Trait and habitat filters shape absences of wild bee and butterfly species from road verges

Road verges can support high densities of flowers and could therefore provide new opportunities for the conservation of flower‐visiting insects. One way of optimizing road verges for vascular plant diversity is to adjust mowing regimes, but to date it is unclear how this affects flower‐visiting insects. Furthermore, for mobile organisms like wild bees and butterflies, there is a risk that the benefit of increased habitat quality in road verges is limited by the proximity to traffic, but this is poorly studied. In a crossed study design, we separated mowing time and frequency (early summer and autumn, or only late summer) from road verge habitat classification (valuable for biodiversity according to transport authority, or regular). We did so along a gradient of traffic intensity, to investigate if a mowing regime designed to enhance plant diversity can also benefit wild bees and butterflies, and if traffic limits the conservation potential of road verges. Road verges that were mown only in late summer had higher flower densities, and there was a positive relationship between flower density and wild bee abundance and species richness. Butterfly abundance and species richness only benefitted from a late summer mowing in valuable but not in regular road verges. Traffic intensity had a substantial negative impact on abundance and species richness of wild bees and butterflies. Higher traffic intensities limited the positive relationship between plant and butterfly species richness that we observed at lower traffic intensities. Increasing width of the road verges buffered negative effects of the traffic on wild bee as well as butterfly abundances, and on wild bee species richness. Synthesis and applications. Road verges can play a valuable role for the conservation of wild bees and butterflies, but there is a need to consider both traffic intensity and resource availability when implementing management strategies. To support wild bee and butterfly diversity, we recommend actions to enhance plant species richness and flower resource availability, and to focus these conservation efforts on roads with low traffic intensity, or on wide road verges.

Road verges can provide habitat for flower-visiting insects that depend on open grassland habitats for feeding and nesting resources. However, road verges can also be harmful or unattractive as habitat due to impacts from traffic or management. In this thesis, I contribute to understanding how traffic intensity, vegetation management and historical landscape context shape the diversity of wild bees, butterflies, plants and briefly solitary wasps in road verges. My results show that a considerable proportion of a region's wild bee and butterfly species can occur in road verges, including red-listed species. Older road verges exhibited a higher plant species richness than younger verges, and I detected a generally positive relationship between flower-visiting insect diversity and the availability of feeding resources. However, I show that high traffic can limit species abundance and richness of wild bees and butterflies, the attractiveness of potential nesting sites, as well as population growth and reproductive success of cavity-nesting insects. High traffic even limited the abundance of flower visitors in the surrounding landscape, although this did not influence pollination success of wild plants. Wide road verges buffered some of the negative effects of traffic on species abundance and richness. Vegetation management also played a significant role in determining habitat quality for insects. Road verges that were mown only once per year in late summer provided more floral resources and exhibited greater insect diversity, though this effect could vary depending on the specific conditions of the verge. Although road verges have the potential to support flower-visiting insects, I conclude that not all verges are equally suitable habitats. Furthermore, some species are particularly sensitive to the effects of traffic intensity, road verge width and management. Wide road verges along roads with little traffic offer the best opportunities for conservation actions. My findings can provide a roadmap for optimizing management to support the diversity of flower-visiting insects and plants along roads.

