Evolutionary emergence of size-structured food webs

Abstract

Explaining the structure of terrestrial and aquatic food webs remains one of the most important challenges of ecological theory. Most existing models use emergent properties of food webs, such as diversity and connectance as parameters, to determine other food-web descriptors. Lower-level processes, in particular adaptation (whether by behavioral, developmental, or evolutionary mechanisms), are usually not considered. Here, we show that complex, realistic food webs may emerge by evolution from a single ancestor based on very simple ecological and evolutionary rules. In our model, adaptation acts on body size, whose impact on the metabolism and interactions of organisms is well established. Based on parameters defined at the organism scale, the model predicts emergent properties at the food-web scale. Variations of two key parameters (width of consumption niche and competition intensity) allow very different food-web structures and functionings to emerge, which are similar to those observed in some of the best-documented food webs.

Fig. 1.

Trophic interactions. Interaction strength, as measured by function γ, between two species as a function of the difference between their body sizes is shown. The vertical line indicates a target species. Species whose traits are larger are its potential predators, and species whose traits are smaller are its potential prey. Mutations acting on the body-size trait of the target species will give birth to types whose trophic interactions are slightly different.

Fig. 2.

Food-web structure. The evolution of simulated food webs during 10⁸ time steps for three values of niche width (nw) and competition intensity (α₀) is shown. Trophic position is computed as explained in Model and Methods. Because this measure is strongly correlated with body size, similar patterns would be obtained by using body size. The following parameter values were used in these simulations: I = 10, e = 0.1, v = 0.5, γ₀ = 1, d = 2, f₀ = 0.3, m₀ = 0.1, and β = 0.25.

Fig. 3.

Emergent properties of simulated food webs as functions of niche width (nw) and competition intensity (α₀). Properties were measured at the end of simulations (i.e., after 10⁸ time steps). See Model and Methods for computation of the various properties. Parameter values are as in Fig. 2.

Fig. 4.

Comparisons with documented food webs. Niche width and competition intensity that lead to food-web properties that best fit those of seven well documented food webs (BB, Bridge Brook Lake; CB, Chesapeake Bay; CD, Coachella Desert; LR, Little Rock Lake; SM, St. Martin Island; SP, Skipwith Pond; and YE, Ythan Estuary). We use the emergent properties given in Table 1. The main characteristics of the corresponding model food webs (level of omnivory and trophic structure) are indicated to give a better idea of the structure of the evolved communities. Empirical data are from Cattin et al. (4). Parameters are the same as in Fig. 2.