Size-dependent foraging affects predator-prey interaction strengths and food-web stability

The metabolic theory predicts that metabolism and consumption rates of individuals follow three-quarter power-laws with individual body mass. Optimal foraging theory predicts that these overall consumption rates are unevenly distributed amongst the multiple feeding links of predators, and attack rates follow a hump-shaped relationship with the predator-prey body-mass ratios. Unifying these two bodies of theory, I present experimental data on the rates of metabolism, consumption and attack of ground-dwelling beetles and spiders. By combining the metabolic theory with optimal foraging models we develop a ‘unified flux model’ predicting that (i) per capita biomass fluxes first increase and then decrease with predator mass, (ii) small predators have higher per capita biomass fluxes when attacking small prey, whereas large predators have higher biomass fluxes while consuming large prey, and (iii) total biomass fluxes decrease with predator mass. This ‘unified flux model’ extends the metabolic theory to the level of feeding links with quantitative predictions of biomass fluxes. Model simulations are presented that demonstrate the consequences of these relationships for population and community dynamics. These results demonstrate that optimal foraging strategies of predators have profound implications for population and community stability.