Habitat and population structure determine patterns of plant-pollinator networks of an endangered palm tree in a grassland-forest ecotone

Pollen transport by insects determines patterns of reproductive encounters between plants with flowers that have spatially or temporally segregated sexes. Pollinators show varied responses to environmental gradients such as those found in grassland-forest ecotones. Individual-based interaction networks are useful yet underexplored tools to understand how interactions vary across these gradients. Interactions between plant individuals and their pollinators directly reflect on plants fitness and genetic structure, seminal attributes for the conservation of endangered species. To test how a grassland-forest ecotone gradient can affect these interactions we studied pollination networks of Butia odorata individuals, an extinction-threatened palm tree from remnant palm grove ecosystems in South America. We evaluated how network metrics (specialization and modularity), and pollinator richness respond to gradients of habitat and population structure in a grassland-forest ecotone. Networks with more isolated palm trees showed greater specialization and modularity. Pollinator richness was dependent on the habitat context and pollinator role: peripheral pollinators were negatively affected by palm density, whereas core pollinators were positively affected by tree cover, which in turn was positively associated with palm density and proximity to the forest. Our results indicated that increased tree cover in the grassland matrix can promote pollinator diversity by decreasing the dominance of core species. Palm density may hamper the movement of pollinators pollen transportation, playing a key role for the conservation of B. odorata and for palm grove ecosystems. Finally, we emphasize the need of protocols that include traditional grassland management to achieve tree and palm tree density that maximizes conservation results.