Keeping Raised Bog Remnants Wet Stabilizes Characteristic Pollinator Communities

Keeping Raised Bog Remnants Wet Stabilizes Characteristic Pollinator Communities | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Keeping Raised Bog Remnants Wet Stabilizes Characteristic Pollinator Communities Tia l'Amie, Jan Kuper, Marijn Nijssen, David Scarse, Eelke Jongejans, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8233210/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Recent studies report declines in abundance and species richness of pollinator communities. In response, nature managers aim to promote pollinators primarily through vegetation manipulation. However, long-term studies that evaluate pollinator community development in restored areas under active management are scarce. To assess whether landscape-level conservation efforts are able to sustain a characteristic pollinator community, we study abundance and richness trends of hoverflies (Diptera: Syrphidae), bees (Hymenoptera: Apidae s.l.), and butterflies/diurnal moths (Lepidoptera), as well as their characteristic species over 30 years within the restored and intensively managed Dutch raised bog system Bargerveen. We describe positive and stable trends in the overall abundance and species richness of pollinators, primarily Apidae and Syrphidae. These trends contrast with nation-wide declines. The effects of recent dry years were evident in declining butterfly abundance trends, although not as pronounced as in national trends. In addition, abundance of species characteristic of raised bog systems remained stable, suggesting that benefits also applied to these generally more sensitive species. Implications for insect conservation : Results from this case study show that conservation efforts in raised bog systems can support and improve pollinator abundance and richness, as well as populations of characteristic and vulnerable species. We argue that the positive and stable population trends of pollinators in Bargerveen are primarily a result of large-scale water management, and that landscape-level hydrological management has the potential to act as a buffer against drivers of insect decline even within predominantly agricultural landscapes. Long-term pollinator trends hoverflies wild bees butterflies wet heathland Nardus grassland Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Abundance and species richness have declined in pollinator communities in recent decades (Biesmeijer et al. 2006 ; Barendregt et al. 2022 ), even within natural areas (Hallmann et al. 2017 ). While pollinator decline is worrisome in and of itself, the loss of pollinators is considered especially alarming due to their crucial role in pollination of both crop and wild plant species (Kremen et al. 2002 ; Vanbergen and Insect Pollinators Initiative 2013). To exemplify, Ollerton et al. ( 2011 ) estimated that roughly 78% of the world's wild flowering plants rely on pollinators for reproduction. Habitat degradation, fragmentation, climate change and the increased abundance of alien species and pathogens are generally hypothesized to be the main drivers behind pollinator decline (Kearns et al. 1998 ; Kremen et al. 2002 ; Potts et al. 2010 ). Overall, there is a clear need for effective conservation and restoration strategies for pollinators. Protected nature areas serve as an important refuge for both native and managed pollinator species (Garibaldi et al. 2011 ; Cooke et al. 2023 ). While many studies show how local measures, such as increasing floral richness, leads to short-term benefits for pollinator communities, this effect generally does not apply to vulnerable insect communities within natural areas. It is known that within natural areas landscape-level nature conservation can promote these communities (Sexton and Emery 2020 ; Larkin and Stanley 2021 ; Uhl et al. 2021 ). However, to our knowledge, the long-term benefits of landscape-level conservation efforts on pollinator communities are rarely assessed. Given the many landscape level threats on natural areas in North-Western Europe, it is essential to assess these long-term effects, as positive effects could well be undone over larger timescales. Raised bog systems in the Netherlands are an excellent example of such a vulnerable natural system. They harbour characteristic pollinator communities that are not found anywhere else in the Netherlands (van Duinen et al. 2013 ; Wallis de Vries et al. 2022 ; Speelman and Kalkman 2023 ). Although raised bogs were once widespread, currently only small remnants remain in an otherwise primarily agricultural landscape. Restoration and conservation efforts within these remnants are therefore crucial for maintaining these characteristic pollinator communities. Two major landscape-level threats for raised bog plant and insect communities in the Netherlands are nitrogen deposition and desiccation (Casparie 1993 ; Tomassen et al. 2004 ). Whereas nitrogen deposition is an ongoing process caused by landscape level drivers beyond the control of local managers, hydrological regimes can be effectively managed at the local scale via the installation of weirs, dams, and buffers (Smolders et al. 2002 ; Brown et al. 2017 ). This in turn has been shown to promote vulnerable plant communities of raised bog systems, despite the context of nitrogen deposition (Bijkerk et al. 2015 ; Adema et al. 2017 ). Although it has been shown that restoring dry grassland vegetation can promote both pollinator abundance and richness (Sexton and Emery 2020 ), to our knowledge the response of pollinator communities from wet systems to hydrological management, especially in the long term, is understudied. Here we analyse two long-term (1994–2024) datasets documenting pollinator and detailed butterfly counts in Dutch raised bog reserve Bargerveen. During the study period several landscape-level hydrological interventions occurred, which gave us the opportunity to study changes in pollinator abundance and richness within the context of restoration efforts. Specifically, we aim to answer the following research questions: 1) Has the abundance of pollinators in Bargerveen changed over the past three decades?; 2) Has pollinator species richness changed?; 3) What are the population trends of selected vulnerable and characteristic species? 4) Do hoverfly abundance trends differ between species with contrasting larval strategies? Materials and Methods Brief Area Description and Management The study was conducted in the Natura2000 area Bargerveen, currently the largest raised bog in the Netherlands. Having been subject to peat extraction until 1992, the area is now actively managed to promote peat formation. It hosts a number of rare and red-listed species typical for raised bogs (van Guldener et al. 2016 ; Speelman and Kalkman 2023 ). Management in Bargerveen includes area-wide mowing and grazing. In addition, significant system-level interventions have been taken to restore wet bog conditions and the natural hydrological regime. These include the infilling of the Noordersloot watershed in 1997 (Rossenaar et al. 2017 ), restructuring and placement of additional weirs and dams which primarily took place between 2001 and 2006, and the placement of hydrological buffers in 2014, 2017, 2018 and 2020 (Fig. 1 A). These measures have rewetted the system and led to more stable water levels in the area (Bijkerk et al. 2015 ; Stroet 2019 ), thereby facilitating the recovery of characteristic raised bog vegetation (Bijkerk et al. 2015 ). Nevertheless, the nature reserve is still subjected to eutrophication, acidification, drainage, and climate change that threaten the naturally nutrient-poor system (van Guldener et al. 2016 ; Adema et al. 2017 ), which are often linked to pollinator declines. In this study, we analyse transect data from two characteristic habitat types found in raised bog systems (Fig. S1 ); Nardus grassland (H6230, Fig. 1 B) and Northern Atlantic wet heath with Erica tetralix (H4010_A) (Fig. 1 C). Grasslands Nardus Grasslands The Nardus grasslands of Bargerveen are semi-natural, and developed from old agricultural fields. A well-developed Nardus grassland can be recognized by the presence of plant species such as Platanthera bifolia , Dactylorhiza maculata , Galium saxatile and Potentilla erecta (Bijkerk et al. 2015 ). To maintain the preferred low to medium nutrient availability in the grasslands, management focuses on soil impoverishment, which is realized via yearly mowing and removal of the clippings. Acidification, primarily due to high N-deposition in the area, poses a significant threat to the ideally weakly buffered grasslands (de Graaf et al. 2009 ; Adema et al. 2017 ). The Nardus grasslands in Bargerveen have their own apparent water table, dependent on precipitation, evaporation and seepage. Maintaining a high water level is essential for maintenance of a characteristic vegetation community (van Duinen et al. 2013 ). Likely as a result of hydrological interventions within the area, the water table in the Nardus grasslands of Bargerveen increased between 2006 and 2007, and remained relatively stable up to 2018, after which a slight drop in the surface water level was observed (Fig. S2). The Nardus grasslands of Bargerveen house a relatively high number of characteristic butterfly species (van Duinen et al. 2013 ), including the grizzled skipper ( Pyrgus malvae ) and the sooty copper ( Lycaena tityrus ). Both butterfly species are sensitive to decline due to their life-history traits, and are closely associated with Nardus grasslands, making them important targets for management in the area. Wet Heathlands The second area is classified as a Northern Atlantic heath with Erica tetralix , found on acidic, nutrient-poor soils. This habitat is characterized by the presence of Erica tetralix, Calluna vulgaris , grasses such as Molinia caerulea , sedges such as Rhynchospora alba , and Sphagnum mosses. Hydrology plays an important role in wet heathland systems, as most characteristic vegetation and insect species of this habitat type rely on high water tables. Within the wet heathlands of Bargerveen, no significant long-term developments in surface water table level were identified (Fig. S2). A major challenge in the wet heathlands of Bargerveen is preventing encroachment of grasses and other vegetation due to high N-deposition and acidification. To counter this, non-heath vegetation is removed via grazing by sheep and cattle, mowing with clipping removal, and controlled burning (Bijkerk et al. 2015 ; Adema et al. 2017 ). These measures aim to promote wet heathland vegetation, but also create open patches which benefit nesting wild bee species (Bijkerk et al. 2015 ; Wallis de Vries et al. 2022 ). The pollinator communities of wet heaths are not very diverse, but contain a high proportion of characteristic and rare species. One such species is the vulnerable silver-studded blue ( Plebejus argus ), which is locally abundant in wet heaths of Bargerveen (van Duinen et al. 2013 ; Wallis de Vries and Oteman 2019 ). The critically endangered brown-banded carder bee ( Bombus humilis ) is also commonly found in the wet heaths of Bargerveen. While once widespread in the Netherlands, its distribution is limited to a few sites in Drenthe (Speelman and Kalkman 2023 ), with Bargerveen likely supporting the largest B. humilis population in the Netherlands, making it an important target species for management. B. humilis is also a valuable indicator species