Degree distribution in plant /animal mutualistic networks: forbidden links or random interactions?

Recent studies show that ecological interaction networks depart from the ‘‘scale-free’’ topologies observed in many other real world networks. Such a departure has been hypothesized to result from non-matching biological attributes of species, such as phenology or morphology, that prevent the occurrence of certain interactions (‘‘forbidden links’’). Here I compare the topology of 17 plant /animal mutualistic networks with that predicted by a simple null model that assumes that a species’ degree (number of interspecific interactions) is a function of its frequency of interaction. The topology predicted by this null model is strikingly close to that observed in the real networks. Thus, this null model provides a simple alternative interpretation of patterns observed in ecological interaction networks that does not require the existence of non-matching species traits.