Response of native bees to landscape structure in a perennial agroecosystem

Background/Question/Methods

Habitat loss and fragmentation, changes in land use, and intensive farming practices are leading causes of biodiversity loss around the world.  Of particular importance in agroecosystems is the loss of beneficial insects that provide services to farmers including pest suppression and pollination.  Native bees have been shown to provide the majority of pollination services in some farming systems and have the potential to act as a buffer against the current decline in managed honey bees.  In order to ensure the persistence and enhance the conservation of native bees in agroecosystems, it is important to understand how they respond to features of the surrounding landscape.  Therefore, the objective of this study was to determine to what extent native bees in a perennial cropping system respond to landscape structure.  Landscape structure was measured using edge density and proportion of habitat cover.  To address our objective we pan trapped bees at 45 commercially managed cranberry bogs in central Wisconsin.  Prior to site selection we used a GIS to classify the landscape in central Wisconsin using the Cropland Data Layer and chose sites to encompass a gradient of surrounding landscape from highly agricultural to highly wooded.

Results/Conclusions

We found that native bee abundance was negatively correlated with agriculture (i.e. cranberry, r = -0.54, p = 0.046) and positively correlated to forest edge density (r = 0.72, p < 0.01) in the surrounding landscape.  Species richness was also negatively correlated with cranberry (r = -0.55, p = 0.042), positively correlated with forest cover (r = 0.66, p = 0.011), and positively correlated with forest edge density (r = 0.68, p < 0.01) in the surrounding landscape.  These results show that native bees respond positively to the fragmentation of wooded habitat but also to the amount of wooded habitat in the surrounding landscape.  That is, as the edge density as well as the proportion of wooded habitat in the landscape increases, native bee abundance and diversity also increase.  These findings suggest that the preservation of even small patches of woodland in agroecosystems may have important implications for the conservation of native bees and the pollination services they provide.  While habitat fragmentation may in general cause a decline in biodiversity, this pattern seems to be organism, context, and scale dependent.