due to its close association with wet heathlands and sensitivity to competition pressure by other pollinating species (Saunders 2008 ; Speelman and Kalkman 2023 ). Pollinator Transects Hoverflies (Diptera: Syrphidae), bees (Hymenoptera: Apidae s.l.), and butterflies/diurnal moths (Lepidoptera) were counted and identified up to species-level during nine years: 1994–1995, 1997–2001, 2017 and 2024. Sampling dates varied between the years, but always took place between May and October, with 93.7% of sampling events taking place between June and August. Transect distance was fixed with permanent poles in the field, and transects were sampled while walking at a constant pace. All individuals were counted and identified to species-level in the field if possible. In case of uncertainty, the specimen was collected and preserved for later identification. Other pollinators passing during this handling time were not included in the counts. Specimens that could not be identified at the species-level were identified to the lowest possible taxonomic rank. For species richness analyses, we included only individuals identified to the species-level, treating additional genera or families as a single species. The Bombus terrestris species complex was treated as one species as accurate identification based on field characteristics is not possible (Williams et al. 2012 ). The Nardus grassland pollinator transect (NG), was located on the east end of Bargerveen (Fig. 1 ), and was sampled a total of 75 times. Transect length of NG was 250m from 1994–2001 and 150 m in 2017 and 2024. Transect width remained constant at 5 m (Fig. 1 B). The wet heath pollinator transect (WH), is located in a wet heath area on the west-side of Bargerveen (Fig. 1 C), and was sampled a total of 67 times. The length of 200 meters and width of 5 meters remained constant during all sampling dates before 2024. In 2024 half of the original transect was inaccessible due to the high water level. Therefore, another nearby transect (100 x 5m) within the same habitat type was sampled in addition to the accessible half of the original transect (100 x 5m) (Fig. 1 C). During the first year (1994) sampling in WH was focused on specific species. This year was excluded from abundance and species richness analyses, but included for species-specific analyses of those species. Butterfly Transects Butterfly counts were consistently performed in accordance with the guidelines set by “ Handleiding Landelijke Meetnetten Vlinders en Libellen ” (van Swaay et al., 2011 ). This data was collected in the framework of the Dutch Butterfly Monitoring Scheme. The butterfly transects overlapped partially with the pollinator transects in both sites. The Nardus grassland butterfly transect (NG_B) had a length of 250 meters and width of 5 meters (Fig. 1 B). In the NG_B transect all butterfly species were counted and identified to the species-level where possible. Sampling took place between April and October during 13 years: in 1995, 1999–2017, 2019, 2021, and 2023. NG_B was sampled a total of 198 times. The wet heathland butterfly transect (WH_B) had a length of 200 and width of 5 meters (Fig. 1 C). Butterfly counts in WH_B focused solely on P. argus , and took place between June and September, during peak activity of the species. Sampling in WH_B took place a total of 96 times over the course of 20 years: in 1999, 2001–2009, 2011–2017, 2019, 2021 and 2023. Weather Data To account for the influence of bioclimatic circumstances on pollinator abundance trends, data on temperature, irradiance, humidity, and wind speed were collected from the Royal Netherlands Meteorological Institute (Koninklijk Nederlands Meteorologisch Instituut: KNMI). Since sampling dates were chosen based on favorable weather, no hourly data on precipitation were included as precipitation was assumed to have been absent. Weather data were collected from the nearest KNMI meteorological station in Hoogeveen (approximately 30 km from Bargerveen). For the WH, WH_B and NG_B transects the starting time of the transect counts was consistently recorded, and hourly data corresponding to the starting hour, as well as averaged daily meteorological data were used for analysis (Table S1 -2). For the NG transect averaged daily meteorological data were used since start time was not always recorded. Hydrological Data Hydrological data describing surface water levels for the Nardus grasslands (ID = P23A0058 and P23A0087) and wet heath (ID = P23A0102 and P23A0103) were retrieved from Dinoloket, a public Dutch national database of groundwater head observations (TNO – Geological Survey of the Netherlands). Data Analysis All analyses and visualizations were conducted in R V4.3.2 (R Core Team 2023 ). Prior to analysis, counts from NG were corrected for transect length by adjusting species counts to a standardized transect length of 150 meters, by removing individuals randomly based on percentual increase in transect length. As transect length remained constant, this was not necessary for WH, WH_B and NG_B. The western honeybee ( Apis mellifera ) is a managed species and was therefore excluded from the abundance analyses, but included in the richness analyses. Pollinator Abundance Trends To test whether the abundance of pollinators has changed within the transects the daily total pollinator counts and species-group specific (Syrphidae, Apidae and Butterflies) daily counts were analysed. To correct for seasonality a similar approach to Barendregt et al. ( 2022 ) was taken. A generalized additive model (GAM) was fitted to daily abundance counts irrespective of year. This model was then used to predict relative day-of-the-year pollinator abundance, resulting in a so-called season score. The season score was normalized and ranges from zero to one, with values close to one representing abundance measures taken during the peak of the season. Then, to assess changes in pollinator abundance over the years separate generalized linear mixed-effect models (GLMM) were fitted over total daily pollinator counts and species-group specific counts in WH and NG. The basic structure of the models included the season score and a continuous year variable as fixed effects, as well as year as a random categorical variable to correct for nestedness within years. Models were fitted over daily abundances over the entire study period (1994–2024) to analyse overall changes in abundance, as well as a subset encompassing 1994–2001, to assess pollinator abundance trends prior to the installation of hydrological buffers. Models including weather variables were explored, but did not significantly improve model AIC (Table S1 -S2). Hence, in the main text we present the results from the basic model. A similar approach was followed to analyse the daily count of butterfly abundances in NG_B and WH_B. However, visual inspection of the data distribution revealed that the daily abundance of butterflies did not follow a linear trend. Therefore, in addition to GLMMs, GAMs including date and season score were fitted to assess non-linear abundance changes over the years in the butterfly transects. Species Richness Trends In order to gain insight into species richness trends, species accumulation curves (SAC) were generated using the R package vegan V2.6.4 (Oksanen et al. 2024 ). The function specpool was used to calculate Chao-estimates, a measure used to estimate the extrapolated species richness within a species-pool based on the observed species accumulation. The calculated Chao-estimates were plotted and visually inspected to assess changes in species richness over the years. Abundance Trends Characteristic Species The characteristic butterfly species Pyrgus malvae , Lycaena tityrus , Plebejus argus , and bumblebee species Bombus humilis were selected for species-specific analysis. GLMMs including year and season score as well as GAMs including date and season score were fitted to analyse P. malvae (NG_B), L. tityrus (NG_B) and P. argus (WH_B) abundance trends. To analyse B. humilis abundance (WH), GLMMs including year and season scores were fitted. Abundance Trends Larval Strategies Syrphidae In order to assess whether abundance trends of hoverflies differed per larval strategy, GLMMs including year and season score were used to analyse trends of subsets of hoverfly species. Syrphidae species were classified as either aquatic saprophagous, terrestrial saprophagous, phytophagous or zoophagous based on classification as described by Reemer et al. ( 2009 ). Due to the low abundances of Syrphidae in the wet heath transect, the larval-stage specific model failed to converge, and no significant trends were observed in the abundances of the various Syrphidae larval types in the wet heath. Results Within the pollinator transects (WH and NG) a total of 15,284 butterfly, 3,956 hoverfly and 3,385 bee individuals (1,897 excluding A. mellifera ) were counted, representing 30, 42 and 19 species respectively. In the pollinator grassland transect, a total of 3,625 individuals from 42 hoverfly species, 682 individuals (313 excluding A. mellifera ) from 17 bee species, and 5,645 individuals from 27 butterfly species were recorded. In comparison, in the wet heath transect 331 individuals from 21 hoverfly species, 2,703 individuals (1,584 excluding A. mellifera ) from 13 bee species, and 9,639 individuals representing 20 butterfly species were recorded. A total of 47,584 butterflies were counted within the butterfly transects (NG_B and WH_B). In the butterfly grassland transect NG_B 20,774 butterflies representing 31 species were counted. In butterfly wet heathland transect WH_B 26,691 Plebejus argus individuals were sampled. For a more detailed species list, refer to table S3 in the supplementary information. Pollinator Transects Abundance Trends Total pollinator abundance within pollinator Nardus grassland transect NG increased significantly (0.93% per year, CI 95% [0.53%, 1.33%] Fig. 2 A) over the course of the entire study (1994–2024). This significant increase in abundance in NG translated to the increased abundance of Apidae (3.28% per year, CI 95% [1.82%, 4.76%], Fig. 2 C) and Syrphidae (2.90% per year, 95% CI [1.40%, 4.42%], Fig. 2 B). The abundance of butterflies on the other hand declined significantly between 1994 and 2024 (-2.16% per year, CI [-3.33%, -1.09%], Fig. 2 D). Analyses using separate GLMMs on smaller data subsets identified no significant changes in total pollinator abundance or group-level abundances in NG between 1995 and 2001 (Figs. 2 A-D). In contrast, no significant changes were found in pollinator abundance in the wet heathlands over the course of the entire study period (1994–2024), either in total pollinator abundance or at the group level (Figs. 2 E-H). However, total pollinator abundance in the wet heathland pollinator transect WH decreased significantly between 1995 and 2001 (-4.96% per year, CI [-5.72%, -4.19%], Fig. 2 E), primarily due to a drop in butterfly abundance (-5.80% per year, CI [-5.95%, -5.66%], Fig. 2 H). No significant pollinator abundance trends were identified for either Syrphidae or Apidae in WH. The majority of the Syrphidae encountered in the Nardus grasslands have (semi-) aquatic saprophagous and terrestrial zoophagous larval stages. The significant increase in Syrphidae mainly was mainly due to an increase in zoophagous species, which saw a significant increase in abundance (4.51% per year, 95% CI [0.11%, 9.10%], Fig. 3B). A significant increase in phytophagous species