Honeybee presence restructures pollination networks more than landscape context by reducing foraging breadths of wild bees

Abstract

Wild bee populations are threatened by habitat fragmentation and land-use change, but few development plans consider resource competition. However, managed honeybees (Apis mellifera) are often introduced to residential areas by hobbyist beekeepers, placing potential competitive pressure on wild bees. We sampled bee-plant interactions from natural reserves across the peri-urban landscape of Fort Collins, Colorado (U.S.A.) and modeled pollination networks and bee foraging breadth in response to road and natural area density, reserve size, and background honeybee abundance. Four key findings emerged: (1) honeybees dominated pollination networks, representing 26% of recorded bee-plant interactions. High frequencies of honeybees reduced network modularity, but reserve size, natural area density, and seasonality did not predict network structure. (2) Honeybee frequency increased with road density and declined as natural area cover increased, indicating that higher residential densities drive honeybee pressure. (3) Patterns of floral visitation by honeybees and wild bees indicate substantial niche overlap, and foraging breadths of Megachilidae (leafcutter bees) and Colletidae (plasterer bees) declined as honeybee frequency increased. (4) Noxious weed species (e.g., Convulvulus arvense and Carduus nutans) had high rates of visitation by wild bees and were identified as both pollination network ‘hubs’ and ‘connectors’. We conclude that honeybee presence alters topology of bee-plant networks, and prioritizing creation of natural area reserves to maximize density of non-impervious cover may indirectly reduce competition with honeybees. Conservation practitioners can offset honeybee effects by favoring floral species that support wild bee foraging and are not visited by honeybees, especially in regions where honeybees are not native.