Early spring orchard pollinators spill over from resource-rich adjacent forest patches.
Abstract
Pollinator communities are more abundant and diverse in agricultural matrices with more natural habitat, although the reasons for these correlations remain unclear. It is possible that forest fragments and edges provide resources for pollinators in important early weeks of spring, after which time those insects can then 'spill over' into crops such as apple orchards during bloom. To explore how forest edges may feed and therefore promote flower visitor communities in adjacent agricultural habitats, we sampled springtime pollinators in nine orchards and their adjacent forest edge canopies and understories. We identified pollen consumed by pan-trapped bees and flower flies to assess if pollinators ate pollen where they were caught, and if their diets similarly 'spill over' from forest to orchard. We further explored sex differences in habitat usage. Our spatially replicated sampling found that bee and flower fly abundance peaks first in the forest understorey, then in the forest canopy and finally in the orchard. Analysis of digestive tracts showed significant usage of forest canopy pollen throughout the spring, especially before apple bloom. Pollinators had often eaten pollen from a different habitat than the one in which they were caught, suggesting frequent movement between habitats. Digestive tract pollen is an underused but powerful avenue for ecological insight. In Andrena, which are important orchard pollinators and one of the most abundant wild bee taxa in this study, male bees were primarily found in the woods but not the orchards where conspecific females were later active. Synthesis and applications: Forested areas, especially forest canopy trees, provide large amounts of early spring resources that facilitate build-up and spillover of wild pollinator populations into apple orchards during bloom. Forests also provide critical habitat for male bees, which were rarely found in orchards. Despite their importance for bee reproduction, the needs of male bees are usually not considered in conservation planning. Overall, our data indicate that ensuring there is adequate forest habitat adjacent to orchards can improve the long-term sustainability of pollinator populations that provide essential crop pollination services.