Habitat Quality and Connectivity in Kettle Holes Enhance Bee Diversity in Agricultural Landscapes

The decline of bees compromises pollination services and connectivity of plant populations, both wild and domesticated. The aim of this study was to evaluate bee diversity in natural wetland island-like habitats - called kettle holes - embedded in an agricultural landscape and its relationship with spatial parameters (kettle-hole area, isolation, neighboring land-use diversity), as well as further bee relevant and local factors (flower, vegetation, and tree cover). We sampled 36 kettle holes in an agricultural landscape north of Berlin, Germany, using color traps and identified 77 bee species, representing ca. 20% of the total diversity of the region. Our results showed that neither kettle-hole area nor density of neighboring kettle holes (isolation) alone affected bee diversity; however, the combination of both did: larger and less isolated kettle holes enhanced bee diversity. In addition, habitat quality (i.e. higher flower availability) within the kettle holes positively influenced bee diversity. These results suggested that flowering plants occurring in the kettle holes are an important foraging resource for bee species. Bee communities respond differently to the environment depending on their functional traits related to sociality. In particular eusocial bee abundance was negatively affected by land-use types at 100 and 500 m distance, as well as vegetation cover, area and isolation, suggesting that eusocial bees might depend more on the quality of the habitat within and surrounding the kettle holes than solitary bees. Surprisingly, bee body size was not associated with any environmental factor tested. Our study identified that a combination of large, high-quality kettle holes providing a high cover of insect pollinated flowers within a high density network of kettle holes, enhances bee diversity in intensively managed agricultural landscapes.