Breaking the spell of nestedness

TL;DR

This study reveals that nestedness in mutualistic networks is primarily driven by degree sequences rather than being an inherent feature, challenging previous assumptions about its ecological significance.

Contribution

The paper demonstrates through analysis of real networks that nestedness arises from degree sequences, suggesting a need to reconsider its ecological importance and origins.

Findings

Most networks do not show significant nestedness.

Nestedness is largely explained by degree sequences.

Revises previous views on the role of nestedness in ecosystems.

Abstract

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.