Cities provide opportunities for biodiversity conservation through the design of urban greenspaces as wildlife habitat. A significant proportion of urban public land is narrow linear road verges, though their small size and harsh environmental conditions (e.g. high soil temperatures) may limit their ability to support plants and animals. We worked with a municipal government in a highly urbanised area to test whether conversion of standard road verges (e.g. lawn) to predominantly native understorey plants (forbs, grasses and shrubs) selected for their abilities to tolerate harsh growing conditions and provide habitat increased the abundance and richness of bees, butterflies and birds. We used a before‐after‐control‐impact experiment and characterised temporal dynamics of biodiversity responses 1 year prior to planting and 4 years post‐planting. We also tested whether traits known to influence species responses to urbanisation (body size and feeding specialisation) mediated responses to road verge plantings. Bee species richness and abundance increased at experimental plantings in the first post‐planting year and remained stable thereafter despite fluctuations at control sites. Butterfly abundance but not richness increased, and there was no evidence of bird responses to plantings. Larger bee species, which are known to be most negatively impacted by urbanisation, benefited more from the road verge plantings, while there was no effect of feeding specialisation. Bird and butterfly traits did not mediate responses to plantings. Synthesis and applications. Road verges comprise a significant proportion of urban green spaces, and our results suggest that despite their small sizes and harsh environmental conditions they can provide habitat for bees and to a lesser extent butterflies. We demonstrated that habitat value can be rapidly enhanced by converting standard road verges to native understorey plants and that these benefits may be greatest for the bee species most negatively impacted by urbanisation. Modifications such as additional plant species or wider verges may be required for urban road verges to provide habitat for a greater range of taxa including birds and butterflies.

Urban expansion continues to impact natural landscapes and biodiversity, yet cities are implementing measures to mitigate some of these negative effects. One such measure is the use of vertical greenings, which aim to enhance urban ecosystems and support biodiversity. Given the limited knowledge of pollinators in these vertical green spaces, we conducted a pilot study in a Central European city (Vienna, Austria) focusing on butterfly and wild bee species richness and their interactions with foraging plants during the year 2022. In total, we recorded 32 wild bee species and eight butterfly species. Our results show that vertical greenings with high floral richness (i.e., comprising more than four flowering entomophilous plant species) supported significantly more wild bee (p = 0.005) and butterfly (p = 0.002) species compared to vertical greenings with low floral richness (i.e., comprising two or fewer flowering plant species). The majority of bee-plant interactions were observed on Nepeta faassenii, Sedum sp., and Parthenocissus quinquefolia, while most butterfly-plant interactions were observed on Nepeta faassenii. Implications for insect conservation: Our study demonstrates that vertical greenings may serve as valuable nectar and pollen sources in urban environments. However, their effectiveness most likely depends on floral diversity, which we observed to be, at least partly, influenced by the maintenance practices of the greenings.

The impact of heavy metal pollution on wild bee communities in smallholder farmlands

Collaborative multi-actor conservation has been heralded as an effective way to address historical biodiversity loss because it makes landscape-level multi-habitat management strategies possible. However, its ecological effectiveness is not well understood. We examine a multi-actor approach in which 11 organisations collaborated to enhance wild bees in a 30 km 2 landscape in the south of the Netherlands. Using a novel study design for landscape-level conservation initiatives, we compared six-year trends in wild bee abundance and species richness and flower cover and species richness in 47 sites with bee-friendly management aimed at increasing the spatio-temporal availability of flowers in five habitats (extensive pastures, road verges, field margins, hedgerows, water retention sites) with trends in similar numbers of conventionally managed controls inside and outside the landscape. Overall, wild bee abundance and species richness increased in sites with bee-friendly management relative to controls, though effectiveness varied by habitat. Across all sites, bee-friendly management resulted in significantly more positive trends in flower cover than in control sites, yet trends in managed sites were stable rather than increasing and flower cover declined by approximately 46 % in control sites. The implementation success of bee-friendly management varied by habitat type and year, and was a key factor underlying the ecological effectiveness of said management. Our results suggest that coordinated collaborative approaches across complementary habitats can produce conservation benefits, but that success depends on effective communication with and consistent participation of actors, guidance by a coordinator, and continuous monitoring of management implementation and ecological outcomes.