was identified as well (2.55% per year, 95% CI [2.51%, 2.59%], Fig. 3C), though these species were only present in very low abundance in all years except 2017. The observed increase in phytophagous syrphids is mainly a result of the remarkably high number of Eumerus in 2017, which accounted for 58 of the 59 phytophagous hoverflies found that year. No significant trends were found for the other larval types in the Nardus grasslands. Similar to in the Nardus grasslands, the larval types of Syrphidae in the wet heath consisted primarily of (semi-) aquatic saprophagous and terrestrial zoophagous species. No terrestrial phytophagous species were observed in the wet heath throughout the entire study period. Terrestrial saprophagous species were recorded in the wet heath only in 1998 and 2001. Within the wet heathland pollinator transect the characteristic bumblebee B. humilis was found in relatively high abundance, accounting for over a quarter of all recorded Apidae individuals. B. humilis abundance in WH showed a near-significant decline (p = 0.061) between 1995 and 2001 of -6.21% per year, CI 95% [-9.20%, -3.11%] (Fig. 4 ). However, no significant change in B. humilis abundance over the course of the entire study (1995–2024) was found. Butterfly Transects Abundance Trends A GLMM including year and season score as fixed effects fitted to the daily count of butterfly abundance in the butterfly Nardus grassland transect NG_B showed a significant yearly increase of 4.29%, CI 95% [1.88%, 6.47%]. A statistically significant non-linear relationship between sampling date and daily NG_B butterfly abundance was identified (Fig. 5 A). NG_B butterfly abundance remained relatively stable at the start of the study between 1995 and 2006, and then showed an increase up to roughly 2016. In the past decade butterfly abundance in NG_B gradually declined. Species-specific analyses using GLMM of L. tityrus and P. malvae in NG_B also found a significant increase in abundance for both butterfly species of 8.80%, CI [1.17% − 16.48%] and 5.79%, CI [1.23% − 10.60%] respectively. However, further analysis using GAMs reveals that abundance does not increase linearly, but instead follows a roughly hump-shaped pattern (Fig. 5 ). A similar increasing period as seen in NG_B total butterfly abundance between 2006 and 2016 for both species. P. malvae abundance remains roughly stable after this increasing period (Fig. 5 C), whereas L. tityrus abundance steeply declines (Fig. 5 B). No significant change in P. argus abundance within the wet heathland butterfly transect WH_B was found over the course of the entire study period. Further analysis using GAM found that abundance trends of P. argus were variable over the years. P. argus daily abundance in WH_B showed an initial increasing trend from 1999 to 2008, after which abundance declined. From 2014 onwards, coinciding with the installation of nearby buffer systems, P. argus daily abundances showed a steady increase (Fig. 6 ). P. argus abundances did not increase during the most recent sampling years. Pollinator Transects Species Richness Trends Pollinator species richness in the pollinator grassland transect NG fluctuated notably more (Fig. 7 A) compared to species richness within the wet heathland pollinator transect WH (Fig. 7 B), but was consistently higher than species richness in WH. Both transects seem to have been subject to a decline in species richness prior to 2001. However, chao estimates in 2017 and 2024 are relatively similar, or even high, compared to richness estimates during the start of the study. Apidae species richness remains relatively low and stable over the course of the study in both pollinator transects (Fig. S3). Species richness in the wet heathland was particularly low in 2017, primarily due to low butterfly and hoverfly richness. In the same year however, species richness within the Nardus grassland pollinator transect was estimated to be relatively high, especially the species richness of Apidae. Butterfly Transect Species Richness Trends The species richness of butterflies in NG_B generally ranged between an estimated 15–30 species. The estimates fluctuated over the years, but an overall increase in species richness was observed (Fig. 8 ). Richness was estimated to be the highest in 2017 (39.75 ± 20.72) and 2023 (33.67 ± 15.16), though both estimates have a large standard deviation. During the peak of butterfly abundance in NG_B between 2010 and 2015 (Fig. 5 A) species richness in NG_B remains very stable. Discussion Pollinators are declining in North-Western Europe (Biesmeijer et al. 2006 ; Warren et al. 2021 ; Barendregt et al. 2022 ), even within natural areas (Hallmann et al. 2017 ). Interestingly, our study shows that pollinator populations of two typical raised bog habitats within the natural area of Bargerveen remained stable or increased over a thirty-year long period. We argue that this is most likely a result of landscape-level nature management, including the intensive large-scale hydrological interventions in Bargerveen which were previously shown to restore typical bog vegetation (Bijkerk et al. 2015 ). The same pattern was observed in selected characteristic species of the studied bog habitats, which depend on specific host plants and open habitat structure (preventing shrub or grass encroachment) that in bog-systems are typically linked to moist conditions (Bos et al. 2006 ; Speelman and Kalkman 2023 ). This further suggests that landscape-level management has the potential to support characteristic pollinator species, and to contribute to the long-term conservation of pollinator communities in raised bog systems. Abundance and Species-Richness In both the Nardus grasslands and the wet heathlands we observed stable to increasing abundance and species richness of pollinators. This finding contrasts with studies describing long-term arthropod biomass and species decline that describe declines even within natural areas. For instance, Hallmann et al. ( 2017 ) studied arthropod biomass within German natural areas during the same time period. They described a 75% decline over 25 years. In later work, they scale this to hoverfly species richness decline (Hallmann et al. 2021 ). The natural areas studied by Hallmann et al. ( 2017 ) were often mown or grazed, but were rarely subjected to land-scape level hydrological management. Therefore, even though management in Bargerveen is not limited to hydrological interventions, we hypothesize that the land-scape level hydrological interventions could explain the differences in pollinator trends. This hypothesis is further supported by the fact that a lot of species that reside in raised bog systems rely on high water tables and are sensitive to drought (Wallis de Vries and Oteman 2019 ; Wallis de Vries et al. 2022 ). In addition, drought has been identified as a major contributor to butterfly decline both nationally and internationally (Warren et al. 2021 ; Westra et al. 2022 ; van Swaay et al. 2022 ; van Swaay and Borkent 2024 ). This sensitivity to drought was also reflected by our results, where the effects of dry years after 2017 (Fig. S2) were evident in the butterfly abundance in the Nardus grassland pollinator transect, which showed a small but significant decline. In spite of this, the declines in butterfly abundance in our study seem less strong than the nationwide pattern (van Swaay and Borkent 2024 ), suggesting that although hydrological measures were not able to fully compensate for the adverse effects of exceptionally dry years, they were able to buffer them. Abundance Trends Characteristic Wet Heathland Species In spite of declines of characteristic bumblebees and butterflies in north-western Europe (Goulson et al. 2008 ; Warren et al. 2021 ), and their decline on a national level (De Vlinderstichting 2017; Speelman and Kalkman 2023 ), no overall decline in abundance was observed in either P. argus or B. humilis . It has been described that the hydrological interventions in Bargerveen have led to the recovery of characteristic heathland flora (van Duinen et al. 2003 ; van Duinen et al. 2006 ; Bijkerk et al. 2015 ). Our results illustrate that this vegetation recovery also benefits the vulnerable pollinating species of wet heathland systems, further suggesting that the focus on hydrological management in the Bargerveen area is an important reason for pollinator community conservation. Nevertheless, population trends of both species did fluctuate throughout the study period. Notably, both B. humilis and P. argus show particularly low abundance at the beginning of the monitoring period. This may have been a result of habitat disturbances resulting from the infilling of the nearby watershed Noordersloot in 1997, one of the early important hydrological interventions in Bargerveen (Rossenaar et al. 2017 ). As Bargerveen is a relatively large area, it is likely that suitable habitat was available elsewhere in the reserve, and these species survived these local interventions and populations later recovered along the transect. However, this does illustrate that hydrological interventions can have significant adverse effects during implementation that should be considered when dealing with small, isolated and fragile populations. Another decline in P. argus abundance was observed between 2007 and 2014. This may have been a result of encroachment of grasses like Molinia caerulea in Bargerveen. The grass reportedly occurred in high abundance during this time at the expense of typical heathland vegetation like E. tetralix , an important host plant (Bijkerk et al. 2015 ). The increase in P. argus abundance after 2014 coincides with the installation of hydrological buffers. Together with the partially stabilized encroachment of M. caerulea , these conditions support typical heathland vegetation (Bijkerk et al. 2015 ). This again suggests that abundance trends are linked to improved hydrological conditions. Overall, these findings suggest that the stable population trends of the characteristic species P. argus and B. humilis observed in Bargerveen are a result of continuous management efforts. In addition, these results highlight the need to track population trends over time. Positive effects of management interventions, such as the stable population trends we observed in spite of national declining trends, may only become clear in the longer term. Abundance Trends Characteristic Nardus Grassland Species Both P. malvae and L. tityrus are known to be sensitive to drought (van Turnhout et al. 2003 ; Wallis de Vries and Oteman 2019 ). Despite P. malvae remaining more abundant than at the start of national monitoring in 1990, the effect of dry years can clearly be seen in recent national abundance trends, with declines in abundance seen after 2017 in both the Netherlands and Belgium (Westra et al. 2022 ; van Swaay and Borkent 2024 ). We found that the P. malvae population in Bargerveen experienced no significant decline during this period. This stable population trend of is likely a result of the combined effect of the large population size (Wallis de Vries and Oteman 2019 ), high abundance of its host plant tormentil Potentilla erecta (Bijkerk et al. 2015 ), and hydrological interventions, which likely helped mitigate the negative impacts of dry years. Though the L. tityrus population in Bargerveen was thought to be stable (van Duinen 2013), our results suggest a recent decline, potentially because of several dry years between 2017 and 2023. A