Trade-off between competitive ability and antipredator adaptation in a freshwater amphipod species complex

Body size influences many physiological and ecological processes, and thus adaptive change in body size may hold diverse consequences for individuals. For amphipods in the Hyalella azteca species complex, divergence in body size appears to be driven by adaptation to disparate regimes of size-biased mortality experienced in different habitat types. Here I ask how size and size-related traits influence competitive ability of large-bodied and small-bodied species. I examined competitive ability in both species using a factorial experiment in which the performance of each species was assessed in all combinations of the presence and absence of the heterospecific amphipod species crossed with the presence and absence of a snail species. Results demonstrated that the large and small Hyalella species differ substantially in competitive ability, with the large species being the superior general competitor. The competitive effect of the large species on the small species was substantial and caused significant reductions in abundance, body size, and fecundity of the small species. In contrast, performance of the large species was not affected significantly by presence of the small species. This competitive asymmetry was also seen in competitive interactions with snails. The small species appears to "trade off" competitive ability in order to survive in habitats with predatory fish, whereas the large species. which is able to obtain a size refuge from predation, has traits that confer strong competitive ability. Because of habitat differences in the cost-benefit structure of the trade-off, however, competition-mediated natural selection may be more intense in the large species than in the small species, a condition that may enhance evolutionary divergence in body size.