Competition in Two Species of Larval Salamanders: A Test of Geographic Variation in Competitive Ability

Abstract

We studied the competitive interactions of two species of larval salamanders (Ambystoma maculatum and A. talpoideum) which occur either syntopically or allotopically in parts of their common geographic range (Mississippi and South Carolina, respectively). Larvae from both geographic regions were reared alone and combined at two total densities in artificial ponds to test a hypothesis of geographic variation in competitive ability. Although both species exhibited negative responses to increasing total density, neither showed geographic variation according to species origin or competitor origin. Ambystoma talpoideum exhibited reduced growth rate, final body size, and metamorphosis at high total density; A. maculatum exhibited only reduced growth rate and final body size. At equal total density, the body size of A. talpoideum was higher when reared with heterospecifics compared to those reared with conspecifics; whereas A. maculatum showed a slightly reduced body size when reared with heterospecifics compared to conspecifics. Our data indicate that A. talpoideum from both regions were competitively superior, and supporting studies suggest that the aggressive behavior of this species can be used to explain this outcome, independent of body size differences or other mechanisms. The lack of geographic variation in species responses suggests to us that the competitive environments in both regions are similar and that interactions of other factors in the aquatic or the terrestrial environment may influence their local distributions.