Intensification of agriculture has resulted in a simplification and fragmentation of agroecosystems. Yet, its impact on the reproductive success and population dynamics of wild bees, and how adverse effects can be mitigated, remains poorly understood. We established populations of seven solitary bee species varying in body size in experimentally sown wildflower strips (WFS), existing semi‐natural habitats (SNH; forest edges) and isolated sites lacking WFS and SNH in the local surrounding (350 m radius) to test (a) whether the wild bee species use planted flowers of WFS to provision offspring with pollen, (b) whether vicinity to WFS reduces their foraging trip duration, (c) parasitism rate and (d) whether it increases bee reproductive success of nesting populations. Furthermore, we tested whether the effect size depends on the body size of the bee species considered. We show that wild bees nesting in WFS provisioned their offspring primarily with pollen from plants of WFS. This led to shorter foraging trips of all bee species nesting in WFS compared to bees nesting in isolated (unrestored) sites and overall increased reproductive success (i.e. more viable offspring). Reproductive success of bees nesting in WFS was also higher, and parasitism rates lower, compared to bees nesting at forest edges. Smaller—potentially less mobile—bee species benefitted more than larger ones from WFS plantings in terms of reproductive success. Synthesis and applications. Our findings demonstrate that diverse WFS can enhance the reproductive success of multiple solitary wild bee species, thereby mitigating negative impacts of agroecosystem simplification through the provision of suitable floral resources and reduced foraging times required for offspring provisioning. Our study further indicates that a relatively fine‐meshed network of wildflower plantings and nesting habitats is required to most effectively enhance reproduction and populations of solitary bees in agricultural landscapes.

(1) Land use intensification has led to serious declines in biodiversity, including in forage production systems for dairy cows. Agri-environmental schemes, such as enriching grasslands in floral species, were shown to be an effective tool to promote biodiversity in higher trophic levels. Here, we studied an innovative pasture-based dairy production system with floral-species-enhanced temporary grasslands, with respect to the effect on wild bee abundance and species richness. (2) We studied three grass-clover mixtures with perennial ryegrass and clover species with different levels of plant diversity for flower cover and wild bees. The grass-clover pastures were rotationally stocked with cattle and parts of the pastures were excluded from grazing. Intensively managed conventional permanent grasslands were studied as the common land use type. Wild bees were caught by sweep netting. Wild bee diversity was analysed with a general linear mixed model. For species richness, an incidence-based coverage estimator was calculated. (3) In total, 541 wild bees from 10 species were found. No wild bees were caught on the conventional grasslands. Wild bee abundance and species richness did not differ among the three grass-clover mixtures, but with increasing flower cover of white clover (Trifolium repens), wild bee abundance increased. Except for one solitary wild bee individual, the recorded community exclusively consisted of bumblebees. While generalist species that are commonly found on farmland dominated, rare long-tongued bumblebees made up 10% on the grazed sites of the multispecies mixture and made up 20% on the ungrazed strips of the binary mixture and multispecies mixture. (4) We conclude that multispecies mixtures can provide resources for generalist bumblebee species, especially when compared with conventional grasslands that offer no resources. Considering that the multispecies mixture has been also shown to reduce greenhouse gas emissions and nitrate leaching, while maintaining high forage yields, our findings add to the potential to promote a wide range of ecosystem services. Yet, should their full potential be enfolded, grazing should partially be excluded, and the mixture should be extended by plant species with more open flowers, suitable for solitary wild bees.

