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

Abstract

1. 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.2. 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.3. 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.4. 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. 5. 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.

Asymmetrical gene flow between coastal and inland dunes in a threatened digger wasp

“…However, in a fragmented and urbanized landscape, the current management framework focuses on grazing used as a tool to locally revitalize sand dynamics (Provoost et al 2004). It is recommended to use a heterogenous approach for grazing and grazer type in space and time to reconcile short-term negative (trampling) and long-term positive (open dune landscape) effects of grazing on B. rostrata (Bonte 2005; Batsleer et al 2022b). The genetically well-connected landscape and large metapopulation context ensures population recovery and persisting connectivity when implementing such a dynamic management approach.…”

“…We suggest that the primary focus should be to enlarge population sizes, as demographics seem highly important for this species’ connectivity. Local and directly surrounding habitat should be improved in quality, by creating pioneer open, sand dune habitat, preferably manually as grazers should be used with caution in small, isolated populations (Batsleer et al 2022b). Creating extra stepping stones to increase landscape connectivity, which may already be partially present but vacant, is only effective when combined with increasing population sizes to have sufficient occasional short and long-distance dispersers.…”

“…The copyright holder for this preprint this version posted September 21, 2022. ; https://doi.org/10.1101/2022.09.20.508247 doi: bioRxiv preprint reconcile short-term negative (trampling) and long-term positive (open dune landscape) effects of grazing on B. rostrata (Bonte 2005;Batsleer et al 2022b). The genetically well-connected landscape and large metapopulation context ensures population recovery and persisting connectivity when implementing such a dynamic management approach.…”

“…In parallel, loss of agricultural practices (including grazing of livestock) and obstruction of sand dynamics due to urbanization stimulated the succession and shrub development, at the cost of the open, early-succession or pioneer dune habitats (Provoost et al 2011). Large herbivores have been introduced in many coastal dune reserves during the last three decades to revitalize dune dynamics (Provoost et al 2004), but might have mixed effects on local arthropod species due to intense trampling (Bonte and Maes 2008; van Klink et al 2015; Batsleer et al 2022b). Second and contrastingly, inland sandy soils in Flanders are large postglacial relicts.…”

Strong gene flow across an urbanized coastal landscape in a dune specialist digger wasp

“…the amount of open dune habitats (Provoost et al 2011). Large herbivores have been introduced in many coastal dune reserves to revitalize dune dynamics as a substitute for natural wind dynamics, but have-due to trampling-a variable effect on local arthropod species (Bonte and Maes 2008;van Klink et al 2015;Batsleer et al 2022c). Altogether, coastal dunes in Flanders are a human-altered landscape with complex nature management considerations to take into account in both landscape scale and site-based nature management.…”