report by Wallis de Vries & Oteman ( 2019 ) provides a possible explanation as to why the effect of the dry years is seen in L. tityrus , but not P. malvae . They found that L. tityrus abundance is primarily driven by high spring precipitation, whereas P. malvae primarily depends on high soil moisture, with spring precipitation the year prior actually reducing its abundance. Combined with our results, this suggests that the hydrological management implemented in Bargerveen is effective in maintaining favourable soil hydrological conditions, even during extended periods of unfavourable weather. Study Limitations and Implications Our results are in line with our hypothesis that the nature conservation efforts within Bargerveen would aid in sustaining pollinator abundance and richness long-term. They align with short-term studies showing that system-level conservation efforts, e.g. via improving local conditions for vegetation, can lead to pollinator increases (Sexton and Emery 2020 ; Larkin and Stanley 2021 ; Uhl et al. 2021 ). Where these studies focus on dry grassland or forest systems, our results indicate that this positive effect is also seen in raised bog habitats. In addition, most studies on pollinator restoration outcomes focus on short-term effects, whereas our analysis reveals that system-level restoration can have long lasting, stabilising effects for pollinator populations. Although the benefits of both species-specific and system-level conservation approaches are a long-standing debate in ecology (Tews et al. 2004 ; Harvey et al. 2017 ; Kremen and Merenlender 2018 ), this distinction has received less attention in pollinator conservation, where most measures remain group-focused (e.g., flower strips) (Albrecht et al. 2020 ; Pérez-Sánchez et al. 2023 ). Our results contribute to this debate by showing that pollinator communities, including characteristic species, can benefit from restoring system-wide abiotic conditions, particularly over longer time scales. This likely reflects the fact that most pollinators are partial habitat users that rely on a mosaic of resources such as different foraging and nesting sites. While targeted interventions can alleviate specific limitations, more recent work describes that they rarely recreate the full habitat complexity required to support diverse pollinator assemblages (Wood et al. 2015 ; Requier and Leonhardt 2020 ). In contrast, bottom-up restoration of entire systems appears to support this complexity. Our findings therefore suggest that landscape-level restoration may yield broader benefits for pollinators. Nevertheless, specifically for restoring hydrological conditions, our study provides only an initial indication that rewetting positively affects pollinator communities. Further research linking hydrological data to pollinator trends is needed to confirm this finding, and to better understand the mechanisms through which hydrology influences pollinators, such as through host plant recovery, changes in vegetation structure, or moisture buffering during dry periods. Our results indicate a positive long-term impact of hydrological interventions. However, nature managers should be aware that such interventions may also lead to an increase in (temporary) disturbances as well as substantial changes within the landscape. Large-scale hydrological interventions may therefore also result in habitat loss in certain locations, with possible adverse effects on characteristic species (Verberk et al. 2010 ; van Duinen et al. 2013 ). For instance, the increased water table of Bargerveen is likely to lead to the disappearance of Nardus grasslands in the center of the nature area, a conscious decision which was made to favor the development of active peat-forming raised bog habitats (Bijkerk et al. 2015 ; van Guldener et al. 2016 ). Therefore, in order to preserve the characteristic species which reside in these Nardus grasslands, it is important to compensate for local habitat losses by developing and supporting suitable alternative sites elsewhere within the nature reserve. Conclusions Although declines in pollinator populations have been reported in North-West Europe, even within protected natural areas (Biesmeijer et al. 2006 ; Hallmann et al. 2021 ; Warren et al. 2021 ; Barendregt et al. 2022 ; van Swaay and Borkent 2024 ), we show that pollinator communities in Bargerveen are relatively stable. This suggests that landscape-level conservation efforts, and in this case hydrological buffering specifically, are able to act as a buffer against drivers of insect decline. As we observe positive and improving trends, both at the community and species-level, we conclude that management in Bargerveen is effective in supporting pollinator communities. Though much of the management in Bargerveen is focussed on restoring the natural hydrological regime to promote typical raised bog vegetation, we argue that this also benefits pollinator communities. Declarations Author Contribution Tia l’Amie : Conceptualization, Data Curation, Formal analysis, Investigation, Visualization, Writing - Original draft preparation. Jan Kuper : Data Curation, Funding acquisition, Writing - Review & Editing. Marijn Nijssen : Data curation, Funding acquisition, Writing - Review & Editing. David Scarse: Investigation Eelke Jongejans: Supervision, Writing - Review & Editing. Constant Swinkels : Conceptualization, Supervision, Writing - Review & Editing Acknowledgement We are grateful to Klaas van den Berg and Piet Ursem from Staatsbosbeheer for granting access to the sites. A sincere thanks to all the volunteers, especially Jan Rocks, who have collected transect data over the years. We are also grateful to Jos Mensen for sharing his expertise on hydrological data. Thanks go to Marten Geertsma and Lars Willighagen for field assistance. The butterfly transect data used in this study was collected as a part of a larger monitoring scheme, namely the Dutch Butterfly Monitoring Scheme. The Dutch Butterfly Monitoring Scheme is a co-operation between Dutch Butterfly Conservation (De Vlinderstichting) and Statistics Netherlands (CBS), in the context of the Network Ecological Monitoring (NEM), and financed by the Ministry of Agriculture, Nature and Food Quality. 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09:11:58","extension":"png","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":30835,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/ce1483feedd7782d9179918d.png"},{"id":100567476,"identity":"bebf032a-03a2-4609-a9eb-fc513ebd00f1","added_by":"auto","created_at":"2026-01-19 09:11:58","extension":"xml","order_by":20,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":153148,"visible":true,"origin":"","legend":"","description":"","filename":"9c9109e3ab6646ea8a8f2da4206ca33e1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/44a9d348d5749d8ece3d87d8.xml"},{"id":100595607,"identity":"af7a7beb-ac2e-45ea-8459-1669720ab326","added_by":"auto","created_at":"2026-01-19 13:48:54","extension":"html","order_by":21,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":164961,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/6bf201d59d1fea344b585f79.html"},{"id":100595104,"identity":"6b6fbce7-d227-4894-8dd1-28e57f50885e","added_by":"auto","created_at":"2026-01-19 13:47:25","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1620562,"visible":true,"origin":"","legend":"\u003cp\u003eOverview of sampling sites and buffer zones within Bargerveen. Buffers installed during the course of the study are shown as solid green lines: Buffer West - Maarsingh and Buffer East 1, Buffer East 2, Buffer South-West - Weiteveen, Buffer North - Swarte Meer, and Buffer North-East. The Noordersloot which was infilled is indicated with a dashed blue line. The two sampling sites, the wet heathland and the \u003cem\u003eNardus\u003c/em\u003e grasslands are indicated by the dashed black lines (A). A more detailed view of the \u003cem\u003eNardus \u003c/em\u003egrassland (pollinator transect NG and butterfly transect NG_B), and wet heathland (pollinator transect WH and butterfly transect WH_B) sites are provided in 1B and 1C respectively. Dashed black lines represent pollinator transects, dashed purple lines indicate butterfly transects, and dashed orange lines show the alternative wet heathland transect from 2024 (the high water table in 2024, prevented access to the original transect)\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/c704d726d6b72c9c77f214ee.png"},{"id":100567462,"identity":"84db6f2c-f686-45f8-8616-76007bdb0fa9","added_by":"auto","created_at":"2026-01-19 09:11:57","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":385303,"visible":true,"origin":"","legend":"\u003cp\u003eMultiyear abundance trends of pollinators in the \u003cem\u003eNardus \u003c/em\u003egrassland transect (GL, left column) and the wet heathland pollinator transect (WH, right column). GLMM’s including year and season score as fixed effects were fitted to analyse NG total pollinator daily abundance (A), NG Syrphidae daily abundance (B), WH Apidae daily abundance (C), NG butterfly daily abundance (D), WH total pollinator daily abundance (E), WH Syrphidae daily abundance (F), WH Apidae daily abundance\u003cem\u003e \u003c/em\u003e(G), and WH butterfly daily abundance (H). GLMM’s were fitted over the entire study period (depicted in blue), as well as a smaller subset of the data encompassing 1994-2001 in GL, and 1995-2001 in WH (depicted in red). Significant relationships are represented by solid lines, and non-significant relationships are represented by dashed lines. The managed species \u003cem\u003eApis mellifera \u003c/em\u003eis not included in this graph\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/e8e8751a3206f84555c428a2.png"},{"id":100594889,"identity":"c5e05b50-cfca-419f-b8da-6d09b9121cf2","added_by":"auto","created_at":"2026-01-19 13:46:12","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":240346,"visible":true,"origin":"","legend":"\u003cp\u003eMultiyear abundance trends of hoverflies with varying larval stages in the \u003cem\u003eNardus\u003c/em\u003egrassland pollinator transect (NG). The coloured lines and surrounding shaded area indicate the fitted GLMM model including confidence intervals, which account for random effects of year and season score. Solid lines indicate a significant relationship, whereas dashed lines indicate an insignificant relationship. The trends of phytophagous (C) and terrestrial saprophagous species also were plotted on a log-scale, shown in the small upper-left panels in C and D)\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/50b878dcbb2f7a86ed05b2ed.png"},{"id":100595836,"identity":"6ec70b7e-d62b-4480-ab54-6a970a7448fb","added_by":"auto","created_at":"2026-01-19 13:49:30","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":174932,"visible":true,"origin":"","legend":"\u003cp\u003eMultiyear \u003cem\u003eBombus humilis\u003c/em\u003e abundance trend in the wet heathland pollinator transect (WH). The colored lines and surrounding shaded area indicate the fitted GLMM including confidence intervals, which account for random effects of year and season score. Solid lines indicate a significant relationship, whereas dashed lines indicate an insignificant relationship. GLMM’s were fitted over the entire study period (depicted in blue), as well as a smaller subset of the data encompassing 1995-2001 (depicted in red). Significant relationships are represented by solid lines, and non-significant relationships