Hybrid structural arrangements mediate stability and feasibility in mutualistic networks

The debate about ecosystems persistence, namely how ecological communities sustain a large number of species, holds considerable importance since the middle of the last century. It has been recognized that the persistence of a system requires stability, but also feasibility, defined as the capability to sustain positive abundances for all biotas. So far, the relationship between the emergence of ecological variables and the species interaction networks has been studied by restricting to single structural patterns, such as nestedness or modularity. Focusing on mutualistic communities, the current paper addresses the role of the interaction architecture in the emergence and maintenance of both properties. Specifically, we propose a new approach relying on hybrid architectural configurations, motivated by the intuition that the co-occurrence of multiple variables cannot be properly grasped by considering different network configurations distinctly. We launch in-block nestedness in the field of ecology, compound by disjoint subsets of species (modules) with internal nested character, and prove that it trades off stability and feasibility. Remarkably, we analyze a large amount of empirical communities and find that a relevant fraction of them exhibits a marked in-block nested structure. We elaborate on the implications of these results, arguing that they provide useful insights about the key properties ruling community assembly.