  Mutualistic interactions, which are beneficial for both interacting species,
are recurrently present in ecosystems. Observations of natural systems showed
that, if we draw mutualistic relationships as binary links between species, the
resulting bipartite network of interactions displays a widespread particular
ordering called nestedness. On the other hand, theoretical works have shown
that a nested structure has a positive impact on a number of relevant features
ranging from species coexistence to a higher structural stability of
communities and biodiversity. However, how nestedness emerges and what are its
determinants, are still open challenges that have led to multiple debates to
date. Here, we show, by applying a theoretical approach to the analysis of 167
real mutualistic networks, that nestedness is not an irreducible feature, but a
consequence of the degree sequences of both guilds of the mutualistic network.
Remarkably, we find that an outstanding majority of the analyzed networks does
not show statistical significant nestedness. These findings point to the need
of revising previous claims about the role of nestedness and might contribute
to expand our understanding of how evolution shapes mutualistic interactions
and communities by placing the focus on the local properties rather than on
global quantities.