are represented by dashed lines\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/5c61e7595b5317db65a302df.jpeg"},{"id":100595535,"identity":"fb12528e-ba76-419b-ba02-8a1262ba24f7","added_by":"auto","created_at":"2026-01-19 13:48:43","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":296589,"visible":true,"origin":"","legend":"\u003cp\u003eMultiyear butterfly abundance \u0026nbsp;\u0026nbsp;trends in the \u003cem\u003eNardus \u003c/em\u003egrassland butterfly transect (NG_B). GAMs were \u0026nbsp;\u0026nbsp;fitted over NG_B daily butterfly abundance (A), NG_B \u003cem\u003eL. tityrus\u003c/em\u003e \u0026nbsp;abundance (B), and NG_B \u003cem\u003eP. malvae \u003c/em\u003eabundance (C), represented by the red line\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/c197dcce7eedea44ea96fce9.jpeg"},{"id":100595683,"identity":"671115b2-c49b-412b-b5bc-618c8c10d6a9","added_by":"auto","created_at":"2026-01-19 13:49:07","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":223434,"visible":true,"origin":"","legend":"\u003cp\u003eMultiyear \u003cem\u003ePlebejus argus\u003c/em\u003e abundance trend in the wet heathland butterfly transect (WH_B). Buffers were installed during the study period in 2014 (W = Buffer West - Maarsingh, and E1 = Buffer East 1), 2016 (E2 = Buffer East 2), 2017 (SW = Buffer South-West - Weiteveen), 2018 (N = Buffer North - Swarte Meer), and 2020 (NE = Buffer North-East), and are represented with vertical black dashed lines\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/bc11c102a4e6ce5263070edd.jpeg"},{"id":100567455,"identity":"9654d84f-1de4-4166-8492-61f83ff4104e","added_by":"auto","created_at":"2026-01-19 09:11:57","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":187446,"visible":true,"origin":"","legend":"\u003cp\u003eSpecies richness of pollinators in the \u003cem\u003eNardus \u003c/em\u003egrassland pollinator transect (NG) and the wet heathland pollinator transect (WH). Chao-estimates are depicted including standard deviation of NG total pollinator richness (A) and WH total pollinator richness (B)\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/dd3e8eb1ed088314b848ee7f.jpeg"},{"id":100567466,"identity":"0487f323-47ca-4ef7-a2ae-0fdeea9f07ed","added_by":"auto","created_at":"2026-01-19 09:11:58","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":184697,"visible":true,"origin":"","legend":"\u003cp\u003eSpecies richness in the \u003cem\u003eNardus \u003c/em\u003egrassland butterfly transect (NG_B). Chao-estimates including standard deviation of NG_B butterfly species richness\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/35e6fb36c1a70c594fe1b3b4.jpeg"},{"id":108006566,"identity":"f4c48842-a90e-4f90-9bec-16a39bab8e9c","added_by":"auto","created_at":"2026-04-28 12:56:06","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3196363,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/6f89f8bf-0e79-4c54-b497-d28b57ed044e.pdf"},{"id":100595276,"identity":"9bea3348-3363-469e-9ceb-c8b0875950f2","added_by":"auto","created_at":"2026-01-19 13:48:06","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":1880590,"visible":true,"origin":"","legend":"","description":"","filename":"SIKeepingRaisedBogRemnantsWetStabilizesCharacteristicPollinatorCommunities.docx","url":"https://assets-eu.researchsquare.com/files/rs-8233210/v1/6cebeff72b2af2c4a8ec2dbc.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Keeping Raised Bog Remnants Wet Stabilizes Characteristic Pollinator Communities","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAbundance and species richness have declined in pollinator communities in recent decades (Biesmeijer et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Barendregt et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), even within natural areas (Hallmann et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). While pollinator decline is worrisome in and of itself, the loss of pollinators is considered especially alarming due to their crucial role in pollination of both crop and wild plant species (Kremen et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Vanbergen and Insect Pollinators Initiative 2013). To exemplify, Ollerton et al. (\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) estimated that roughly 78% of the world's wild flowering plants rely on pollinators for reproduction. Habitat degradation, fragmentation, climate change and the increased abundance of alien species and pathogens are generally hypothesized to be the main drivers behind pollinator decline (Kearns et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Kremen et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Potts et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Overall, there is a clear need for effective conservation and restoration strategies for pollinators.\u003c/p\u003e \u003cp\u003eProtected nature areas serve as an important refuge for both native and managed pollinator species (Garibaldi et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Cooke et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). While many studies show how local measures, such as increasing floral richness, leads to short-term benefits for pollinator communities, this effect generally does not apply to vulnerable insect communities within natural areas. It is known that within natural areas landscape-level nature conservation can promote these communities (Sexton and Emery \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Larkin and Stanley \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Uhl et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). However, to our knowledge, the long-term benefits of landscape-level conservation efforts on pollinator communities are rarely assessed. Given the many landscape level threats on natural areas in North-Western Europe, it is essential to assess these long-term effects, as positive effects could well be undone over larger timescales.\u003c/p\u003e \u003cp\u003eRaised bog systems in the Netherlands are an excellent example of such a vulnerable natural system. They harbour characteristic pollinator communities that are not found anywhere else in the Netherlands (van Duinen et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Wallis de Vries et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Speelman and Kalkman \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Although raised bogs were once widespread, currently only small remnants remain in an otherwise primarily agricultural landscape. Restoration and conservation efforts within these remnants are therefore crucial for maintaining these characteristic pollinator communities.\u003c/p\u003e \u003cp\u003eTwo major landscape-level threats for raised bog plant and insect communities in the Netherlands are nitrogen deposition and desiccation (Casparie \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1993\u003c/span\u003e; Tomassen et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). Whereas nitrogen deposition is an ongoing process caused by landscape level drivers beyond the control of local managers, hydrological regimes can be effectively managed at the local scale via the installation of weirs, dams, and buffers (Smolders et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Brown et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). This in turn has been shown to promote vulnerable plant communities of raised bog systems, despite the context of nitrogen deposition (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Adema et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Although it has been shown that restoring dry grassland vegetation can promote both pollinator abundance and richness (Sexton and Emery \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), to our knowledge the response of pollinator communities from wet systems to hydrological management, especially in the long term, is understudied.\u003c/p\u003e \u003cp\u003eHere we analyse two long-term (1994\u0026ndash;2024) datasets documenting pollinator and detailed butterfly counts in Dutch raised bog reserve Bargerveen. During the study period several landscape-level hydrological interventions occurred, which gave us the opportunity to study changes in pollinator abundance and richness within the context of restoration efforts. Specifically, we aim to answer the following research questions: 1) Has the abundance of pollinators in Bargerveen changed over the past three decades?; 2) Has pollinator species richness changed?; 3) What are the population trends of selected vulnerable and characteristic species? 4) Do hoverfly abundance trends differ between species with contrasting larval strategies?\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eBrief Area Description and Management\u003c/h2\u003e \u003cp\u003eThe study was conducted in the Natura2000 area Bargerveen, currently the largest raised bog in the Netherlands. Having been subject to peat extraction until 1992, the area is now actively managed to promote peat formation. It hosts a number of rare and red-listed species typical for raised bogs (van Guldener et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Speelman and Kalkman \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Management in Bargerveen includes area-wide mowing and grazing. In addition, significant system-level interventions have been taken to restore wet bog conditions and the natural hydrological regime. These include the infilling of the Noordersloot watershed in 1997 (Rossenaar et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), restructuring and placement of additional weirs and dams which primarily took place between 2001 and 2006, and the placement of hydrological buffers in 2014, 2017, 2018 and 2020 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). These measures have rewetted the system and led to more stable water levels in the area (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Stroet \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), thereby facilitating the recovery of characteristic raised bog vegetation (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Nevertheless, the nature reserve is still subjected to eutrophication, acidification, drainage, and climate change that threaten the naturally nutrient-poor system (van Guldener et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Adema et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), which are often linked to pollinator declines. In this study, we analyse transect data from two characteristic habitat types found in raised bog systems (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e); \u003cem\u003eNardus\u003c/em\u003e grassland (H6230, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB) and Northern Atlantic wet heath with \u003cem\u003eErica tetralix\u003c/em\u003e (H4010_A) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003e Grasslands\u003c/h3\u003e\n\u003cdiv class=\"Heading\"\u003e\u003cem\u003eNardus\u003c/em\u003e Grasslands\u003c/div\u003e \u003cp\u003eThe \u003cem\u003eNardus\u003c/em\u003e grasslands of Bargerveen are semi-natural, and developed from old agricultural fields. A well-developed \u003cem\u003eNardus\u003c/em\u003e grassland can be recognized by the presence of plant species such as \u003cem\u003ePlatanthera bifolia\u003c/em\u003e, \u003cem\u003eDactylorhiza maculata\u003c/em\u003e, \u003cem\u003eGalium saxatile\u003c/em\u003e and \u003cem\u003ePotentilla erecta\u003c/em\u003e (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). To maintain the preferred low to medium nutrient availability in the grasslands, management focuses on soil impoverishment, which is realized via yearly mowing and removal of the clippings. Acidification, primarily due to high N-deposition in the area, poses a significant threat to the ideally weakly buffered grasslands (de Graaf et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Adema et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The \u003cem\u003eNardus\u003c/em\u003e grasslands in Bargerveen have their own apparent water table, dependent on precipitation, evaporation and seepage. Maintaining a high water level is essential for maintenance of a characteristic vegetation community (van Duinen et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Likely as a result of hydrological interventions within the area, the water table in the \u003cem\u003eNardus\u003c/em\u003e grasslands of Bargerveen increased between 2006 and 2007, and remained relatively stable up to 2018, after which a slight drop in the surface water level was observed (Fig. S2).