Role of abiotic factors and habitat heterogeneity in the interactions between stream salamanders and crayfish in the southern Appalachians

Species interactions are important for determining the biological organization of natural communities. Presumably, as interacting organisms become more similar in resource niches, the magnitude of competition strength increases. However, the strength of interactions can be context dependent and mitigated by abiotic factors. In western North Carolina, headwater-stream salamanders and crayfish coexist across broad and fine spatial scales throughout their ranges. These dissimilar taxa occupy similar ecological niches within streams, yet there is limited understanding of the role that species interactions play in their local coexistence at fine spatial scales. We used both in-situ and ex-situ methods to evaluate the mechanisms promoting coexistence between stream salamanders (Desmognathus quadramaculatus [Holbrook, 1840] and Desmognathus marmoratus [Moore, 1899]) and Appalachian Brook Crayfish (Cambarus bartonii [Fabricius, 1798]). We conducted field surveys and documented factors potentially associated with refuge cohabitation between crayfish and salamanders within natural streams. We also used a stream mesocosm experiment to test if refuge density and competitor identity affected salamander and crayfish growth and behavior. In our in-situ approach, we found low interspecific cohabitation among desmognathan salamanders and C. bartonii with no detected effects of abiotic or biotic factors. Similarly, in our ex-situ experiment, we found that neither refuge density nor the presence of hetero- or conspecifics influenced the frequency of cohabitation and refuge use, growth, and mortality of D. quadramaculatus and C. bartonii. Although it is possible that local adaptation facilitates coexistence between our focal species, it is more likely that other abiotic (e.g., flow or temperature) and biotic factors (e.g., predators and other community members) in headwater streams besides the presence or absence of our focal taxa affect their distributions, or their interactions are size structured. These results support existing concepts that distantly related species are less likely to compete because of their morphological and phylogenetic dissimilarities, but future evaluations of interactions through time, space, and ontogeny would be useful to fully understand how these 2 taxa interact in headwater streams.