Understanding the Web of Life: The Birds, the Bees, and Sex with Aliens

W hen I was in school, I learned about a linear food chain in which, for example, flowers provide food for bees, which in turn are eaten by birds. The implications of this model are clear: if bees were to vanish, birds would starve and flowers would not be pollinated. Whether this concept was a hangover from the old idea of the great chain of being (scala naturae) or a simplification deemed necessary for unsophisticated school children is unclear. Nevertheless, despite the appeal of this simple caricature, it couldn't be further from the truth in most of the world's ecosystems. Birds feed on a variety of plants and animals, and are themselves fed upon by mammals, other birds, a diverse array of parasites, and eventually carrion feeders. It takes little more than a passing glance at the natural world to notice that complexity is the rule, rather than the exception. Describing this complexity and understanding its importance, however, is anything but simple. Ecologists have for a long time struggled to find consistent patterns in the structure of complex webs of interacting species from disparate ecosystems. However, recent empirical and theoretical breakthroughs have begun to shed light on the structure of the web of life that connects living things, and the vulnerability of this web to perturbations such as the introduction of invasive alien species. Interaction webs have a long pedigree in ecological research. For decades, theoretical work has linked the structure of food webs to their ability to resist perturbations [1–3]. In particular, the proportion of weak versus strong interactions within a web can determine its resistance to the extinction of particular species [4]. In these cases, " interaction strength " is measured roughly as the proportion of individuals of a species at the lower trophic level fed upon by, or interacting with, a species at the higher trophic level. Despite the solid grounding and importance of this concept in theory, quantifying interaction strength in diverse, real-world webs presents practical hurdles, and a major breakthrough in empirical food web research was the quantification and graphical representation of community-scale interaction webs (e.g., [5]) (Figure 1). This work paved the way for research examining changes to the structure of real-world biological communities brought about by human activities. Metrics that describe interaction structure can allow us to detect subtle shifts in entire communities of organisms better than coarse metrics such as diversity (the …