\u003c/p\u003e \u003cp\u003eThe \u003cem\u003eNardus\u003c/em\u003e grasslands of Bargerveen house a relatively high number of characteristic butterfly species (van Duinen et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), including the grizzled skipper (\u003cem\u003ePyrgus malvae\u003c/em\u003e) and the sooty copper (\u003cem\u003eLycaena tityrus\u003c/em\u003e). Both butterfly species are sensitive to decline due to their life-history traits, and are closely associated with \u003cem\u003eNardus\u003c/em\u003e grasslands, making them important targets for management in the area.\u003c/p\u003e\n\u003ch3\u003eWet Heathlands\u003c/h3\u003e\n\u003cp\u003eThe second area is classified as a Northern Atlantic heath with \u003cem\u003eErica tetralix\u003c/em\u003e, found on acidic, nutrient-poor soils. This habitat is characterized by the presence of \u003cem\u003eErica tetralix, Calluna vulgaris\u003c/em\u003e, grasses such as \u003cem\u003eMolinia caerulea\u003c/em\u003e, sedges such as \u003cem\u003eRhynchospora alba\u003c/em\u003e, and \u003cem\u003eSphagnum\u003c/em\u003e mosses. Hydrology plays an important role in wet heathland systems, as most characteristic vegetation and insect species of this habitat type rely on high water tables. Within the wet heathlands of Bargerveen, no significant long-term developments in surface water table level were identified (Fig. S2).\u003c/p\u003e \u003cp\u003eA major challenge in the wet heathlands of Bargerveen is preventing encroachment of grasses and other vegetation due to high N-deposition and acidification. To counter this, non-heath vegetation is removed via grazing by sheep and cattle, mowing with clipping removal, and controlled burning (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Adema et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). These measures aim to promote wet heathland vegetation, but also create open patches which benefit nesting wild bee species (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Wallis de Vries et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2022\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe pollinator communities of wet heaths are not very diverse, but contain a high proportion of characteristic and rare species. One such species is the vulnerable silver-studded blue (\u003cem\u003ePlebejus argus\u003c/em\u003e), which is locally abundant in wet heaths of Bargerveen (van Duinen et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Wallis de Vries and Oteman \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The critically endangered brown-banded carder bee (\u003cem\u003eBombus humilis\u003c/em\u003e) is also commonly found in the wet heaths of Bargerveen. While once widespread in the Netherlands, its distribution is limited to a few sites in Drenthe (Speelman and Kalkman \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), with Bargerveen likely supporting the largest \u003cem\u003eB. humilis\u003c/em\u003e population in the Netherlands, making it an important target species for management. \u003cem\u003eB. humilis\u003c/em\u003e is also a valuable indicator species due to its close association with wet heathlands and sensitivity to competition pressure by other pollinating species (Saunders \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Speelman and Kalkman \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e\n\u003ch3\u003ePollinator Transects\u003c/h3\u003e\n\u003cp\u003eHoverflies (Diptera: Syrphidae), bees (Hymenoptera: Apidae s.l.), and butterflies/diurnal moths (Lepidoptera) were counted and identified up to species-level during nine years: 1994\u0026ndash;1995, 1997\u0026ndash;2001, 2017 and 2024. Sampling dates varied between the years, but always took place between May and October, with 93.7% of sampling events taking place between June and August. Transect distance was fixed with permanent poles in the field, and transects were sampled while walking at a constant pace. All individuals were counted and identified to species-level in the field if possible. In case of uncertainty, the specimen was collected and preserved for later identification. Other pollinators passing during this handling time were not included in the counts. Specimens that could not be identified at the species-level were identified to the lowest possible taxonomic rank. For species richness analyses, we included only individuals identified to the species-level, treating additional genera or families as a single species. The \u003cem\u003eBombus terrestris\u003c/em\u003e species complex was treated as one species as accurate identification based on field characteristics is not possible (Williams et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2012\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe \u003cem\u003eNardus\u003c/em\u003e grassland pollinator transect (NG), was located on the east end of Bargerveen (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), and was sampled a total of 75 times. Transect length of NG was 250m from 1994\u0026ndash;2001 and 150 m in 2017 and 2024. Transect width remained constant at 5 m (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB). The wet heath pollinator transect (WH), is located in a wet heath area on the west-side of Bargerveen (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC), and was sampled a total of 67 times. The length of 200 meters and width of 5 meters remained constant during all sampling dates before 2024. In 2024 half of the original transect was inaccessible due to the high water level. Therefore, another nearby transect (100 x 5m) within the same habitat type was sampled in addition to the accessible half of the original transect (100 x 5m) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC). During the first year (1994) sampling in WH was focused on specific species. This year was excluded from abundance and species richness analyses, but included for species-specific analyses of those species.\u003c/p\u003e\n\u003ch3\u003eButterfly Transects\u003c/h3\u003e\n\u003cp\u003eButterfly counts were consistently performed in accordance with the guidelines set by \u0026ldquo;\u003cem\u003eHandleiding Landelijke Meetnetten Vlinders en Libellen\u003c/em\u003e\u0026rdquo; (van Swaay et al., \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). This data was collected in the framework of the Dutch Butterfly Monitoring Scheme. The butterfly transects overlapped partially with the pollinator transects in both sites. The \u003cem\u003eNardus\u003c/em\u003e grassland butterfly transect (NG_B) had a length of 250 meters and width of 5 meters (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB). In the NG_B transect all butterfly species were counted and identified to the species-level where possible. Sampling took place between April and October during 13 years: in 1995, 1999\u0026ndash;2017, 2019, 2021, and 2023. NG_B was sampled a total of 198 times.\u003c/p\u003e \u003cp\u003eThe wet heathland butterfly transect (WH_B) had a length of 200 and width of 5 meters (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC). Butterfly counts in WH_B focused solely on \u003cem\u003eP. argus\u003c/em\u003e, and took place between June and September, during peak activity of the species. Sampling in WH_B took place a total of 96 times over the course of 20 years: in 1999, 2001\u0026ndash;2009, 2011\u0026ndash;2017, 2019, 2021 and 2023.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eWeather Data\u003c/h2\u003e \u003cp\u003eTo account for the influence of bioclimatic circumstances on pollinator abundance trends, data on temperature, irradiance, humidity, and wind speed were collected from the Royal Netherlands Meteorological Institute (Koninklijk Nederlands Meteorologisch Instituut: KNMI). Since sampling dates were chosen based on favorable weather, no hourly data on precipitation were included as precipitation was assumed to have been absent. Weather data were collected from the nearest KNMI meteorological station in Hoogeveen (approximately 30 km from Bargerveen). For the WH, WH_B and NG_B transects the starting time of the transect counts was consistently recorded, and hourly data corresponding to the starting hour, as well as averaged daily meteorological data were used for analysis (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e-2). For the NG transect averaged daily meteorological data were used since start time was not always recorded.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eHydrological Data\u003c/h3\u003e\n\u003cp\u003eHydrological data describing surface water levels for the \u003cem\u003eNardus\u003c/em\u003e grasslands (ID\u0026thinsp;=\u0026thinsp;P23A0058 and P23A0087) and wet heath (ID\u0026thinsp;=\u0026thinsp;P23A0102 and P23A0103) were retrieved from Dinoloket, a public Dutch national database of groundwater head observations (TNO \u0026ndash; Geological Survey of the Netherlands).\u003c/p\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eData Analysis\u003c/h2\u003e \u003cp\u003eAll analyses and visualizations were conducted in R V4.3.2 (R Core Team \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Prior to analysis, counts from NG were corrected for transect length by adjusting species counts to a standardized transect length of 150 meters, by removing individuals randomly based on percentual increase in transect length. As transect length remained constant, this was not necessary for WH, WH_B and NG_B. The western honeybee (\u003cem\u003eApis mellifera\u003c/em\u003e) is a managed species and was therefore excluded from the abundance analyses, but included in the richness analyses.