Range Limits and Demography of a Mountaintop Endemic Salamander and Its Widespread Competitor

Range limits can be caused by a multitude of abiotic or biotic factors, but all of these must act through the demography of range-edge populations. Woodland salamanders of the genus Plethodon often exhibit distinct range boundaries where the distributions of competing species meet. Because of their high densities and low mobility, Plethodon are well suited for studies of how fitness-related traits change as species approach their range limits. Across contact zones between the mountaintop endemic Peaks of Otter Salamander (Plethodon hubrichti) and the widespread Eastern Redback Salamander (Plethodon cinereus), we measured changes in three salamander traits: 1) body condition, 2) frequency of tail loss, and 3) proportion of hatchlings. We then used hierarchical Bayesian models to compare these traits among five site types: allopatric sites for both species, sites where one of the species was dominant and the other was rare, and mixed sites containing high densities of both species. For P. hubrichti, we found no consistent changes in body condition across contact zones. However, frequency of tail loss increased continuously from allopatric sites (21%) to rare sites (54%). We also found evidence of reduced hatchling proportions at sites outside of allopatric areas (15–16% versus 30% at allopatric sites). For P. cinereus, body condition was higher at allopatric sites compared to sites within the contact zone. Similar to P. hubrichti, frequency of tail loss in P. cinereus increased continuously from allopatric sites (27%) to sites where P. cinereus were rare (50%). However, for P. cinereus, we did not find evidence of reduced hatchling numbers towards the edge of their range margin. Overall, our results suggest that both species likely have reduced fitness as they approach their range margin. Tail loss, which may reflect interference competition, effects of predation, or interactions between these, could potentially act as a density-dependent factor that stabilizes the range boundary between these species, at least over shorter time scales.