Physical Factors and Size-structure of Food Webs
Sebastien Portalier, Mehdi Cherif, Gregor Fussmann, Michel Loreau
MCGILL UNIVERSITY

Studies on food webs usually focus on species interactions but omit the role played by the surrounding medium. The present study investigates how physical factors from the medium can constrain the size structure of food webs. Hence, we built a model in which species motion and species interactions are constrained by physical properties of the medium and biological traits (e.g., metabolism). As key physical factors of the medium, we consider gravity, medium density, body density, medium viscosity, and light availability. These factors, in relation with body size, constrain species interactions (i.e., the presence of a link between two species) and the energy gain associated to this link (i.e., net energy intake for the predator). These factors also constrain photosynthesis production. Hence, the overall dynamic emerges from these constraints. Preliminary results show that a hunting predator usually has to be larger than its prey. Different structures can be explored. Based on these energetic features, the model can show which structures can occur and which are unlikely. Moreover, the model also shows which size ratios between species allow a given structure to persist. The present study recombines biological and physical aspects of ecosystems into a whole unified framework. This study emphasizes the central role played by body size (here as an interface between biology and physics) in the structure of food webs. Results from this study should highlight the need to consider physical factors from the medium as explanatory variables of community structure.