Edge effects on flower-visiting insects in grapefruit plantations bordering premontane subtropical forest.
Over the last decade, there has been much concern about the decline in pollinator abundance and diversity caused by different types of anthropogenic disturbances, including deforestation and habitat fragmentation. However, little empirical information exists documenting this decline and its consequences for cultivated flowering crops. We tested the hypothesis that remnants of natural habitats act as a source of flower-visiting insects for neighbourhood crops. Over 3 consecutive years we evaluated flower-visiting insect diversity, visitation frequency and composition in four grapefruit Citrus paradisi Macf. plantations at increasing distances (edge, 10, 100, 500 and 1000 m) from remnants of subtropical premontane forest in NW Argentina. The frequency of visits to grapefruit flowers decreased by more than twofold as distance to the forest increased and the flower-visiting fauna became more depaupurate. Even the feral africanized honeybee Apis mellifera, the dominant flower visitor to grapefruit flowers, showed a decline at distances >500 m from the forest edge. However, the greatest relative declines occurred among stingless and solitary bees as well as other native flower visitors, which were rarely seen a few hundred metres inside the plantations. In addition, flower-visiting insect faunas among plantations became more homogeneous as distance from the edge increased. These trends were consistent over years and among plantations up to 50 km apart. Thus, we can conclude that negative forest edge effects on flower-visiting insects inside grapefruit plantations are widespread in the increasingly deforested landscape of NW Argentina. Synthesis and applications. This study provides empirical evidence for considering remnants of natural habitats as a source of both native and alien flower-visiting insects that can be potential pollinators for agriculture. Increasing edge density in agricultural lands, through preservation and restoration of natural habitats, can foster stocks of diverse and abundant insect pollinators.