The compound topology of host–parasite networks is explained by the integrative hypothesis of specialization

What is the prevalent topology among interaction networks? How do consumers balance between generalism and performance when exploiting different resources? These two long-standing, still open questions have been unified under a common framework by the integrative hypothesis of specialization (IHS). According to the IHS, ecological specialization is structured by different processes at small and large network hierarchical levels, from an entire network to its modules and nodes. From those hierarchical processes, two patterns are expected. First, a modular network with internally nested modules, i.e. a compound topology. Second, different relationships between consumer performance and generalism on different network hierarchical levels. We confirmed those predictions using an extensive data set of host–parasite interactions, compiled from several studies, and spanning decades of fieldwork in the Palearctic Region. We used a set of topological analyses combined in a novel protocol based on the IHS to disentangle the complexity of this data set at different geographic scales, from local to regional. As predicted, the studied network indeed has a compound topology at both local and regional geographic scales. In addition, the relationship between parasite generalism and performance changes from negative in an entire network to positive within its modules. But, as expected, this shift in the signal of the generalism versus performance relationship happens only in local networks with a compound structure. Our results shed light on two central debates about topology and performance and provide insight into their solution.