The interactions between bottom-up and top-down forces in regulating consumer biomass have interested ecologists for decades. Yet, experiments to test these interactions have relied heavily on metrics of resource abundance to assess bottom-up effects, rather than resource quality. This is particularly true for research on interactions between bottom-up forces and top-down control from non-consumptive effects (NCEs) of predation. We investigated the interaction between bottom-up forces from litter quality and top-down control from NCEs in a simplified freshwater benthic food web. We collected litter from three populations of Phragmites australis (common reed) that varied widely in litter quality. We assembled simplified pond communities in 100 L containers with litter, pond slurry, zooplankton, amphipods, and Lithobates palustris (pickerel frog) tadpoles. Lithobates palustris tadpoles feed largely off the biofilms that form on decaying litter; the quality and abundance of these biofilms is proportional to litter quality. We added tadpoles at a density of 1 tadpole for every 10 liters of water – a density that heightened interspecific competition. We then added a caged dragonfly larvae (Anax sp.) to half of the mesocosms to provide a predator cue.
Results/Conclusions
Phragmites australis litter played an important role in structuring species interactions in our experimental food web, and the direction and magnitude of these effects was a function of litter quality. In general, L. palustris performed poorly with low quality litter compared to medium and high quality litter, and fared even worse when we added a predator. Conversely, amphibians performed well in the medium and high quality litter, and tended to perform even better with a predator cue. We surmise that this apparent interaction between resource quality and predator cue arose because of competition among tadpoles. Without a predator cue, all tadpoles foraged freely, reducing resources such that very few individuals gained enough energy to complete metamorphosis. Conversely, tadpoles reduced foraging when a predator was present – a common NCE of predation – allowing resources to rebound. With medium and high quality P. australis litter, individuals were able to gain enough energy to metamorphose. However, with low quality litter, resources were still limited, and few individuals metamorphosed. Our results reveal complex interactions among species in this simplified food web. They support the view that ecosystems, and the species within them, are dynamic entities, whose properties are a function of species traits, environmental context, and the interaction between the two.
