Competition leads to an extinction-prone species of salamander: Interspecific territoriality in a metapopulation

The Shenandoah salamander, Plethonon shenandoah, is a federally endangered species that is restricted to Pleistocene-age talus on three mountains in Shenandoah National Park, Virginia, USA. Each population is surrounded by the red-backed salamander, P. cinereus, which defends intraspecific territories on the forest floor. On Hawksbill Mountain, P. shenandoah consists of a metapopulation, with a large "source" subpopulation on the top of the mountain and at least five small "sink" subpopulations below. Here we report that P. shenandoah is constrained in escaping from the talus into deep soil, and thus in emigrating from source to sink subpopulations, by interspecific territoriality from P. cinereus. We performed competitive release experiments during two summers by removing P. cinereus from under experimental rocks while leaving territorial residents in place under control rocks. Plethodon shenandoah responded by invading experimental rocks significantly more frequently than control rocks in both years. Quadrat analyses indicated that only a small number of individuals escaped from the talus at any given time. Behavioral experiments revealed no significant differences in propensity to bite by the two species, indicating that more subtle tactics are employed by P. cinereus in excluding P. shenandoah from territorial sites. One sink subpopulation of P. shenandoah became extinct in 1970 as a consequence of a severe drought, which affected areas of talus more drastically than areas of deep soil containing P. cinereus. Our data now suggest that interspecific territoriality by P. cinereus can contribute to the extinction-prone status of P, shenandoah by inhibiting movements from source to more distant sink subpopulations.