Abstract
Many drivers of ecological systems exhibit regular scaling relationships, yet the mechanisms explaining these relationships are often unknown. Trophic interaction strengths are no exception, exhibiting scaling relationships with predator and prey traits that lack evolutionary explanations. We propose two rules to explain the scaling of trophic interaction strengths through the relationship between a predator's feeding rate and its prey's density-the so-called predator functional response. First, functional responses allow predators to meet their energetic demands when prey are rare. Second, functional responses approach their maxima near the highest prey densities predators experience. We show that equations derived from these rules predict functional response parameters across over 2100 functional response experiments and make additional predictions such as their allometric scaling. The two rules thereby offer a potential ultimate explanation for the determinants of trophic interaction strengths, revealing ecologically realised constraints to the complex, adaptive nature of functional response evolution.