Verena Riedinger

Delivery of crop pollination services is an insufficient argument for wild pollinator conservation

There is compelling evidence that more diverse ecosystems deliver greater benefits to people, and these ecosystem services have become a key argument for biodiversity conservation. However, it is unclear how much biodiversity is needed to deliver ecosystem services in a cost-effective way. Here we show that, while the contribution of wild bees to crop production is significant, service delivery is restricted to a limited subset of all known bee species. Across crops, years and biogeographical regions, crop-visiting wild bee communities are dominated by a small number of common species, and threatened species are rarely observed on crops. Dominant crop pollinators persist under agricultural expansion and many are easily enhanced by simple conservation measures, suggesting that cost-effective management strategies to promote crop pollination should target a different set of species than management strategies to promote threatened bees. Conserving the biological diversity of bees therefore requires more than just ecosystem-service-based arguments.

Local and landscape‐level floral resources explain effects of wildflower strips on wild bees across four European countries

Growing evidence for declines in wild bees calls for the development and implementation of effective mitigation measures. Enhancing floral resources is a widely accepted measure for promoting bees in agricultural landscapes, but effectiveness varies considerably between landscapes and regions. We hypothesize that this variation is mainly driven by a combination of the direct effects of measures on local floral resources and the availability of floral resources in the surrounding landscape. To test this, we established wildflower strips in four European countries, using the same seed mixture of forage plants specifically targeted at bees. We used a before-after control-impact approach to analyse the impacts of wildflower strips on bumblebees, solitary bees and Red List species and examined to what extent effects were affected by local and landscape-wide floral resource availability, land-use intensity and landscape complexity. Wildflower strips generally enhanced local bee abundance and richness, including Red-listed species. Effectiveness of the wildflower strips increased with the local contrast in flower richness created by the strips and furthermore depended on the availability of floral resources in the surrounding landscape, with different patterns for solitary bees and bumblebees. Effects on solitary bees appeared to decrease with increasing amount of late-season alternative floralresources in the landscape, whereas effects on bumblebees increased with increasing early-season landscape-wide floral resource availability.Synthesis and applications. Our study shows that the effects of wildflower strips on bees are largely driven by the extent to which local flower richness is increased. The effectiveness of this measure could therefore be enhanced by maximizing the number of bee forage species in seed mixtures, and by management regimes that effectively maintain flower richness in the strips through the years. In addition, for bumblebees specifically, our study highlights the importance of a continuous supply of food resources throughout the season. Measures that enhance early-season landscape-wide floral resource availability, such as the cultivation of oilseed rape, can benefit bumblebees by providing the essential resources for colony establishment and growth in spring. Further research is required to determine whether, and under what conditions, wildflower strips result in actual population-level effects. Our study shows that the effects of wildflower strips on bees are largely driven by the extent to which local flower richness is increased. The effectiveness of this measure could therefore be enhanced by maximizing the number of bee forage species in seed mixtures, and by management regimes that effectively maintain flower richness in the strips through the years. In addition, for bumblebees specifically, our study highlights the importance of a continuous supply of food resources throughout the season. Measures that enhance early-season landscape-wide floral resource availability, such as the cultivation of oilseed rape, can benefit bumblebees by providing the essential resources for colony establishment and growth in spring. Further research is required to determine whether, and under what conditions, wildflower strips result in actual population-level effects. (Less)

Early mass-flowering crops mitigate pollinator dilution in late-flowering crops

Previous studies focused mainly on the provision of ecosystem services by species movements between semi-natural and managed habitats, whereas data on spillover effects between two managed habitats or between habitats that provide target resources in non-overlapping time periods are lacking. We studied densities of three pollinator groups on sunflower fields as a late mass-flowering crop in 16 landscapes that differed in the relative cover of oil-seed rape as an early mass-flowering crop, in the relative cover of sunflowers and in the relative cover of semi-natural habitats. Our aim was to evaluate dynamics between two crops with non-overlapping flowering periods. Densities of bumble bees in late-flowering sunflower fields were enhanced by early-flowering oil-seed rape. Highest bumble bee densities in the late-flowering crop were reached in landscapes that combined high relative covers of oil-seed rape and semi-natural habitats. Further, low relative covers of oil-seed rape in spring led to decreased bumble bee densities in late-flowering sunflower fields in landscapes with high relative covers of sunflower fields (dilution effect), whereas in landscapes with high relative covers of oil-seed rape, no dilution of bumble bees was found. Thus, our results indicate that early mass-flowering crops can mitigate pollinator dilution in crops flowering later in the season. We conclude that the management of landscape-scale patterns of early and late mass-flowering crops together with semi-natural habitats could be used to ensure crop pollination services. Similar processes could also apply for other species groups and may be an important, but so far disregarded, determinant of population densities in agroecosystems.

