Age Structure in a Population of Red-Spotted Newts from the Allegheny Plateau of Maryland

Abstract

The red-spotted newt, Notophthalmus viridescens, is widely distributed in North America east of a line stretching from southern Quebec to the upper Gulf Coast of Texas (Mecham, 1967; Conant, 1975). The species exhibits a complex life history which includes an aquatic larva, a terrestrial juvenile (eft), and an aquatic adult. The duration of these life history stages is variable. In upland populations, larvae characteristically hatch during the spring and summer and engage in post-larval migrations during the late summer and early fall (Hurlbert, 1970; Healy, 1975a; Gill, 1978a, b). Juveniles spend from 2-7 years as terrestrial efts before returning to the aquatic habitat to spend 1-9 years as mature, sexually active adults (Hurlbert, 1969; Pough, 1971; Healy, 1975b; Gill, 1978a, b, 1985). In coastal plain populations, the terrestrial eft phase is rare and in some localities may be omitted altogether (Noble, 1926, 1929; Healy, 1975a). In these populations, larvae metamorphose directly into adults and reach sexual maturity after two years. Historically, estimations pertaining to the age structure of various populations of the red-spotted newt have been based on one of two techniques: (1) a comparison of size-frequency histograms derived from sequential field collections; or (2) multi-year mark and recapture techniques. In the first technique, accuracy is influenced by variability in individual growth rate. The growth rate of newts comprising each of the life history stages is directly proportional to the amount of food consumed (Springer, 1909). Food consumption depends upon resource availability and the intensity and duration of individual activity. The activity of terrestrial efts is positively correlated with humidity and temperature (Healy, 1973), while that of aquatic juveniles and adults is inversely correlated with temperature (Springer, 1909). The use of a mark and recapture technique to estimate age structure maximizes accuracy, but requires a prolonged and often impractical time commitment (see Gill, 1985). In this report we apply the technique of skeletochronology to estimate the age structure and relative duration of the eft and adult life history stages of redspotted newts taken during a single sampling event. Seventy-six red-spotted newts (10 larvae, 36 efts and 30 adults) were collected from the Blue Ridge Physiographic Province of western Maryland (elevation 800 m) during June, 1984. The site consisted of a eutrophied farm pond surrounded by steep slopes covered by a second-growth, mixed mesophytic forest. Larvae and adults were collected from the shal-