Distinct responses of species structural roles in multi‐trophic interactions to island area and isolation

Habitat loss and fragmentation reshape ecological communities and alter species interactions. However, it remains largely unknown how the roles that species play within interaction networks vary across local communities and trophic contexts in fragmented landscapes. We addressed this question using a plant–aphid–ant system sampled across 18 land‐bridge islands in Thousand Island Lake, China. Aphids occupy an intermediate trophic position by feeding on plants and engaging in mutualistic interactions with ants, thereby linking antagonistic and mutualistic interaction layers. We quantified their structural roles from motif positions in plant–aphid, aphid–ant, and integrated tri‐trophic networks, and examined how island area and isolation shape community‐level structural distinctiveness and structural dissimilarity among island communities. Aphid structural roles in mutualism better predicted those in tri‐trophic interactions than herbivory. Isolated islands hold structurally distinct species in the plant–aphid network, while small islands hold those distinct species in the aphid–ant and tri‐trophic networks. While role redundancy was prevalent, aphid communities exhibited higher role turnover in multi‐trophic interactions, with island area and isolation exerting stronger effects than in single‐trophic interactions. Together, these changes in species structural roles may alter the organization and redundancy of ecological networks across local habitats, with potential consequences for network resilience and functioning at the landscape level. Our findings underscore the value of multi‐trophic perspectives for understanding how reduced habitat area and increased isolation reorganize ecological communities and reshape species roles across interaction layers, as conservation strategies focusing solely on single‐trophic interactions may overlook critical trophic dynamics.