ECOLOGY OF ECOTONES: INTERACTIONS BETWEEN SALAMANDERS ON A COMPLEX ENVIRONMENTAL GRADIENT

We investigated factors that define boundaries between assemblages of streamside and terrestrial plethodontid salamanders by manipulating the moisture gradient in the ecotone between headwater streams and upland forests in the Allegheny Mountains of Virginia. Two experiments tested the hypothesis that microhabitat distributions of species change in response to competitors whose distributions are controlled by variation across the landscape at a larger scale. In the first experiment we created replicated seeps (continuously moist patches) 3 m and 15 m from streams. Desmognathus fuscus , a streamside species, began to increase in density at streamside seeps and to colonize distant seeps within 1.5 mo of their construction. Plethodon cinereus , a small terrestrial species, initially responded favorably to distant seeps but was displaced as seeps were colonized by streamside species. The D. fuscus colonists remained on distant plots with seeps 22 mo after water to seeps was turned off and the habitat had returned to pretreatment conditions. On streamside plots P. cinereus occurred at low densities initially and was quickly displaced by D. fuscus from portions of the plots near seeps. Gyrinophilus porphyriticus , a large streamside species, and Plethodon glutinosus , a large terrestrial species, were less abundant but exhibited responses that were similar to those of D. fuscus and P. cinereus , respectively. In the second experiment we created seeps in upland forests >30 m from the nearest stream or natural seep, more than twice as far from a source of colonists as the distant seeps in the first experiment. The creation of seeps was crossed with the addition of logs and rocks as cover objects in a replicated factorial design. Plethodon cinereus became more abundant and had greater mass adjusted for body size on plots with seeps than on control plots. Abundance of P. cinereus also increased with the addition of cover objects. Plethodon glutinosus was unaffected by the seeps. Streamside salamanders were rare or absent on all plots. These experiments demonstrate that the response of a species to an environmental perturbation can depend on the spatial context and presence of competitors or predators, and that the response may persist much longer than the perturbation itself.