Fig. 2

Complex networks of ecological interactions can vary in their shape, link density, and component structure depending on the type of interaction they embed. While food webs typically describe all the interactions occurring in a given ecosystem (a) with multiple trophic levels (Dunne et al. 2002), most plant–animal interactions can be displayed as bipartite graphs (b) describing the pairwise pattern of mutual interdependencies (Jordano 1987) among two distinct sets of animals (orange nodes) and plants (yellow). Interactions among species with a higher degree of intimacy, such as ant-plants show a distinct pattern of structure (c), often with multiple distinct groups (modules) of closely intimate associations (Guimarães et al. 2007). The three types of webs share a complex pattern of interactions made up of multiple simple “building blocks” or motifs (Bascompte and Melián 2005) that vary in shape and frequency across these networks (d, e). Motifs in food webs (d) include simple trophic chains, omnivory, apparent competition, and intraguild predation (from left to right); those in bipartite graphs (e) include different forms of generalization/specialization, with more specialized (e, top) and more generalized (e, bottom) motifs. Images a, b, and c produced with FoodWeb3D, written by R. J. Williams and provided by the Pacific Ecoinformatics and Computational Ecology Lab (www.foodwebs.org)