CONSEQUENCES OF RATIO-DEPENDENT PREDATION FOR STEADY-STATE PROPERTIES OF ECOSYSTEMS

Ratio-dependent and prey-dependent models of trophic interactions make very different predictions about the steady-state (equilibrial) properties of ecosystems, such as the response of equilibrium abundance and biomass of organisms in each trophic level to increased primary productivity or nutrient input. Prey-dependent theory predicts alternating positive, negative, and zero responses of trophic levels to increased productivity, whereas ratio-dependent theory predicts proportional increase in all trophic level biomasses. We analyzed data on the nutrient input and the biomasses of phytoplankton, zooplankton, and fish across lakes to distinguish between ratio-dependent and prey-dependent models. The results show parallel increases in all trophic levels as a result of increased nutrient input, demonstrating that natural systems are closer to ratio dependence than to prey dependence.