DENSITY-DEPENDENT AND DENSITY-INDEPENDENT CAUSES OF EXTINCTION OF A SALAMANDER POPULATION.

Abstract

Most of the species that have ever existed have gone extinct, but except for species affected by human disturbances, the causes of these extinctions are poorly understood. It is commonly stated that interspecific competition often may lead to the local or total extinction of a species (e.g., Pielou, 1974; Pianka, 1978). Indeed MacArthur and Wilson's (1967) theory of island biogeography rests largely on the assumption that the rate of extinction is directly related to the number of competing species on an island. However, the competition-to-extinction idea still depends more on theory than on observations of its occurrence in natural habitats. Even when it is possible to observe the extinction of a population, it is usually difficult to ascertain the causal factors involved. Terrestrial forest salamanders of the genus Plethodon provide an instructive study of encroaching extinction, because at least one species (P. cinereus) appears to be expanding its geographic range at the expense of several other species (Highton, 1972; Jaeger, 1974). The Shenandoah salamander, P. shenandoah, occurs on only three mountains in the Blue Ridge Mountains of Virginia. It occupies north-facing talus slopes above 900 m on The Pinnacles, Stony Man Mountain and Hawksbill Mountain (Highton andWorthington, 1967). On Hawksbill Mountain it was found, before 1970, as six isolated populations (see map in Jaeger, 1970): one population occupied a large talus slope of about 1 km2 while the others were partitioned among five smaller taluses. A sibling species, the redbacked salamander P. cinereus, is widely distributed in eastern North America and completely surrounds each of the populations of P. shenandoah (Highton and Worthington, 1967). Previous research (Jaeger, 1970, 1971a, 1971b, 1972) showed that P. cinereus has competitively excluded P. shenandoah from forests where the soil is relatively deep (Ct = 46 cm, from Jaeger, 1970) and thus moist, resulting in the Shenandoah salamander's confinement to the drier talus slopes. This competitive exclusion apparently is maintained by interspecific aggression and territoriality (Thurow, 1976; Jaeger and Gergits, 1979). Consequently interspecific competition has determined the distributional limits and the habitat-types of P. shenandoah. I previously proposed (Jaeger, 1970) that a long-term erosion of the talus slopes would lead to the eventual extinction of the Shenandoah salamander. It now ap-