EXPERIMENTAL AQUATIC FOOD WEBS - INTERACTIONS BETWEEN 2 PREDATORS AND 2 PREY

Performed 4 replicates of all 16 combinations of the presence and absence of 2 predators (larvae of the dragonfly Anax junius and adult salamanders Notophthalmus viridescens) and 2 anuran prey (Bufo americanus and Rana palustris larvae). The anurans were introduced at densities high enough to cause density-dependent reductions in survival and body size at metamorphosis and to increase larval period. The 2 species were in competition when raised together. Notophthalmus reduced the density of Bufo and caused the survivors to metamorphose early and at a small size. Anax caused an even greater reduction in survival but not as strong an acceleration of metamorphosis. Newts also reduced the density of Rana, but survivors benefited by growing rapidly. The effects of Anax were even stronger: the only Rana tadpoles that were able to metamorphose in the 2 mo of the experiment were from ponds in which Anax reduced densities enough to permit rapid growth of the surviving tadpoles. In systems with 1 predator and 2 prey, the anurans shared the risk of predation and were mutualists rather than competitors. In systems with 2 predators and 1 prey, the risks of predation were additive. In systems with all 4 species, risks were again spread over both anurans, making them mutualists. One- and 2-species systems predicted low and nearly equal survival of the two prey at the end of the experiment (4% for Bufo and 7% for Rana). The observed survival in the 4-species systems was 35% for Bufo and 59% for Rana. -from Authors