FUNCTIONAL REDUNDANCY, NONADDITIVE INTERACTIONS, AND SUPPLY-SIDE DYNAMICS IN EXPERIMENTAL POND COMMUNITIES

Factorial manipulations of the abundance of two predator species (presence or absence of six adult Notophthalmus viridescens, and/or six larval Ambystoma opacum per pond) and a guild of three prey species (larvae of Pseudacris crucifer, Bufo woodhousii fowleri, and Hyla andersonii) tested the equivalence, additivity, and dependence on initial prey density of predation by both predator species in 1-m(3) artificial ponds. The two predators were functionally equivalent, eliminating Bufo and Hyla, while leaving Pseudacris to numerically dominate ponds with salamanders. Effects of both predators also cascaded down to algal primary producers, which became more abundant where predators reduced the abundance of herbivorous tadpoles. Combined impacts of the two predator species on the survival of Pseudacris were less than that predicted by an additive model of the effects of each predator measured in the absence of the other, but greater than predicted by a multiplicative model of joint predator effects. Reduced growth of Ambystoma with Notophthalmus provides one possible mechanism for the observed shift to less than additive per capita predator effects. Initial variation in prey density influenced prey species composition only in ponds without predators. Increased prey density in ponds without predators reduced the survival of Hyla, apparently through increased interspecific competition among tadpoles.