Factors Facilitating Co-occurrence at the Range Boundary of Shenandoah and Red-Backed Salamanders

Abstract

The transition from species in allopatry to sympatry, that is, the co-occurrence zone, allows for investigation of forces structuring range limits and provides evidence of the evolutionary and population responses of competing species, including mechanisms facilitating co-occurrence (e.g., character displacement). The Shenandoah Salamander (Plethodon shenandoah), an endangered plethodontid, is limited to three mountaintops in Shenandoah National Park, Virginia, USA. This species' distributional limits are attributed to competitive exclusion by the Red-Backed Salamander (Plethodon cinereus). Recent work showed range overlap between these species is greater than previously thought, requiring investigation of species morphology, behavior, and demographic measures in single-species and co-occurrence zones that might facilitate such overlap. We analyzed individual characteristics from 2 yr of transect surveys to see whether traits differed within and outside co-occurrence zones. Measures showed species- and zonal-specific differences, but we found limited support for character displacement. Both species were larger in the co-occurrence zone, indicating larger salamanders might better compete for resources or that symmetric competition restricts dispersal or recruitment processes at the co-occurrence zone. Microhabitat use also differed by species across transects, with Red-Backed Salamanders using more rock microhabitats in the co-occurrence zone, potentially because of competition for microclimates that minimize physiological stress. The lack of strong evidence of differentiation in situ at the range edge suggests weaker character displacement and potentially weaker competition than previously thought, with other factors contributing to the range limits of Shenandoah Salamanders.