Theoretical evolutionary ecology generates testable predictions concerning the effects of species interactions on ecological, behavioral, and morphological traits. When patterns of morphology, resource use, or behavior covary as predicted from theory, this is treated as evidence that a particular interaction is an important force in the community. Throughout Ohio, two closely related and ecologically similar salamander species, Plethodon cinereus and P. electromorphus, occur in similar habitats and can be found in sympatry. However, the occurrence of sympatry is infrequent compared to the range overlap of each species and seems to be limited by factors other than broad geographic factors. Behavioral studies between these two species reveal that aggression of both species is greater in sympatry than where they occur in allopatry, consistent with the hypothesis of alpha-selection, where increased aggression is the result of interspecific competition. Here I examined head shape variation among allopatric and sympatric populations of P. cinereus and P. electromorphus in Ohio. In addition, I compared resource use (food and cover objects) in each species from allopatry and sympatry.
Results/Conclusions

I found significant phenotypic differentiation in head shape between species, between environments, and a significant interaction between species and environment. Significant phenotypic variation across all populations was observed.When patterns among allopatric and sympatric populations were examined, I found that the average sympatric divergence at within a site was significantly greater than expected by chance. Overall, P. cinereus and P. electromorphus from allopatry or sympatry did not differentiate types of cover objects used or the types of prey consumed. However, I did find that when species in sympatry showed convergence in head shape, food use was similar between species and when species in sympatry were different in head shape, food use differed as well. Head morphology differences are suggested to be a result of difference in resource use between the two species. These results suggest that sympatry may represent an unstable equilibrium where neither species can gain the competitive advantage, but that each species is able to prevent the other from expanding into allopatric areas. Furthermore, the intensity of competition may differ between sympatric sites resulting in different observable outcomes.