Food as a Limited Resource in Competition between Two Species of Terrestrial Salamanders

Abstract

The terrestrial salamander Plethodon richmondi shenandoah occurs primarily in areas of talus, apparently excluded from surrounding areas of deep soil by Plethodon cinereus. Although the two species are closely related P. r. shenandoah grows to a longer body length and has a broader head. All ages classes of P. r. shenandoah emigrate out of the talus and are found within 3 m of the talus edge, but only adults which are as large as or larger than the largest P. cinereus are able to survive farther than about 3 m from the talus. This distribution is not explained by a tendency for large adults to wander more than juveniles. The large P. r. shenandoah in the soil take prey that are significantly larger than prey taken by conhabiting P. cinerueus, indicating that food may be the limited resource for which competition occurs. Both species are euryphagic and prey upon the same taxa of small animals. Food is periodically limited in availability, but not in abundance. Following a rain, salamanders forage freely and consume a large quantity of prey, based on stomach contents, but prey become increasingly unavailable as the soil and leaf litter dry and the salamanders are restricted to isolated pockets of moisture. Stomachs remain virtually empty until the next rain, at which time they are full again indicating that the abundance of prey is not greatly affected by the intervening dryness. Food may periodically become a limited resource during long dry seasons and as such may affect the survival of the salamanders or their ability to produce eggs. It is not clear whether interspecific competition is through differential exploitation of the limited resource or through interference. Lack of evidence for interspecific aggression may indicate that the former occurs, although it could not be demonstrated experimentally. Contiguous allopatry between the two species seems to result from an interaction of competitive exclusion and differential tolerance to dryness; P. cinereus is presumably a superior competitor for food in the soil and excludes P. r. shenandoah to areas of talus which P. cinereus cannot enter due to its tolerance of drier conditions in the rocks.