Factors Facilitating Co-occurrence at the Range Boundary of Shenandoah and Red-Backed Salamanders
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.Abstract
Hypotheses to test for measured traits and behaviors of Shenandoah (gray) and Red-Backed (dark gray) Salamanders across the Shenandoah Salamander (PSHE), co-occurrence, and Red-Backed Salamander (PCIN) zones. For the pictured example of salamander body size (SVL), larger Shenandoah and smaller Red-Backed Salamanders in only the co-occurrence zone would support H1. Species differentiation (H2) indicates species are consistently different across all zones. Character convergence by zone (H3) occurs when both species are similar in only certain zones. Zonal differences (H4) indicate size differs by zone but similarly for both species. Last, complete convergence (H5) indicates no difference across zones or species. Shenandoah Salamanders are depicted as the larger species in this example based on Jaeger (1972).
Morphological traits and population parameters differed by species (Shenandoah [PSHE, triangles] and Red-Backed Salamanders [PCIN, circles]), transect zone (diamonds), or both. The co-occur zone indicates the co-occurrence zone where there was a high probability of both species vs. the single-species zones. Shown are proportions with SEs of salamanders (A) with a striped dorsal surface, indicating differences by species (B) with damaged tails, indicating differences by species by zone, (C) that are female, indicating differences by species, and (D) that are adults, indicating differences by zone.
Morphological stripe traits and microhabitat selection behavior differed either by species (Shenandoah [PSHE] and Red-Backed Salamanders [PCIN]) or species by transect zone. The co-occur zone indicates the co-occurrence zone where there was a high probability of both species vs. the single-species zones. Percentages of salamanders (left panels) with different stripe colors, indicating differences by species (except in the case of yellow stripes where differences were non-significant) and (right panels) using microhabitat types, indicating differences by zone by species (log) or by zone and species (leaf litter, rock).
Mean SVL of captured salamanders. Bars represent SEs. An interaction between species (Shenandoah [PSHE] and Red-Backed [PCIN] Salamander) and zone best explained differences in body size for both metrics. The co-occur zone indicates the co-occurrence zone where there was a high probability of both species vs. the single-species zones.
General linear model results from our best-supported microhabitat temperature-use model indicated support for an interaction of species (Shenandoah [PSHE] and Red-Backed [PCIN] Salamander) and zone. The co-occur zone indicates the co-occurrence zone where there was a high probability of both species vs. the single-species zones. The 95% confidence intervals are shown in gray. Each salamander species used microhabitats more frequently in each species' respective home range, but microhabitat use similarly declined as temperature increased for both species.
