A detailed analysis of three species-rich ecosystem food webs has shown that they display scale-free distributions of connections. Such graphs of interaction are in fact shared by a number of biological and technological networks, which have been shown to display a very high homeostasis against random removals of nodes. Here we analyse the response of these ecological graphs to both random and selective perturbations (directed to most connected species). Our results suggest that ecological networks are extremely robust against random removal but very fragile when selective attacks are used. These observations can have important consequences for biodiversity dynamics and conservation issues, current estimations of extinction rates and the relevance and definition of keystone species.