Roads may have an important negative effect on animal dispersal rate and mortality and thus the functioning of local populations. However, road verges may be surrogate habitats for invertebrates. This creates a conservation dilemma around the impact of roads on invertebrates. Further, the effect of roads on invertebrates is much less understood than that on vertebrates. We studied the effect of roads on butterflies by surveying abundance, species richness and composition, and mortality in ten grassland patches along high-traffic roads (∼50–100 vehicles per hour) and ten reference grassland patches next to unpaved roads with very little traffic (<1 vehicle per day) in southern Poland. Five 200-m transects parallel to the road were established in every grassland patch: at a road verge, 25 m from the verge, in the patch interior, and 25 m from the boundary between the grassland and field and at the grassland-arable field boundary. Moreover, one 200-m transect located on a road was established to collect roadkilled butterflies. The butterfly species richness but not abundance was slightly higher in grassland patches adjacent to roads than in reference grassland patches. Butterfly species composition in grasslands adjacent to roads differed from that in the reference patches. Proximity of a road increased variability in butterfly abundances within grassland patches. Grassland patches bordering roads had higher butterfly abundance and variation in species composition in some parts of the grassland patch than in other parts. These effects were not found in reference grassland patches, where butterfly species and abundance were more homogenously distributed in a patch. Plant species composition did not explain butterfly species. However, variance partitioning revealed that the presence of a road explained the highest proportion of variation in butterfly species composition, followed by plant species richness and abundance in grassland patches. Road mortality was low, and the number of roadkilled butterflies was less than 5% of that of all live butterflies. Nevertheless, the number and species composition of roadkilled butterflies were well explained by the butterfly communities living in road verges but not by total butterfly community structure in grassland patches. This study is the first to show that butterfly assemblages are altered by roads. These results indicate that: (1) grassland patches located near roads are at least as good habitats for butterflies as reference grassland patches are, (2) roads create a gradient of local environmental conditions that increases variation in the abundance of certain species and perhaps increases total species richness in grassland patches located along roads, and (3) the impact of roads on butterflies is at least partially independent of the effect of plants on butterflies. Furthermore, (4) the direct impact of road mortality is probably spatially limited to butterflies living in close proximity to roads.

Roads and road verges can potentially have opposite impacts on organisms that move through the landscape. While road verges can be habitat for a large number of species, and have been proposed to act as dispersal corridors, roads can act as barriers to movement. This duality of roads and road verges has however rarely been assessed simultaneously, and it is unknown to what degree it depends on the amount of traffic on the road and on the habitat quality of the road verge. We used fluorescent powdered dye to track movements of flower‐visiting insects along roads with varying traffic intensity, and in verges with contrasting habitat quality (flowering plant species richness and flower density). Insect movements along road verges were more frequent than movements into the adjacent habitats, indicating that verges act as corridors. The frequency of movements that required crossing the road was lower compared to the frequency of movements that did not, suggesting that roads are barriers. The movement patterns were independent of traffic intensity, but the barrier effect was stronger when the road verge had a higher density of flower resources. The effect of roads as a barrier and of the road verges as corridors were independent of each other. Our results suggest that flower‐visiting insects tend to remain longer in road verges with high density of flowers and we therefore suggest that managing road verge habitats for an increased plant diversity will mitigate the known negative impacts of roads on insect populations.

Abstract1. The severe and ongoing decline in semi‐natural grassland habitat during the past two centuries means that it is important to consider how other, marginal grassland habitat elements can contribute to landscape‐level biodiversity, and under what circumstances.2. To examine how habitat age and the amount of core grassland habitat in the surrounding landscape affect diversity in green infrastructure, we carried out inventories of 36 rural road verges that were either historical (pre‐1901) or modern (established post‐1901 and before 1975), and were surrounded by relatively high (&gt;15%) or low (&lt;5%) levels of grassland habitat. We recorded the number of plant species, grassland specialists, grassland conservation species and the fraction of the landscape's species and specialists found in the road verge.3. Road verge communities were characterised by high levels of grassland specialist species (35% of the 161 species recorded), with road verge sites supporting 15–20% of the specialist species found in the surrounding 25 km2 landscape.4. Richness of species and specialists were more closely related to road age than to the amount of surrounding habitat. Higher diversity in historical roads, despite the majority of modern roads being at least 60 years old, suggests a long time lag in the establishment of grassland communities in marginal grassland habitats. We identified no effect of historical surrounding land use on present day diversity in road verges.5. Road verge richness was not affected by the amount of surrounding grassland. This could be due to the relatively low amounts of grassland remaining in all landscapes, together with dispersal limitation commonly found in grassland plant communities contributing to a potential time lag.6. We identified road verges as potentially very important habitats for grassland communities. Because of the high levels of grassland specialists present, these and other marginal grasslands and grassland green infrastructure should be explicitly considered in landscape‐scale conservation management. Practitioners looking to identify the most species‐rich road verges should aim to find the oldest possible, while long time lags in community assembly suggests that seed sowing could be appropriate to enhance roadside diversity, even in decades‐old road verges.

