Habitat diversity and floral density at different spatial scales influence wild bee pollinators of orchards in a multi-year study

Background/Question/Methods

Native, wild bees provide the important ecosystem service of pollination to crop plants, without which our reliance on managed pollinators would increase. The diversity and abundance of wild bees within an agroecosystem has been found to vary with both local, farm-scale characteristics, and with broad-scale landscape structure surrounding the farm. The objectives of this research were to examine how vegetation and landscape structure interact to affect wild bee abundance and diversity in apples, a pollinator-dependent crop. This research was done for two years in order to examine the temporal consistency in wild bee populations. Thirty orchards were sampled in 2010, and 16 of these sites were resampled in 2012. Landscape metrics at each site were determined with land-cover data, and floral density within each site was measured weekly during apple bloom. The abundance and diversity of wild bees were determined through bee bowl sampling in 2010 and 2012. We hypothesized that the abundance of wild bees would be positively related to the total natural habitat surrounding the farm, and to the density of flower blossoms within the farm. We also hypothesized that the diversity of wild bees would be positively related to the diversity of the surrounding landscape.

Results/Conclusions

Wild bee abundance and diversity were in relative terms highly consistent across the two years at the sampling sites. This suggests that the relative wild bee population is a stable characteristic of an agroecosystem. In both years, wild bee abundance was positively correlated to the density of flower blossoms within the orchard. Furthermore, in both years, wild bee diversity was positively correlated to the diversity of land-cover types surrounding the orchard at a 1 Km radius. These results suggest that while bee abundance is affected primarily by the fine-scale density of blossoms within the farm, bee diversity is dependent on a source pool from the broader landscape. One explanation for these findings is that habitat diversity may increase the availability of nesting and foraging resources for different bee species, including specialists, thereby increasing overall bee diversity within a region. Bee abundance within a site, however, is driven by the relative attractiveness of the floral display at the farm-scale. This study illustrates that landscape features at multiple spatial scales interact to influence the wild bee community within a farm.