Food-web structure and network theory: The role of connectance and size

TLDR

Ecological food webs have been debated as to whether they exhibit the small‑world and scale‑free properties characteristic of many physical, biological, and social networks. The authors examined 16 high‑quality food webs (25–172 nodes) from diverse aquatic and terrestrial ecosystems to assess network structure. Their analysis shows that food webs are generally more complex (higher connectance) and smaller than other networks, and that most do not display small‑world or scale‑free structure unless connectance is low; degree distributions and clustering patterns are systematically linked to connectance and size, revealing a continuum that aligns with known network classes.

Abstract

Networks from a wide range of physical, biological, and social systems have been recently described as “small-world” and “scale-free.” However, studies disagree whether ecological networks called food webs possess the characteristic path lengths, clustering coefficients, and degree distributions required for membership in these classes of networks. Our analysis suggests that the disagreements are based on selective use of relatively few food webs, as well as analytical decisions that obscure important variability in the data. We analyze a broad range of 16 high-quality food webs, with 25–172 nodes, from a variety of aquatic and terrestrial ecosystems. Food webs generally have much higher complexity, measured as connectance (the fraction of all possible links that are realized in a network), and much smaller size than other networks studied, which have important implications for network topology. Our results resolve prior conflicts by demonstrating that although some food webs have small-world and scale-free structure, most do not if they exceed a relatively low level of connectance. Although food-web degree distributions do not display a universal functional form, observed distributions are systematically related to network connectance and size. Also, although food webs often lack small-world structure because of low clustering, we identify a continuum of real-world networks including food webs whose ratios of observed to random clustering coefficients increase as a power–law function of network size over 7 orders of magnitude. Although food webs are generally not small-world, scale-free networks, food-web topology is consistent with patterns found within those classes of networks.