Hyalella species. Further, patterns of species exclusions differed among the species, which suggests that one species is an especially poor competitor. Finally, these patterns were consistent regardless of whether mesocosms were fertilized with low or high levels of phosphorus. Our results, suggest that species differences in resource competition and predator avoidance ability found in previous studies using these Hyalella species may not be sufficient to allow for coexistence. Moreover, our study demonstrates the importance of using a variety of empirical approaches to test species coexistence theory.