Mass-flowering crops dilute pollinator abundance in agricultural landscapes across Europe

Abstract Mass‐flowering crops (MFCs) are increasingly cultivated and might influence pollinator communities in MFC fields and nearby semi‐natural habitats (SNHs). Across six European regions and 2 years, we assessed how landscape‐scale cover of MFCs affected pollinator densities in 408 MFC fields and adjacent SNHs. In MFC fields, densities of bumblebees, solitary bees, managed honeybees and hoverflies were negatively related to the cover of MFCs in the landscape. In SNHs, densities of bumblebees declined with increasing cover of MFCs but densities of honeybees increased. The densities of all pollinators were generally unrelated to the cover of SNHs in the landscape. Although MFC fields apparently attracted pollinators from SNHs, in landscapes with large areas of MFCs they became diluted. The resulting lower densities might negatively affect yields of pollinator‐dependent crops and the reproductive success of wild plants. An expansion of MFCs needs to be accompanied by pollinator‐supporting practices in agricultural landscapes.

Annual dynamics of wild bee densities: attractiveness and productivity effects of oilseed rape

Mass-flowering crops may affect long-term population dynamics, but effects on pollinators have never been studied across several years. We monitored wild bees in oilseed rape fields in 16 landscapes in Germany in two consecutive years. Effects on bee densities of landscape oilseed rape cover in the years of monitoring and in the previous years were evaluated with landscape data from three consecutive years. We fit empirical data to a mechanistic model to provide estimates for oilseed rape attractiveness and its effect on bee productivity in comparison to the rest of the landscape, and we evaluated consequences for pollinator densities in consecutive years. Our results show that high oilseed rape cover in the previous year enhances current densities of wild bees (except for bumble bees). Moreover, we show a strong attractiveness of and dilution on (i.e., decreasing bee densities with increasing landscape oilseed rape cover) oilseed rape for bees during flowering in the current year, modifying the effect of the previous years oilseed rape cover in the case of wild bees (excluding Bombus). As long as other factors such as nesting sites or natural enemies do not limit bee reproduction, our findings suggest long-term positive effects of mass-flowering crops on bee populations, at least for non-Bombus generalists, which possibly help to maintain crop pollination services even when crop area increases. Similar effects are conceivable for other organisms providing ecosystem services in annual crops and should be considered in future studies.

Managing trap‐nesting bees as crop pollinators: Spatiotemporal effects of floral resources and antagonists

Summary 1.The decline of managed honeybees and the rapid expansion of mass-flowering crops increase the risk of pollination limitation in crops and raise questions about novel management approaches for wild pollinators in agroecosystems. Adding artificial nesting sites, such as trap nests, can promote cavity-nesting bees in agroecosystems, but effectiveness could be limited by the availability of floral resources in the surrounding landscape and by natural antagonists. 2.In two European regions, we exposed artificial trap nests in paired field boundaries adjacent to oilseed rape (OSR) fields or non-flowering crops for two years within 32 landscapes covering two independent gradients of OSR cover and semi-natural habitat (SNH) cover in the landscape. We analysed the effects of local and landscape-wide floral resource availability, land-use intensity, landscape complexity and natural antagonists on community composition and population dynamics of trap-nesting bees. 3.Number of brood cells showed a strong, three-fold increase in response to the additional nesting sites. Species richness and abundance of cavity-nesting bees that were active during OSR flowering increased significantly with increasing amount of early-season landscape-wide floral resource availability, such as the cultivation of OSR. Later foraging species benefited instead from the availability of late-season alternative flower resources or SNH cover once the mass-flowering had ceased. Density-dependent parasitism increased following mass-flowering, while no density-dependent effect was found during mass-flowering. 4.Structural equation modelling revealed that the influence of floral resource availability on community growth rate was mediated by community size. Community size showed a strong negative effect on community growth rate. Despite positive density-dependent parasitism, antagonists had only weak regulating effects on community growth rate. 5.Synthesis and applications. Trap-nesting bee populations grow markedly with the increasing availability of food resources in the landscape and effectiveness of trap nests is only marginally limited by natural antagonists. Thus, trap nests could be a simple pollinator-supporting strategy to accompany the current expansion of mass-flowering crops, and to ensure pollination services for insect-pollinated crops. Trap nests benefit not only early season active generalist bees during oilseed rape flowering but also species with later phenology if accompanied by other pollinator-supporting practices. This article is protected by copyright. All rights reserved.

Plant-pollinator networks in semi-natural grasslands are resistant to the loss of pollinators during blooming of mass-flowering crops

Mass-flowering crops lead to spatial redistributions of pollinators and to transient shortages within nearby semi-natural grasslands, but the impacts on plant–pollinator interactions remain largely unexplored. Here, we characterised which pollinator species are attracted by oilseed rape and how this affected the structure of plant–pollinator networks in nearby grasslands. We surveyed 177 networks from three countries (Germany, Sweden and United Kingdom) in 24 landscapes with high crop cover, and compared them to 24 landscapes with low or no oilseed rape during and after crop blooming. On average 55% of grassland pollinator species were found on the crop, which attracted 8–35% of individuals away from grasslands. However, networks in the grasslands were resistant to these reductions, since mainly abundant and highly mobile species were attracted. Nonetheless, simulations indicated that network structural changes could be triggered if > 50% of individuals were attracted to the crop (a value well-above that found in our study system), which could affect community stability and resilience to further disturbance.