Document: GAR-3-71-4

Tradeoff between competitive ability and antipredator adaptation in a freshwater amphipod species complex.

WELLBORN, G.A.*

University of Oklahoma, Norman, OK 73019, U.S.A. ¹

Abstract:
Because body size influences many physiological and ecological processes, tradeoffs associated with adaptive change in body size will produce manifold effects for individuals. Within the Hyalella azteca (Crustacea: Amphipoda) species complex, there is a striking pattern of diversification in body size, with large-bodied species occurring in fishless habitats and small-bodied species occurring in habitats with centrarchid fish. This divergence in body size appears to be driven by adaptation to the disparate regimes of size-biased mortality experienced in the two habitat types. I explored some consequences of this predation-mediated adaptation in body size by asking how size and size-related traits influence competitive ability of the large and small species. I examined competitive ability in large and small Hyalella species by conducting a factorial experiment in which the performance of each amphipod species was assessed in all combinations of the presence and absence of the heterospecific amphipod species crossed with the presence and absence of a pulmonate snail, Physella virgata. Results of this study indicate that the large and small Hyalella species differ substantially in competitive ability, with the large species being the superior general competitor. The competitive impact of the large species on the small species was substantial and caused significant reductions in abundance, body size, and fecundity of the small species. For example, relative to the treatment in which the small species was alone, the presence of the large species caused a 7-fold reduction in the final abundance and a 25-fold reduction in fecundity of the small species. In contrast, performance of the large species was not significantly affected by presence of the small species. This competitive asymmetry was also seen in competitive interactions with snails. For example, snail density was significantly lower in the presence of the large Hyalella species than in the presence of the small species. The large and small Hyalella species also differed significantly in two size-dependent traits, foraging rate and activity level, and these trait differences may form the mechanistic basis of the competitive asymmetry between the species.

Keywords: Competition, Hyalella, predation

This abstract is being presented at: 1:45 PM in session:
Oral Session #48: Anti-Predator Responses: Fish to Sagebrush.