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003ePollinator Abundance Trends\u003c/h2\u003e \u003cp\u003eTo test whether the abundance of pollinators has changed within the transects the daily total pollinator counts and species-group specific (Syrphidae, Apidae and Butterflies) daily counts were analysed. To correct for seasonality a similar approach to Barendregt et al. (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) was taken. A generalized additive model (GAM) was fitted to daily abundance counts irrespective of year. This model was then used to predict relative day-of-the-year pollinator abundance, resulting in a so-called season score. The season score was normalized and ranges from zero to one, with values close to one representing abundance measures taken during the peak of the season. Then, to assess changes in pollinator abundance over the years separate generalized linear mixed-effect models (GLMM) were fitted over total daily pollinator counts and species-group specific counts in WH and NG. The basic structure of the models included the season score and a continuous year variable as fixed effects, as well as year as a random categorical variable to correct for nestedness within years. Models were fitted over daily abundances over the entire study period (1994\u0026ndash;2024) to analyse overall changes in abundance, as well as a subset encompassing 1994\u0026ndash;2001, to assess pollinator abundance trends prior to the installation of hydrological buffers. Models including weather variables were explored, but did not significantly improve model AIC (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e-S2). Hence, in the main text we present the results from the basic model.\u003c/p\u003e \u003cp\u003eA similar approach was followed to analyse the daily count of butterfly abundances in NG_B and WH_B. However, visual inspection of the data distribution revealed that the daily abundance of butterflies did not follow a linear trend. Therefore, in addition to GLMMs, GAMs including date and season score were fitted to assess non-linear abundance changes over the years in the butterfly transects.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eSpecies Richness Trends\u003c/h2\u003e \u003cp\u003eIn order to gain insight into species richness trends, species accumulation curves (SAC) were generated using the R package vegan V2.6.4 (Oksanen et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The function specpool was used to calculate Chao-estimates, a measure used to estimate the extrapolated species richness within a species-pool based on the observed species accumulation. The calculated Chao-estimates were plotted and visually inspected to assess changes in species richness over the years.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eAbundance Trends Characteristic Species\u003c/h2\u003e \u003cp\u003eThe characteristic butterfly species \u003cem\u003ePyrgus malvae\u003c/em\u003e, \u003cem\u003eLycaena tityrus\u003c/em\u003e, \u003cem\u003ePlebejus argus\u003c/em\u003e, and bumblebee species \u003cem\u003eBombus humilis\u003c/em\u003e were selected for species-specific analysis. GLMMs including year and season score as well as GAMs including date and season score were fitted to analyse \u003cem\u003eP. malvae\u003c/em\u003e (NG_B), \u003cem\u003eL. tityrus\u003c/em\u003e (NG_B) and \u003cem\u003eP. argus\u003c/em\u003e (WH_B) abundance trends. To analyse \u003cem\u003eB. humilis\u003c/em\u003e abundance (WH), GLMMs including year and season scores were fitted.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eAbundance Trends Larval Strategies Syrphidae\u003c/h2\u003e \u003cp\u003eIn order to assess whether abundance trends of hoverflies differed per larval strategy, GLMMs including year and season score were used to analyse trends of subsets of hoverfly species. Syrphidae species were classified as either aquatic saprophagous, terrestrial saprophagous, phytophagous or zoophagous based on classification as described by Reemer et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Due to the low abundances of Syrphidae in the wet heath transect, the larval-stage specific model failed to converge, and no significant trends were observed in the abundances of the various Syrphidae larval types in the wet heath.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003eWithin the pollinator transects (WH and NG) a total of 15,284 butterfly, 3,956 hoverfly and 3,385 bee individuals (1,897 excluding \u003cem\u003eA. mellifera\u003c/em\u003e) were counted, representing 30, 42 and 19 species respectively. In the pollinator grassland transect, a total of 3,625 individuals from 42 hoverfly species, 682 individuals (313 excluding \u003cem\u003eA. mellifera\u003c/em\u003e) from 17 bee species, and 5,645 individuals from 27 butterfly species were recorded. In comparison, in the wet heath transect 331 individuals from 21 hoverfly species, 2,703 individuals (1,584 excluding \u003cem\u003eA. mellifera\u003c/em\u003e) from 13 bee species, and 9,639 individuals representing 20 butterfly species were recorded.\u003c/p\u003e \u003cp\u003eA total of 47,584 butterflies were counted within the butterfly transects (NG_B and WH_B). In the butterfly grassland transect NG_B 20,774 butterflies representing 31 species were counted. In butterfly wet heathland transect WH_B 26,691 \u003cem\u003ePlebejus argus\u003c/em\u003e individuals were sampled. For a more detailed species list, refer to table S3 in the supplementary information.\u003c/p\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003ePollinator Transects Abundance Trends\u003c/h2\u003e \u003cp\u003eTotal pollinator abundance within pollinator \u003cem\u003eNardus\u003c/em\u003e grassland transect NG increased significantly (0.93% per year, CI 95% [0.53%, 1.33%] Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA) over the course of the entire study (1994\u0026ndash;2024). This significant increase in abundance in NG translated to the increased abundance of Apidae (3.28% per year, CI 95% [1.82%, 4.76%], Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eC) and Syrphidae (2.90% per year, 95% CI [1.40%, 4.42%], Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB). The abundance of butterflies on the other hand declined significantly between 1994 and 2024 (-2.16% per year, CI [-3.33%, -1.09%], Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eD). Analyses using separate GLMMs on smaller data subsets identified no significant changes in total pollinator abundance or group-level abundances in NG between 1995 and 2001 (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA-D).\u003c/p\u003e \u003cp\u003eIn contrast, no significant changes were found in pollinator abundance in the wet heathlands over the course of the entire study period (1994\u0026ndash;2024), either in total pollinator abundance or at the group level (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eE-H). However, total pollinator abundance in the wet heathland pollinator transect WH decreased significantly between 1995 and 2001 (-4.96% per year, CI [-5.72%, -4.19%], Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eE), primarily due to a drop in butterfly abundance (-5.80% per year, CI [-5.95%, -5.66%], Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eH). No significant pollinator abundance trends were identified for either Syrphidae or Apidae in WH.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe majority of the Syrphidae encountered in the \u003cem\u003eNardus\u003c/em\u003e grasslands have (semi-) aquatic saprophagous and terrestrial zoophagous larval stages. The significant increase in Syrphidae mainly was mainly due to an increase in zoophagous species, which saw a significant increase in abundance (4.51% per year, 95% CI [0.11%, 9.10%], Fig.\u0026nbsp;3B). A significant increase in phytophagous species was identified as well (2.55% per year, 95% CI [2.51%, 2.59%], Fig.\u0026nbsp;3C), though these species were only present in very low abundance in all years except 2017. The observed increase in phytophagous syrphids is mainly a result of the remarkably high number of \u003cem\u003eEumerus\u003c/em\u003e in 2017, which accounted for 58 of the 59 phytophagous hoverflies found that year. No significant trends were found for the other larval types in the \u003cem\u003eNardus\u003c/em\u003e grasslands.\u003c/p\u003e \u003cp\u003eSimilar to in the \u003cem\u003eNardus\u003c/em\u003e grasslands, the larval types of Syrphidae in the wet heath consisted primarily of (semi-) aquatic saprophagous and terrestrial zoophagous species. No terrestrial phytophagous species were observed in the wet heath throughout the entire study period. Terrestrial saprophagous species were recorded in the wet heath only in 1998 and 2001.\u003c/p\u003e \u003cp\u003eWithin the wet heathland pollinator transect the characteristic bumblebee \u003cem\u003eB. humilis\u003c/em\u003e was found in relatively high abundance, accounting for over a quarter of all recorded Apidae individuals. \u003cem\u003eB. humilis\u003c/em\u003e abundance in WH showed a near-significant decline (p\u0026thinsp;=\u0026thinsp;0.061) between 1995 and 2001 of -6.21% per year, CI 95% [-9.20%, -3.11%] (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e). However, no significant change in \u003cem\u003eB. humilis\u003c/em\u003e abundance over the course of the entire study (1995\u0026ndash;2024) was found.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003eButterfly Transects Abundance Trends\u003c/h2\u003e \u003cp\u003eA GLMM including year and season score as fixed effects fitted to the daily count of butterfly abundance in the butterfly \u003cem\u003eNardus\u003c/em\u003e grassland transect NG_B showed a significant yearly increase of 4.29%, CI 95% [1.88%, 6.47%]. A statistically significant non-linear relationship between sampling date and daily NG_B butterfly abundance was identified (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eA). NG_B butterfly abundance remained relatively stable at the start of the study between 1995 and 2006, and then showed an increase up to roughly 2016. In the past decade butterfly abundance in NG_B gradually declined.\u003c/p\u003e \u003cp\u003eSpecies-specific analyses using GLMM of \u003cem\u003eL. tityrus\u003c/em\u003e and \u003cem\u003eP. malvae\u003c/em\u003e in NG_B also found a significant increase in abundance for both butterfly species of 8.80%, CI [1.17% \u0026minus;\u0026thinsp;16.48%] and 5.79%, CI [1.23% \u0026minus;\u0026thinsp;10.60%] respectively. However, further analysis using GAMs reveals that abundance does not increase linearly, but instead follows a roughly hump-shaped pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e). A similar increasing period as seen in NG_B total butterfly abundance between 2006 and 2016 for both species. \u003cem\u003eP. malvae\u003c/em\u003e abundance remains roughly stable after this increasing period (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eC), whereas \u003cem\u003eL. tityrus\u003c/em\u003e abundance steeply declines (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eB).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNo significant change in \u003cem\u003eP. argus\u003c/em\u003e abundance within the wet heathland butterfly transect WH_B was found over the course of the entire study period. Further analysis using GAM found that abundance trends of \u003cem\u003eP. argus\u003c/em\u003e were variable over the years. \u003cem\u003eP. argus\u003c/em\u003e daily abundance in WH_B showed an initial increasing trend from 1999 to 2008, after which abundance declined. From 2014 onwards, coinciding with the installation of nearby buffer systems, \u003cem\u003eP. argus\u003c/em\u003e daily abundances showed a steady increase (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e). \u003cem\u003eP. argus\u003c/em\u003e abundances did not increase during the most recent sampling years.