Naturally Occurring Networks

As examples of collaborative networks found in nature we used 
Two naturally occurring networks were used were an airport connection network, where directed edges show connections between airports \citep{nr}, and the weights of the edges represent the amount of traffic between the nodes. The second network is an ecology network \citep{nr}, where nodes are species in an environment, edges represent dependence of one species on another, and the weight is the amount of dependence of one species on another.