PREDATION BY TADPOLES OF RANA SYLVATICA ON EMBRYOS OF AMBYSTOMA MACULATUM : IMPLICATIONS OF ECOLOGICAL ROLE REVERSALS BY RANA (PREDATOR) AND AMBYSTOMA (PREY)

Abstract

Wood frog (Rana sylvatica) tadpoles are microphagous filter-feeders that function as primary consumers in vernal pond communities. Nonetheless, field observations suggest that tad- poles may shift temporarily to higher trophic levels and prey upon the eggs of an upper-level predator, the spotted salamander (Ambystoma maculatum). We conducted experiments in both artificial mesocosms and in natural ponds to determine factors that influence tadpole predation rates on white and clear egg masses of A. maculatum. In an artificial pool experiment, tadpoles signifi- cantly reduced embryonic survival of A. maculatum. When supplemental food level was low, rates of egg predation were relatively high and clear egg masses suffered greater mortality than white egg masses. In a field experiment, we used cages to protect egg masses from predators in ponds with and without tadpoles of R. sylvatica. Survival of embryonic A. maculatum was lower for un- caged egg masses in both sets of ponds, but extremely low for egg masses in ponds with tadpoles of R. sylvatica. As in the artificial pool experiment, white egg masses were less vulnerable to tadpole predation than clear egg masses. Results from a correlation analysis of data collected from 84 breeding sites at a bog complex in western North Carolina indicate that the percentage of white egg masses increases with the density of egg masses of R. sylvatica from local breeding sites. Collectively, our data suggest that opportunistic predation by a primary consumer can result in high premetamorphic mortality of an upper-level predator. We hypothesize that tadpole predation is one of several selection pressures maintaining egg color polymorphism in A. maculatum.