Numerous wild bee (Hymenoptera, Apoidea, Anthophila) species show negative population trends, while the knowledge gaps on their occurrences and distributions prevent adequate conservation actions. The need for continuous updating of species records and reconfirmation of their presence has been recognized, especially in understudied areas. The present study presents the results of bee monitoring at 30 Serbian localities, each surveyed three times during 2023. Two sampling methods were used, the transect walks and the pan traps, resulting in a detection of 232 wild bee species. Among them, 13 species found at 13 localities, represent the first published records from Serbia: Andrena ferox Smith, 1847, A. nana (Kirby, 1802), A. praecox (Scopoli, 1763), A. pusilla Pérez, 1903, A. susterai Alfken, 1914, A. angustior (Kirby,1802), A. curvungula Thomson,1870, A. falsifica Perkins, 1915, Hoplitis mitis (Nylander, 1852), Hylaeus friesei (Alfken, 1904), Melitta melanura (Nylander, 1852), Nomada trapeziformis Schmiedeknecht, 1882, and Osmia uncinata Gerstaecker, 1869. This study contributes to an update of the list of bee species in Serbia, that now counts 744 species, and also provides additional data on European distributions. The new information on Melitta melanura is especially noteworthy, since this species has been assessed as Endangered by the European Red List of Bees due to its small area of occupancy and a severely fragmented distribution. Other important findings include the confirmation of the presence of some wild bee species in Serbia, i.e., 10 species reported as new records within the previous update, and 19 species that were without previously available records from the 21st century. Additionally, the present study indicates the effectiveness of both conducted monitoring techniques, in terms of different recorded species and numbers of specimens. These results lead to the conclusion that corroborates the application of complementary sampling methods as an adequate way to survey bee diversity and abundance.

Roads are vital for human societies, yet they can also have negative impacts on the ecological communities that live in close proximity to them. Insect pollinators, which nest and forage in road verges running alongside roads, are a group of particular importance. These verges may act as an "ecological trap," drawing insect pollinators into contact with traffic, increasing the risk of pollinator-traffic collisions. Spanning six European regions, we evaluated the complex relationships between traffic, road verge floral composition, and surrounding land use to understand how these factors influence abundance and richness of bees, butterflies, and hoverflies sampled within road verges. Across the study, we observed 10,960 pollinators belonging to 293 species of bees, butterflies, and hoverflies. We observed greater pollinator abundance in verges with higher flower cover, and greater pollinator richness in verges with more species of flowering plants. Lower abundances of bees and butterflies and lower species richness of bees were observed when traffic speed in the adjacent road was higher. This study indicates that road verges with abundant and diverse floral resources support more abundant and diverse pollinator populations, especially on verges alongside lower speed roads. We recommend that lower speed roads should be prioritized for floral enhancements.

While urbanisation contributes to global biodiversity declines, flower-rich urban habitats may provide beneficial pollinator habitats. We investigated the potential of urban residential areas to contribute to pollinator diversity by analysing wild bee and hoverfly species richness and composition of species assemblages of summer-active species, sampled in 53 gardens across urban and rural landscapes of Malmö, the regional capital of Sweden’s southernmost county. Species richness differed between urban and rural gardens, and between four urban residential types (ranging from low human density and high vegetation cover, to high human density and low vegetation cover), and taxonomic groups responded differently. Solitary bee species richness was higher in urban than rural gardens, driven by a higher richness in low-density urban gardens compared to both high-density urban gardens and rural gardens. In contrast, bumblebee species richness was higher in rural than urban gardens, whereas differences among the urban types were less clear. Hoverfly species richness was consistently higher in rural gardens than any urban garden type. Species richness of all groups was negatively related to human population density at the landscape scale (radius 500 m), but unrelated to vegetation cover. This indicates that population density affects pollinator habitat quality through associated green space management and design. Rural and urban wild bee species assemblages consisted of different species (significant species turnover), whereas urban hoverfly assemblages were a subset of rural ones (significant nestedness). Species nestedness of hoverflies, but not bees, increased with human population density. We show that urban areas can complement the regional wild bee species pool, mainly caused by large variation in tenure and management at small spatial scales, while urbanisation drives a systematic loss of hoverfly species. We suggest alternatives to improve dense residential areas for pollinators.

