Resource Edibility and the Effects of Predators and Productivity on the Outcome of Trophic Interactions

Population regulation of organisms is frequently viewed as being either by predation ("top-down" regulation) or by resource productivity ("bottom-up" regulation). However, these two types of regulation should ultimately balance, and the abundances of interacting organisms that result in any given environment should depend on the densities that can satisfy the balancing conditions. Here I use a simple model to examine interactions among consumers and two types of resources that differ in edibility. I use the model to predict the abundances of all three populations, which should result in environments that differ in consumer mortality (e.g., predation intensity) and resource productivity (e.g., nutrient levels). The model predicts that the effects of predation and productivity will depend strongly on the diet breadth of the consumers. I then test the predictions of the model in a field experiment on the plankton of an oligotrophic lake by manipulating fish predation and nutrient productivity. Finally, I compare these results with those of 46 other similar experiments and show the utility of the model in understanding the results of the experiments and the observed correlations of abundances of phytoplankton and zooplankton in natural lakes. The results of the experiments confirm that the effects of predators and nutrients on consumers (zooplankton) and resources (phytoplankton) predicted by the model differ when the consumer assemblage is dominated by a generalist herbivore (Daphnia) from those expected in the absence of such a generalist herbivore.