Studies on food webs usually focus on topology using network approaches, but these studies are often descriptive. Other studies, such as Optimal Foraging approaches, focus on species traits, thus providing mechanisms from lower levels of organization. Last, many studies argued that stability of interactions between a consumer and its resource is strongly influenced by the strength of their interaction.
But a central question is to know what determines the strength of a given interaction. Body size seems to be a good predictor of trophic position. However, the interplay between body size and the physical properties of the medium has not really been investigated in a context of food webs.
The present study investigates how physical factors from the medium can constrain the size structure of food webs according to the ecosystem considered (e.g., aquatic, terrestrial). 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 turbulent mixing. 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). Hence, the overall dynamic emerges from these constraints.
Preliminary results show that, in aquatic systems, consumers feeding on photosynthetic organisms usually have a small or intermediate size. Larger consumers do not find any convenient prey because either these prey do not provide enough energy, or they cannot persist on their own. Results also show that very small predators cannot persist because they are unable to contact enough prey. Consumers also allow for coexistence of several prey that otherwise would compete for light. These coexistence patterns are constrained by body size ratios between species.
The present study recombines biological and physical aspects of ecosystems into a whole unified framework. Thus, it allows us to investigate new questions (such as the size discrepancy between predator and prey, or the size structure within food webs) that otherwise cannot be efficiently investigated. 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.