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

De moeilijke balans tussen duinbeheer en bescherming van de Harkwesp : valt begrazing in de duinen te verzoenen met het behoud van ongewervelden?

Bij gebrek aan of in afwachting van grootschalige zandverstuivingsdynamiek in de duinen focust duinbeheer in natuurgebieden vaak op het herstel van habitattypen en maatregelen voor bescherming van specifieke soorten. Begrazing is hierbij een cruciaal beheerinstrument om een dynamisch duinlandschap te behouden of te creëren. Insecten en andere ongewervelden die aangepast zijn aan het dynamische karakter van de duinen zijn niet noodzakelijk ongevoelig voor begrazing in het actuele duinenlandschap. De verstoring door grazers is dus niet zomaar een surrogaat voor natuurlijke winddynamiek. In dit artikel bespreken we of en hoe het gebruik van grazers te verzoenen valt met de bescherming van de bedreigde Harkwesp Bembix rostrata en andere ongewervelden in de duinen.Large-scale sand dynamics are generally lacking in North-Western European dunes. Dune management in nature areas is therefore focused on restoration of typical habitat types and conservation of dune-specific species. Grazing with large herbivores is a crucial tool to protect and create dynamic open dune landscapes. In an altered landscape (urbanisation, fragmentation, fixated and shrub-dominated dunes, dikes, water extraction, etc.) the impact of grazers is expected to be altered compared to the historical agro-pastoral use. Especially dune-specific arthropods are vulnerable to grazing impacts, an often neglected species group in nature management, but an important one for typical biodiversity of dynamic dunes. As such disturbance by grazers cannot be used as an alternative for natural wind dynamics and needs to be reconciled with arthropod conservation in the current landscape context. We use the digger wasp Bembix rostrata, a progressive provisioner with a high level of site fidelity, as an example to illustrate this contradiction and conceive possible solutions. Grey dunes are the ideal habitat of this species. We propose the framework of a landscape mosaic to rotate grazers, preferably sheep, in time and in space to ensure open sand areas are proactively created and source populations and expansion areas are sustained