Physics, body size, and the structure of food webs
Studies on food webs usually focus on topology and connectance 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. Others studies consider food web evolution usually using abstract traits, or a concrete trait: body size. Indeed, 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. Although seminal studies (such as studies by Arthur Tansley) emphasized the role played by the medium, this interplay has not received an important focus lately.
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 turbulence. 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 pelagic ecosystems, a hunting predator usually has to be bigger than its prey. It clearly appears that, for such a predator, sinking velocity is not negligible, while primary producers species (phytoplankton) can maintain their populations, even if species are not naturally buoyant. Turbulence can enhance persistence of primary producers, but not persistence of their predators. The conclusion is that a pure planktonic predator that does not adjust its buoyancy cannot survive, which is in accordance with field observations.
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.