Across the edge: Spatial segregation drives community structure in tri-trophic multilayer networks at a forest-grassland edge.

Examining spillover between habitat boundaries offers a key opportunity to understand how neighbouring habitats may affect each other. Although extensively studied, ecological responses at forest-grassland edges are variable across trophic levels and their underlying interactions. Thus, tackling the subject from a multitrophic perspective may yield valuable insights into how energy may flow across forest-grassland edges. We asked whether a forest-grassland edge functions as an ecological barrier or a continuum for species interactions across space and trophic levels. We also examined whether species influence in the network is better explained by their distribution across the edge (spatial structure) or by their connections with other species (modular structure). We studied a tri-trophic (prey-consumer-parasitoid) antagonistic system at Atlantic Forest and Pampa Grassland edges, arranged in fire-prone mosaics in southern Brazil. Using network dissimilarity and multilayer approaches, we investigated species and trophic-level contributions to connectivity across the spatial/modular landscape by sampling cavity-nesting hymenopterans and their interactions across a distance gradient from the habitat edge We found spatially segregated modules confined to each habitat, indicating that the edge likely functions as an ecological barrier. Network dissimilarity peaked in cross-habitat comparisons, reinforcing the separation between forest and grassland ecosystems. While all trophic levels were less adaptable to shifts between habitats and modules, they showed greater adaptability across spatial strata within each habitat. The main factor determining species influence throughout the network was their ability to move across spatial layers, although trophic-level and habitat subgroups also responded to other variables. Cross-edge species had greater influence in connecting habitats internally than in serving as energy pathways between them. Our findings reveal that Atlantic Forest-Pampa Grassland edges likely constitute an ecological barrier network-wise. However, edge effects increasing interaction richness and abundance may highlight the importance of edge proximity to key species promoting within-habitat network cohesion. Our results highlight how network dynamics may span across habitat edges with significant species turnover, calling for active conservation strategies to prevent forest encroachment and maintain grassland habitats-while recognising that disturbances within the roughly 40-m edge effects zone could potentially cascade inward, influencing species and interactions beyond the edge.