Tests of Functional Equivalence: Complementary Roles of Salamanders and Fish in Community Organization

We experimentally tested whether two similarly sized generalist predators found in different habitats had similar effects on prey community patterns. Notophthalmus viridescens (the red-spotted newt) occurs primarily in temporary ponds, while Enneacanthus obesus (the banded sunfish) is restricted to permanent ponds. Larval anurans are an important prey assemblage found in both kinds of ponds. We stocked both predators in artificial ponds together with six species of larval anurans, forming different species assemblages, to assess whether a similar abundance and biomass of each predator would have comparable impacts on community patterns. The predators differed in their effects on prey species composition but had similar impacts on composite community attributes, including prey species diversity and total prey biomass. Enneacanthus eliminated Pseudacris crucifer, Hyla andersonii, Hyla versicolor, and Scaphiopus holbrookii, creating a community dominated by Bufo woodhousii and Rana sphenocephala. Notophthalmus eliminated Bufo, creating an assemblage dominated by Pseudacris but also containing four additional species. Very few anurans survived in ponds containing both newts and fish. The different consequences of predation by newts and fish reflect different effectiveness of anuran antipredator defenses against these predators. Mechanisms that reduced predation by fish, such as unpalatability, were ineffective against newts. Our findings suggest that a patchy distribution of functionally distinct predators across a landscape of discrete habitats will contribute to predictable patterns of spatial variation in community composition and will create a mosaic of communities dominated by different prey species.