The mechanics of predator-prey interactions: first principles of physics predict predator-prey size ratios

1. Robust predictions of predator-prey interactions are fundamental for the understanding of food webs, their structure, dynamics, resistance to species loss, response to invasions and ecosystem function. Most current food web models measure parameters at the food web level to predict patterns at the same level. Thus, they are sensitive to the quality of the data, and may be ineffective in predicting non-observed interactions and disturbed food webs. There is a need for mechanistic models that predict the occurrence of a predator-prey interaction based on lower levels of organization (i.e., the traits of organisms) and the properties of their environment. 2. Here, we present such a model that focuses on the predation act itself. We built a Newtonian, mechanical model for the processes of searching, capturing and handling of a prey item by a predator. Associated with general metabolic laws, we predict the net energy gain from predation for pairs of pelagic or flying predator species and their prey depending on their body sizes. 3. Predicted interactions match well with data from the most extensive predator-prey database, and overall model accuracy is greater than the allometric niche model. 4. Our model shows that it is possible to accurately predict the structure of food webs using only a few mechanical traits. It underlines the importance of physical constraints in structuring food webs.