Prevalence of Chemical Interference Competition in Natural Populations of Wood Frogs, Rana sylvatica

Abstract

When wood frog tadpoles are crowded in the laboratory, their intestines become packed with a unicellular alga (Prototheca) that inhibits growth. We conducted a series of experiments to determine the importance of this phenomenon in natural breeding ponds where local densities often exceed 15,000 wood frog tadpoles/m3 of water. Water from three of 10 breeding sites inhibited the growth of assay tadpoles relative to controls. Although Prototheca was found in the intestines of tadpoles from all sites, several lines of evidence suggest that it was not the causative agent of inhibition. First, Prototheca densities in wild-caught animals were extremely low compared to those that develop in laboratory stock and did not appear to be sufficiently dense to stunt growth. Second, tadpoles from sites where inhibitors were detected did not have disproportionately large numbers of Prototheca cells in their intestines. Third, wild-caught tadpoles which were crowded for three days in the laboratory almost always produced growth inhibitors; however, the production of inhibitors was not associated with a proliferation of Prototheca in the intestines. Tadpoles crowded in small containers placed outdoors did not produce inhibitors. However, tadpoles grown indoors under otherwise similar conditions did. The results of a temperature manipulation experiment suggest that inhibitors are most likely to develop when tadpoles are maintained at room temperature. Tadpoles did not produce inhibitors when maintained at relatively cool temperatures which approximate those of natural breeding sites. Collectively, our data suggest that growth inhibition involvingPrototheca is primarily a laboratory artifact that has little relevance to the ecology of natural populations of wood frogs.