Moisture as a factor influencing the distributions of two species of terrestrial salamanders

Plethodon richmondi shenandoah occurs in at least three geographically isolated talus slopes in Shenandoah National Park, Virginia, U.S.A., each surrounded by a continuous population of Plethodon c. cinereus in the soil outside the talus. Distributions are contiguous but largely non-overlapping. The talus presents a much drier habitat than does the surrounding soil. Four experiments were designed to test the responses of the two species to moisture and substrate. Although shenandoah lives in a habitat generally drier than that of cinereus, both species choose the wet end of a moisture gradient and do not differ significantly in moisture preference. When given choices between a substrate of rock or soil, the two species respond similarly: neither expresses a preference when both substrates are moist and both choose soil over rock as the substrates dry, showing that substrate preference is based on moisture content and not texture. A third experiment demonstrates that cinereus suffers significantly greater mortality and loss of body water when subjected to a drying rock substrate than when subjected to a soil substrate, since the latter holds moisture longer. Thus the talus most likely presents a greater stress of dehydration to salamanders than does the soil. A fourth experiment shows that when forced to dehydrate, shenandoah survives longer, loses significantly less body water per hour, and withstands a greater loss of body water before death than does cinereus.The conclusions drawn are that cinereus inhabits areas of deep soil not due to a preference for that substrate but due to the requirement of a moist substrate, and it cannot enter the talus due to the dry conditions there. P. r. shenandoah, on the other hand, neither prefers the rocky nor the dry conditions of the talus and is probably excluded from the soil by the presence of cinereus. The survival of shenandoah in the talus is due, at least in part, to its ability to withstand the stress of dehydration for a longer period than can cinereus.