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

Factorial manipulations of the abundance of two predator species (pres ence or absence of six adult Notophthalmus viridescens, and/or six lar val Ambystoma opacum per pond) and a guild of three prey species (larv ae of Pseudacris crucifer, Bufo woodhousii fowleri, and Hyla andersoni i) tested the equivalence, additivity, and dependence on initial prey density of predation by both predator species in 1-m(3) artificial pon ds. The two predators were functionally equivalent, eliminating Bufo a nd Hyla, while leaving Pseudacris to numerically dominate ponds with s alamanders. Effects of both predators also cascaded down to algal prim ary producers, which became more abundant where predators reduced the abundance of herbivorous tadpoles. Combined impacts of the two predato r species on the survival of Pseudacris were less than that predicted by an additive model of the effects of each predator measured in the a bsence of the other, but greater than predicted by a multiplicative mo del of joint predator effects. Reduced growth of Ambystoma with Notoph thalmus 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 pred ators. Increased prey density in ponds without predators reduced the s urvival of Hyla, apparently through increased interspecific competitio n among tadpoles.