Complex life-histories and biogeochemical cycles : interactions between amphibian life-history strategies and elemental cycling

Top-down and bottom-up forces shape the ecological roles of both producers and consumers. In many aquatic systems, amphibians with complex life-histories tie together terrestrial and aquatic systems. Studies of aquatic amphibian larvae generally investigate the effects of the larval habitat on survival and fitness of the amphibians themselves. Fewer studies yet investigate or document the effects of amphibians on their aquatic habitats. We use a series of manipulative mesocosm treatments varying in bottom-up and top-down pressures to investigate the effects of amphibian larvae on aquatic productivity and water nutrients. Decreasing the strength of bottom-up effects through increased shading resulted in not only reduced productivity, but switches in the effects of amphibians on their aquatic environments. Top-down effects of predation generally increased productivity through a reduction of primary consumer biomass. Predation prevented the drawdown of phytoplankton in high light tanks and thus maintained total water P in phytoplankton biomass, whereas high light tanks with large primary consumers were most likely supplemented by the translocation of benthic P to the water column as was evident from an increase in dissolved P.