Models of species distributions have historically focused on the role of climate as a primary ecological filter determining species distributions, but research has highlighted the importance of species interactions in determining range boundaries. We test the role of species interactions in structuring range limits by modeling the range boundary and investigating individual traits and microhabitat use of two terrestrial salamanders. The Shenandoah salamander (Plethodon shenandoah) is an endangered salamander endemic to three mountain peaks in Shenandoah National Park, Virginia. The species range is encompassed by the range of the red-backed salamander (P. cinereus), and previous work has focused on the role of competitive exclusion and interspecific aggression in setting this range edge. We analyzed two years of transect surveys to gain better spatial resolution of the range boundary where these two species meet. First, we tested the importance of incorporating species interactions in modeling the range boundary in an occupancy framework by conditioning occupancy and detection probabilities of one species upon the probabilities of another. Additionally, these relationships are frequently spatially structured with occupancy probabilities varying by proximity to occupied neighboring sites. We also tested the inclusion of conditional autoregressive (CAR) random effects that allow for spatial relationships in these species interactions. Second, we investigated individual trait data in the overlap versus single-species zones to see if character displacement could describe co-occurrence of these species. We used chi-square and linear regression analyses to test if trait frequencies differed between zones.
Results/Conclusions
Range overlap was historically assumed to be minimal between the species, and raw data showed that both species were not frequently simultaneously observed. Spatial occupancy models including a CAR random effect that gave a higher resolution view of this range edge and the zone of co-occurrence outperformed non-spatial occupancy models. Cross validation of models showed that single species occupancy models with CAR random effects did the best job in capturing the range boundary, indicating that species interactions do not appear to manifest themselves at the scale of the range boundary. However, detection probabilities of red-backed salamanders were lower in the presence of Shenandoah salamanders, indicating some level of behavioral displacement. Individual characteristics of detected animals in these zones did differ significantly in life stage and cover object use, indicating that character displacement may facilitate co-occurrence at a smaller scale. Understanding the influence and scale of species interactions in setting range limits is necessary for accurate models of species distributions.