Impacts of anthropogenic landscapes on bee community composition and seasonality

Background/Question/Methods

Bees provide the majority of animal-mediated pollination services on which natural systems and food economies depend. However, the impact of landscape-scale changes, including increasing agriculture and urbanization, on bee populations are poorly understood, making it difficult to predict accurately and to plan for how populations will be affected as land development continues. Bees depend on floral resources, yet natural, agricultural, and urban areas present highly variable types of resources and timing of availability. In California, natural areas typically experience large bursts of blooms in the spring and early summer, whereas agricultural areas have homogeneous bloom pulses throughout the season corresponding to crop flowering. Alternatively, urban areas, with high proportions of exotic plants and external inputs, often retain flowers year-round. Here, we investigate the impact of different types of land-use on bee distributions and seasonality.  We collected bees at 24 sites representing a mix of land-use types based on NOAA’s 2006 Pacific Coast Land Cover Dataset classification, and simplified these to: “urban,” “agricultural,” and natural” in East Contra Costa County, CA. Bees were collected via standardized pan trapping in March, May, July, and September 2011.

Results/Conclusions

During 2011, we collected 7883 specimens which were identified to 23 genera. There were significant differences between aggregate bee abundance in different land-use types, but the strength and directionality of these differences changed over the course of the year. Differences were greatest in May, when bees were 4x more abundant in natural areas compared to agricultural and natural sites.  However, towards the end of the season, bee abundance in natural areas decreased to below that in other areas, whereas bee abundance in agricultural areas increased.  As predicted, there were no significant differences between months for bee abundance in urban sites.  Changes in bee diversity paralleled changes in abundance.  We conclude that the impact of land-use change on pollination systems is complex and depends on the type of land-use change and seasonality as well as an interaction between the two.