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Seed-dispersal networks on the Canaries and the Galápagos archipelagos: interaction modules as biogeographical entities

Abstract:

Aim: Mutualistic network parameters, such as modularity and nestedness, show non-random linkage patterns. Both increase network stability in different ways. Modularity hampers extinction cascades, whereas nestedness resists network disassembly. We explore these parameters in seed-dispersal networks in two archipelagos and the significance of life history, habitat, geography and phylogeny as drivers of linkage patterns and the applicability of modules as biogeographical entities. Location: Canaries (Atlantic Ocean) and Galápagos (Pacific Ocean). Methods: We compiled data on plant–seed disperser interactions from own observations and the literature, estimated network parameters describing interaction patterns (connectance, nestedness and modularity) and constructed a backbone phylogeny for the analyses. Results: The Canarian network was highly nested but weakly modular, whereas the Galápagos network showed the opposite characteristics. Most key network species are native and have a favourable conservation status. Modularity in the Canaries is correlated with habitats (indirectly affected by altitude and orientation), whereas in the Galápagos it mainly reflects the functional roles of species. Main conclusions: The divergent link patterns for the archipelagos imply that the highly nested Canarian network is stable against disassembly, whereas the modular Galápagos network may show strong resistance against extinction cascades. This difference may be driven by the specific evolutionary dynamics on the archipelagos.