Network structure of food webs in Mediterranean streams: a tool for conservation

Certain structural properties of food webs have important implications for ecosystem functioning; they provide us with information regarding the recovery capacity of ecosystems confronting different types of perturbations. In this study, we analyse the network structure of food webs in six Mediterranean streams in order to determine whether they display a small-world topology. In these types of network, perturbation effects are distributed rapidly through large, complex and highly clustered systems due to the surprisingly short path lengths between nodes. We focus on the analysis of three topological properties: 1) characteristic path length (D), 2) clustering coefficient (Cl) and 3) cumulative degree distribution P(k). Our results show that the food webs of the studied Mediterranean streams do not display typical small-world topology. Despite presenting low D values, the clustering ratio was quite low and an exponential type cumulative degree distribution was found, rather than the power-law type distribution that is typical of small-world networks. It has been demonstrated, however, that the exponential type distribution p(k) presents similar response patterns to those presented by a small-world network undergoing the loss of nodes (species). It therefore appears that the Mediterranean streams may be robust in the face of random species loss but fragile when faced with the selective loss of more connected species.