  In this work we present a dynamical model that succesfully describes the
organization of mutualistic ecological systems. The main characteristic of
these systems is the nested structure of the bipartite adjacency matrix
describing their interactions. We introduce a nestedness coefficient, as an
alternative to the Atmar and Patterson temperature, commonly used to measure
the nestedness degree of the network. This coefficient has the advantage of
being based on the robustness of the ecological system and it is not only
describing the ordering of the bipartite matrix but it is also able to tell the
difference, if any, between the degree of organization of each guild.
