Ecological food webs: High-quality data facilitate theoretical unification

Despite the enormous diversity and complexity of ecological systems, when data for many individuals of many different species are analyzed, there are emergent regularities in the statistical distributions of numerical abundance, spatial dispersion, trophic relations, and species richness, and in bivariate and multivariate relationships among these variables (e.g., refs. 1–3). These empirical macroecological patterns have been known for many decades, but now ecology is beginning to understand the mechanistic processes that produce them. This conceptual unification is being facilitated by two breakthroughs. First, intensive, technology-assisted empirical studies are generating vast quantities of new and better data. Second, theoretical advances are characterizing the interrelationships among ecological phenomena and explaining them in terms of first principles of physics, chemistry, and biology. There is much still to be done, but recent progress is encouraging.

The paper on food webs in this issue of PNAS (4) offers an excellent example of what can be done by using new data to inspire theoretical exploration. Food webs are representations of trophic relationships, network diagrams in which each node represents a species and each line linking two nodes represents one species feeding on another. Food webs are known to exhibit emergent empirical patterns in number of trophic levels (successive nodes in a food chain from green plant producer through herbivore and carnivore to decomposer), number of links per species, and ratios of predators to prey (5–7). Food webs potentially provide the basis for conceptual unification between two major areas of ecological research: (i) biodiversity theory, which endeavors to explain the number of species that coexist within an ecosystem in terms of the interactions of these species with each other and with their nonliving environment; and (ii) metabolic theory, which endeavors to explain the structure and dynamics of ecosystems in terms of the flux, …