This study examines the mutual competitive effects among larvae of two syntopic salamanders, Ambystoma macrodactylum and Ambystoma gracile. Evidence for competitive equivalence and constant per capita effects among these syntopic species could justify the use of simple models for describing competition. The species were raised alone and together at two densities in replicated artificial ponds. Hatchlings of Ambystoma macrodactylum were smaller than A. gracile, yet larger at the end of the experiment when A. macrodactylum began to metamorphose. Competition was detected as a decrease in growth rate with increasing density, but competition was not so strong as to influence the survival of either species. Ambystoma macrodactylum grew slowly relative to heterospecifics, and growth declined more with increasing conspecific density than with heterospecific density. In contrast, growth of A. gracile was depressed more by increasing density of the slower growing heterospecifics. These differences in effects of competition likely arise from differing breeding phenology—A. macrodactylum breeds earlier than A. gracile, and A. macrodactylum was larger than A. gracile when the latter were added to the experimental ponds. The results provide no evidence for competitive differences or interaction at the low density used here. Assuming a coexistence mechanism exists to offset the competitive asymmetry at high density, I present a testable hypothesis on the mechanism of differential susceptibility of the species to predation.
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Vol. 58 • No. 2