Rapid conservation evidence for the impact of sheep grazing on a threatened digger wasp

Insect populations show strong temporal fluctuations in abundance. This renders classical monitoring studies extremely difficult to provide insights into specific management actions. For rare species of conservation concern, it is not an option to develop large scale experiments to assess and steer landscape-level actions such as grazing management. Bembix rostrata (Linnaeus, 1758) is a threatened digger wasp from coastal dunes and inland sandy regions occurring in a limited number of populations in NW Europe. Since coastal dunes are rapidly being encroached by bushes, grazing management (cattle, sheep, and horses) has been implemented to keep this biotope open. In order to provide insights for local evidence based conservation, a BACI (before/after and control/impact) experiment was set up to assess the impact of sheep grazing on B. rostrata. We quantified the number of nests during 3 years at two grazed sites and a control-site excluded from grazing. We additionally assessed grazing pressure. The BACI design allowed us to directly adjust the current grazing management. The implemented sheep grazing reduced densities of B. rostrata, but did not lead to its local extinction. We discuss these findings in relation to the biology of the species. Our efficient and effective experimental design allowed a fast assessment of the current grazing management and showed that spatially heterogeneous sheep grazing could be used as a management tool to ensure the conservation of the emblematic digger wasp B. rostrata.