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AbstractBody mass-based links between predator and prey are fundamental to the architecture of food webs. These links determine who eats whom across trophic levels and strongly influence the population abundance, flow of energy, and stability properties of natural communities. Body mass links scale up to create predator-prey mass relationships across species, but the origin of these relationships is unclear. Here I show that predator-prey mass relationships are consistent with the idea that body mass evolves to maximize a dependable supply of resource uptake. I used a global database of ~2,100 predator-prey links and a mechanistic optimization model to correctly predict the slope of the predator-prey mass scaling relationships across species generally and for nine taxonomic subsets. The model also predicted cross-group variation in the heights of the body mass relationships, providing an integrated explanation for mass relationships and their variation across taxa. The results suggest that natural selection on body mass at the local scale is detectable in ecological organization at the macro scale.

Food web models parameterised using body size show promise to predict trophic interaction strengths (IS) and abundance dynamics. However, this remains to be rigorously tested in food webs beyond simple trophic modules, where indirect and intraguild interactions could be important and driven by traits other than body size. We systematically varied predator body size, guild composition and richness in microcosm insect webs and compared experimental outcomes with predictions of IS from models with allometrically scaled parameters. Body size was a strong predictor of IS in simple modules (r 2  = 0.92), but with increasing complexity the predictive power decreased, with model IS being consistently overestimated. We quantify the strength of observed trophic interaction modifications, partition this into density-mediated vs. behaviour-mediated indirect effects and show that model shortcomings in predicting IS is related to the size of behaviour-mediated effects. Our findings encourage development of dynamical food web models explicitly including and exploring indirect mechanisms., (© 2018 John Wiley & Sons Ltd/CNRS.)