Bees, important providers of vital environmental functions such as pollination and soil bioturbation, are currently declining both on a local and global scale. Two main causes of this decline are habitat loss and fragmentation and floral scarcity in modern agricultural landscapes. Effective measures are needed to counteract these factors so that bees are preserved and their functions are not impaired. The aim of my dissertation is to investigate the potential of on-field and off-field measures for bee conservation, i.e. measures that target productive arable land and non-productive areas. Chapter 1 targets the on field-measure intercropping with focus on legumes and non-legume mixtures. Bees can benefit from this cultivation system through the provision of pollen and nectar. Characteristics of intercrops between legumes and non-legumes are investigated that may influence the value of this cropping system for bees, but also for agricultural production (i.e. pollination services, legume yields, crop genotype traits), as both aspects need to go hand in hand to create desirable "win-win" situations. Bees were surveyed in 2019 during time-standardized observation walks in a field experiment consisting of small-scale plots (21 sqm) with sole crop stands of six genotypes of the grain legume Vicia faba (L.) and intercrops of V. faba and wheat (replacement design). The number of flower visits by bees in intercrops and in sole crops was compared, assuming that intercrops are less attractive due to a lower density of pollen and nectar-providing flowers. Further, the foraging behavior of bees was observed, i.e. legal (= frontal) flower visits and nectar robbing. Differences between the two cropping systems and V. faba genotypes in terms of the yield per V. faba plant and the number of V. faba inflorescences, leaf area index (LAI) and mean V. faba plant height per plot were analyzed. The latter two traits are important for determining the performance of intercrops of V. faba and wheat and possibly for determining attractiveness for pollinators by moderating flower perceptibility. V. faba-wheat intercrops were characterized by a significantly lower number of inflorescences, a lower LAI and a lower V. faba plant height than V. faba sole crops. Further, V. faba yields were significantly higher in intercrops. LAI, plant height, number of inflorescences and yields also differed significantly between V. faba genotypes. No significant difference in the number of bee flower visits per plot was found between intercrops and sole crops. The total number of flower visits and cases of nectar robbing were positively associated with higher V. faba yields. The results show that intercrops with legumes, even though less rich in inflorescences, can be as attractive as legume sole crops. The lower inflorescences density in intercrops is possibly compensated by higher flower perceptibility or higher investments in floral rewards. However, it needs to be verified whether these findings also apply at larger spatial scales (e.g. field scale, landscape scale). With increasing total number of flower visits, including cases of nectar robbing, higher yields per V. faba plant was observed, suggesting positive effects of bee pollination. Yet it is difficult to distinguish to what extent this increase is due to pollination or to other differences between the genotypes / cropping systems. V. faba genotypes differed in LAI and average plant height per plot, but these differences were not linked to the number of flower visits or foraging behavior of bees. Thus, other traits (e.g. floral architecture and display, scent, and quality and quantity of floral rewards) are probably more important for bees. Higher yields per V. faba plant in intercrops compared to sole crops indicate agronomic advantages of this cropping system. Intercrops were mainly visited by domesticated honeybees and widespread ubiquitous bumblebees. Other wild bee species, including threatened species, were rare. As an on-field measure, intercropping with legumes therefore appears to be most suitable for supporting common bee species and thus promoting the provision of important ecosystem services. To foster rare, threatened and specialized species, off-field measures aiming at preserving semi-natural habitats are required. Limestone quarries are semi-natural habitats that could be targeted by off-field measures to promote demanding bee species. Chapter 2 addresses the question which habitat and landscape characteristics determine habitat quality of quarries for bees and which management measures are needed to conserve and restore them. In 2020, wild bees were surveyed in 19 limestone quarries near Göttingen using time-standardized transects. 