Food chains and food webs: controlling factors and cascading effects

Abstract This chapter uses simple theory and experiments to address the fundamental question of what determines the biomass (abundance) of different trophic levels (plants, herbivores, carnivores) in a community. Theory predicts joint control of trophic-level abundance by bottom-up effects (resources) and top-down effects (predation), with the relative strengths of top-down and bottom-up effects depending on the number of trophic levels and species composition within a trophic level. Observations and experiments support these predictions. Trophic cascades provide evidence for the importance of top-down processes, but the existence of a trophic cascade says little about the relative importance of predator limitation versus resource limitation. Cascading effects result from either the consumptive or non-consumptive effects of predators, or both. Natural systems contain as few as three and as many as six trophic levels, but what determines this number is unknown. Evidence suggests that both productivity and ecosystem size, perhaps in combination, are the primary factors.