Feed two birds with one tree? Multilayer bird-plant networks and the maintenance of biodiversity in urban environments

Urbanization in recent decades has led to changes in the structure of communities and ecosystem functions, including pollination and seed dispersal. Many plants rely on biotic vectors for these stages of their reproduction, while many animals depend on the plant resources acquired through these interactions. However, how these interactions overlap in urban environments is not well known. Here, we built multi-layered bird-plant networks characterizing resource use (nectarivory/frugivory) and ecosystem functions (pollination/seed dispersal) from an urban community in Brazil, asking whether biodiversity conservation can be optimized by considering multiple interactions simultaneously. The multilayer networks involving 86 bird and 108 plant species showed low modularity, indicating weak partitioning of interactions. Nevertheless, bird families showed specific associations across modules, indicating some preferences in resource use. Nevertheless, native and non-native plants were broadly distributed across modules, illustrating how the latter are embedded in urban ecological networks. Few species stood out as important contributors to multilayer network structure, including both native and non-native tree species. We find that no plant simultaneously provided both flower and fruit resources in equal importance, even with considerable overlap in the interaction layers. Our study illustrates the use of a recently developed network approach that can be applied to assess multiple interaction types simultaneously. This approach may be useful in finding species that contribute to multiple ecosystem functions simultaneously, or as in our case, show that although some overlap exists, different plant species are needed to fulfil distinct needs for bird species to promote biodiversity-friendly cities.