CAUSE-EFFECT RELATIONSHIPS IN ENERGY-FLOW, TROPHIC STRUCTURE, AND INTERSPECIFIC INTERACTIONS

Measurements of the efficiency of energy transfer between trophic leve ls are consistent with the hypothesis that it is trophic structure tha t controls the fraction of energy consumed at each trophic level, rath er than energetics controlling trophic structure. Moreover, trophic st ructure is determined by competitive and predator-prey interactions. I n freshwater pelagic communities, the collective efficiency of herbivo rous plankton in consuming primary producers is up to 10 times as grea t as is the efficiency of forest herbivores in consuming their food. C onversely, forest predators are about three times as efficient in cons uming herbivore production. as are zooplanktivorous fish. The presence of an additional level, piscivorous fish, in pelagic communities acco unts for the difference. In the aquatic system, herbivorous zooplankto n are freed from predation by the effect of piscivorous fish on their predators; in the terrestrial system, green plants are freed from herb ivory by predation on the herbivores. We explain the contrast between freshwater pelagic systems and forests and prairies as follows: Pelagi c ecosystems have more trophic levels as a result of selection for sma ll rapidly growing primary producers, which cannot hold space in the f luid medium, in contrast to large space-occupying producers in the ter restrial environment. Consumers in pelagic systems are more frequently gape limited in the size range of food they can ingest than are grasp ing consumers in terrestrial systems. This difference makes for two la rgely distinct levels of predators in pelagic communities. The energy within the living, nondetrital components is more finely divided betwe en trophic levels in pelagic systems than in terrestrial systems. Ecol ogical efficiencies do not determine trophic structure; rather, they a re its product.