114 wild bee species, including 35 Red List species, were found. Richness and abundance increased with increasing habitat age, but only at sites where woody plant encroachment was limited. Further, positive interactions between quarry age and landscape diversity and/or habitat connectivity to neighbouring dry grasslands, another valuable and threatened habitat type, were found. In particular, high connectivity between both habitats ensured stable richness of endangered species in old quarries. The results suggest that large, flower-rich and old quarries should be prioritized by conservation and restoration measures. To ensure positive effects of quarry age, woody plant encroachment must be controlled (e.g. by extensive mowing, grazing, scrub clearance), as otherwise important open patches with favorable microclimate, rich in food and nesting resources, could be lost. Furthermore, high connectivity between old limestone quarries and calcareous grasslands was found to be beneficial. Measures aimed at preserving and restoring both habitat types at a landscape scale could therefore be most successful. In Chapter 3, bee data from 2020 was compared with historical data from 1999 collected at the same quarries as in Chapter 2. This was done to determine how bee community composition and beta diversity (incidence-based Sørensen dissimilarity, abundance-based Bray-Curtis dissimilarity and their turnover and nestedness components) changed over time and to assess whether quarry conservation and restoration could be a suitable tool to ensure long-term stability of bee communities in these habitats. Changes in community composition and beta diversity were linked to local habitat characteristics (age, area, woody vegetation cover) and changes in functional characteristics that make bees particularly sensitive or adaptive to changing environmental conditions (nesting behavior, lecty, thermophily, body size). The composition of the bee communities in the quarries changed between the study years, resulting in higher beta diversity in 2020. This was indicated by increasing incidence-based Sørensen dissimilarity and its turnover component. However, no significant changes in the nestedness component of Sørensen dissimilarity, or of the abundance-based Bray-Curtis dissimilarity and its turnover and nestedness components could be found between study years. In terms of functional characteristics, the richness of thermophilic bee species was higher in 2020 than in 1999. Other functional characteristics did not differ significantly between the years, but were consistently influenced by habitat characteristics. High woody vegetation cover in quarries resulted in a decline of the proportions of bee species that nest obligatory underground in self-dug nests and in a tendency towards bee communities with a higher average body size. Proportions of oligolectic bee species (i.e. bees specialized on a narrow range of pollen sources) and the richness of thermophilic bee species were higher large quarries. These results show that bee communities changed between 1999 and 2020, becoming more dissimilar due to species turnover between quarries. This was caused by the spread of thermophilic bee species that migrated and successfully established in the quarries over the last two decades due to climate change. With regard to the most common bee species, little difference was found between 1999 and 2020, as the bee communities in both years were mainly dominated by widespread and non-threatened bumblebees. Further, the findings highlight the value of large quarries with limited woody vegetation for the two vulnerable functional groups of ground-nesting and oligolectic wild bee species, but also for thermophilic bees. However, large-bodied bees appear to be more attracted to wood-dominated quarries, which could be due associated advantages for bumblebees (e.g. favorable microclimate, nesting opportunities). I conclude that intercrops between legumes and non-legumes as well as the preservation and restoration of limestone quarries are suitable measures for bee conservation. Intercrops with legumes primarily support unthreatened and ubiquitous bee species such as bumblebees and domesticated honeybees and are therefore a suitable on-field measure for supporting ecosystem service providers. For threatened, specialized and rare wild bee species, the conservation and restoration of limestone quarries is a suitable off-field measure when in particular large, flower-rich and old quarries with limited amounts of woody vegetation and high connectivity to nearby calcareous grasslands are considered.