Influence of Amphibian Larvae on Primary Production, Nutrient Flux, and Competition in a Pond Ecosystem

In a pond ecosystem near St. Louis, Missouri, natural variations in tadpole biomass during 1971—1972 were accompanied by shifts in patterns of nutrient cycling and primary production, particularly when metamorphoses caused abrupt removal of these transient consumers. In the field, increased tadpole biomass was associated with: (1) reduced standing crop of suspended particles, including phytoplankton, the tadpoles' major food source; (2) a shift in the state of nitrogen from largely particulate to largely dissolved; (3) reduced rates of primary production, from both H¹ ⁴CO₃ uptake and diurnal oxygen methods; (4) a nonliner effect on phytoplankton specific growth rates; (5) a shift in phytoplankton community structure away from filamentous blue—green algae; and (6) a reduced proportion of active chlorophyll @a in the photosynthetic pigments of phytoplankton. From laboratory experiments, the potential impact of tadpoles on nitrogen flux, through feeding and nutrient release, was estimated. Several conclusions were made: (1) Suspensions feeding by tadpoles reduced concentrations of suspended particles. Under conditions of low particles: high tadpoles, the specific growth rates of tadpoles reduced. Recruitment was absent except under conditions of high particles: low tadpoles; the most diverse community of new tadpole recruits (four species) was observed under such conditions. All four species (three genera) had similarly sized particles in their guts. These field observations are consistent with an hypothesis of competition among the tadpoles. (2) Tadpoles apparently were regulatory consumers; they became a large component with respect to phytoplankton. Nitrogen flux through tadpoles was within the same order of magnitude, and sometimes exceeded the estimated N uptake by phytoplankton. (3) Tadpoles probably regulated primary production by both reducing standing crop and altering specific growth rates of algae. At maximum tadpole biomass, suspended particle concentrations were stabilized near the laboratory—determined threshold concentration for feeding by these Rana tadpoles. When metamorphosis removed these transient consumers, rates of primary production increased dramatically. (4) Interactions within the pond ecosystem apparently determined aquatic—terrestrial nutrient balances for the amphibian communities. Some species deposited more nutrient in their eggs than was assimilated by larvae, but the community as a whole extracted nutrient from the ecosystem. Nutrient input in eggs was much less than that assimilated by autotrophs.