Anuran Responses to Salamander Predation

Abstract

A previous experiment by P. J. Morin demonstrated that the newt, Notophthalmus viridescens (Amphibia: salamandridae) preyed on the species of tadpole that was the competitive dominant in communities of six species of anurans without newts. We studied the mechanism producing this result by combining three densities of the predator with three densities of prey in replicated artificial ponds. Pond communities were established in 27 tanks 1.52 m in diameter and 0.62 m deep. Because the late arrival of rains delayed the start of the planned experiment, 300 hatchlings of the toad Bufo woodhousei fowleri were added to all tanks to control algal populations. To these standardized communities we added all nine combinations of three densities of adult newts (0, 4, or 8 in an equal sex ratio) and three densities of larvae of the frogs Rana sphenocephala (Ranidae), Bufo terrestris (Bufonidae), and Scaphiopus holbrooki (Pelobatidae), in constant relative abundance. These nine treatments were randomly located in each of three blocks. The nine tanks without adult newts (three replicates of three densities of tadpoles: 50 Rana + 100 Bufo americanus + 100 Scaphiopus, 100 + 200 + 200, and 200 + 400 + 400) were used to assay competition within the tadpole guild. In the absence of predators, the anurans had density-dependent survival, lengths of larval period, and sizes at metamorphosis. There was evidence of competition among as well as within species. Rana sphenocephala had wintering tadpoles only in the highest density populations (Figure 1). Overwintering tadpoles were also observed in crowded natural populations with little predation.