Two Degrees of Separation in Complex Food Webs

Feeding relationships can cause invasions, extirpations, and population fluctuations of a species to dramatically affect other species within a variety of natural habitats. Such strong effects rarely propagate through food webs more than three links away from the initial perturbation. However, in large food webs the number of species within these spheres of potential influence are generally unknown. Here we show that the vast majority of species within habitats are three or fewer links from each other. Specifically, food webs from widely different ecosystems are shown to be surprisingly small worlds in which species are only two links from each other on average. Contrary to expectations, species are drawn even closer as network complexity and species richness increase. Our findings are based on seven of the largest and most complex food webs available as well as a food-web model that extends the generality of the empirical results. These results indicate that the dynamics of species within ecosystems may be more highly interconnected and that biodiversity loss and species invasions potentially affect more species than previously thought.