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003ePollinator Transects Species Richness Trends\u003c/h2\u003e \u003cp\u003ePollinator species richness in the pollinator grassland transect NG fluctuated notably more (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003eA) compared to species richness within the wet heathland pollinator transect WH (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e7\u003c/span\u003eB), but was consistently higher than species richness in WH. Both transects seem to have been subject to a decline in species richness prior to 2001. However, chao estimates in 2017 and 2024 are relatively similar, or even high, compared to richness estimates during the start of the study. Apidae species richness remains relatively low and stable over the course of the study in both pollinator transects (Fig. S3). Species richness in the wet heathland was particularly low in 2017, primarily due to low butterfly and hoverfly richness. In the same year however, species richness within the \u003cem\u003eNardus\u003c/em\u003e grassland pollinator transect was estimated to be relatively high, especially the species richness of Apidae.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003eButterfly Transect Species Richness Trends\u003c/h2\u003e \u003cp\u003eThe species richness of butterflies in NG_B generally ranged between an estimated 15\u0026ndash;30 species. The estimates fluctuated over the years, but an overall increase in species richness was observed (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Richness was estimated to be the highest in 2017 (39.75\u0026thinsp;\u0026plusmn;\u0026thinsp;20.72) and 2023 (33.67\u0026thinsp;\u0026plusmn;\u0026thinsp;15.16), though both estimates have a large standard deviation. During the peak of butterfly abundance in NG_B between 2010 and 2015 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eA) species richness in NG_B remains very stable.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003ePollinators are declining in North-Western Europe (Biesmeijer et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Warren et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Barendregt et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), even within natural areas (Hallmann et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Interestingly, our study shows that pollinator populations of two typical raised bog habitats within the natural area of Bargerveen remained stable or increased over a thirty-year long period. We argue that this is most likely a result of landscape-level nature management, including the intensive large-scale hydrological interventions in Bargerveen which were previously shown to restore typical bog vegetation (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The same pattern was observed in selected characteristic species of the studied bog habitats, which depend on specific host plants and open habitat structure (preventing shrub or grass encroachment) that in bog-systems are typically linked to moist conditions (Bos et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Speelman and Kalkman \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). This further suggests that landscape-level management has the potential to support characteristic pollinator species, and to contribute to the long-term conservation of pollinator communities in raised bog systems.\u003c/p\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003eAbundance and Species-Richness\u003c/h2\u003e \u003cp\u003eIn both the \u003cem\u003eNardus\u003c/em\u003e grasslands and the wet heathlands we observed stable to increasing abundance and species richness of pollinators. This finding contrasts with studies describing long-term arthropod biomass and species decline that describe declines even within natural areas. For instance, Hallmann et al. (\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) studied arthropod biomass within German natural areas during the same time period. They described a 75% decline over 25 years. In later work, they scale this to hoverfly species richness decline (Hallmann et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The natural areas studied by Hallmann et al. (\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) were often mown or grazed, but were rarely subjected to land-scape level hydrological management. Therefore, even though management in Bargerveen is not limited to hydrological interventions, we hypothesize that the land-scape level hydrological interventions could explain the differences in pollinator trends.\u003c/p\u003e \u003cp\u003eThis hypothesis is further supported by the fact that a lot of species that reside in raised bog systems rely on high water tables and are sensitive to drought (Wallis de Vries and Oteman \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Wallis de Vries et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). In addition, drought has been identified as a major contributor to butterfly decline both nationally and internationally (Warren et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Westra et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; van Swaay et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; van Swaay and Borkent \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). This sensitivity to drought was also reflected by our results, where the effects of dry years after 2017 (Fig. S2) were evident in the butterfly abundance in the \u003cem\u003eNardus\u003c/em\u003e grassland pollinator transect, which showed a small but significant decline. In spite of this, the declines in butterfly abundance in our study seem less strong than the nationwide pattern (van Swaay and Borkent \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), suggesting that although hydrological measures were not able to fully compensate for the adverse effects of exceptionally dry years, they were able to buffer them.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section2\"\u003e \u003ch2\u003eAbundance Trends Characteristic Wet Heathland Species\u003c/h2\u003e \u003cp\u003eIn spite of declines of characteristic bumblebees and butterflies in north-western Europe (Goulson et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Warren et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), and their decline on a national level (De Vlinderstichting 2017; Speelman and Kalkman \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), no overall decline in abundance was observed in either \u003cem\u003eP. argus\u003c/em\u003e or \u003cem\u003eB. humilis\u003c/em\u003e. It has been described that the hydrological interventions in Bargerveen have led to the recovery of characteristic heathland flora (van Duinen et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; van Duinen et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Our results illustrate that this vegetation recovery also benefits the vulnerable pollinating species of wet heathland systems, further suggesting that the focus on hydrological management in the Bargerveen area is an important reason for pollinator community conservation.\u003c/p\u003e \u003cp\u003eNevertheless, population trends of both species did fluctuate throughout the study period. Notably, both \u003cem\u003eB. humilis\u003c/em\u003e and \u003cem\u003eP. argus\u003c/em\u003e show particularly low abundance at the beginning of the monitoring period. This may have been a result of habitat disturbances resulting from the infilling of the nearby watershed Noordersloot in 1997, one of the early important hydrological interventions in Bargerveen (Rossenaar et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). As Bargerveen is a relatively large area, it is likely that suitable habitat was available elsewhere in the reserve, and these species survived these local interventions and populations later recovered along the transect. However, this does illustrate that hydrological interventions can have significant adverse effects during implementation that should be considered when dealing with small, isolated and fragile populations.\u003c/p\u003e \u003cp\u003eAnother decline in \u003cem\u003eP. argus\u003c/em\u003e abundance was observed between 2007 and 2014. This may have been a result of encroachment of grasses like \u003cem\u003eMolinia caerulea\u003c/em\u003e in Bargerveen. The grass reportedly occurred in high abundance during this time at the expense of typical heathland vegetation like \u003cem\u003eE. tetralix\u003c/em\u003e, an important host plant (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The increase in \u003cem\u003eP. argus\u003c/em\u003e abundance after 2014 coincides with the installation of hydrological buffers. Together with the partially stabilized encroachment of \u003cem\u003eM. caerulea\u003c/em\u003e, these conditions support typical heathland vegetation (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). This again suggests that abundance trends are linked to improved hydrological conditions.\u003c/p\u003e \u003cp\u003eOverall, these findings suggest that the stable population trends of the characteristic species \u003cem\u003eP. argus\u003c/em\u003e and \u003cem\u003eB. humilis\u003c/em\u003e observed in Bargerveen are a result of continuous management efforts. In addition, these results highlight the need to track population trends over time. Positive effects of management interventions, such as the stable population trends we observed in spite of national declining trends, may only become clear in the longer term.\u003c/p\u003e \u003cdiv id=\"Sec23\" class=\"Section3\"\u003e \u003ch2\u003eAbundance Trends Characteristic \u003cem\u003eNardus\u003c/em\u003e Grassland Species\u003c/h2\u003e \u003cp\u003eBoth \u003cem\u003eP. malvae\u003c/em\u003e and \u003cem\u003eL. tityrus\u003c/em\u003e are known to be sensitive to drought (van Turnhout et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Wallis de Vries and Oteman \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Despite \u003cem\u003eP. malvae\u003c/em\u003e remaining more abundant than at the start of national monitoring in 1990, the effect of dry years can clearly be seen in recent national abundance trends, with declines in abundance seen after 2017 in both the Netherlands and Belgium (Westra et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; van Swaay and Borkent \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). We found that the \u003cem\u003eP. malvae\u003c/em\u003e population in Bargerveen experienced no significant decline during this period. This stable population trend of is likely a result of the combined effect of the large population size (Wallis de Vries and Oteman \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), high abundance of its host plant tormentil \u003cem\u003ePotentilla erecta\u003c/em\u003e (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), and hydrological interventions, which likely helped mitigate the negative impacts of dry years. Though the \u003cem\u003eL. tityrus\u003c/em\u003e population in Bargerveen was thought to be stable (van Duinen 2013), our results suggest a recent decline, potentially because of several dry years between 2017 and 2023.\u003c/p\u003e \u003cp\u003eA report by Wallis de Vries \u0026amp; Oteman (\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) provides a possible explanation as to why the effect of the dry years is seen in \u003cem\u003eL. tityrus\u003c/em\u003e, but not \u003cem\u003eP. malvae\u003c/em\u003e. They found that \u003cem\u003eL. tityrus\u003c/em\u003e abundance is primarily driven by high spring precipitation, whereas \u003cem\u003eP. malvae\u003c/em\u003e primarily depends on high soil moisture, with spring precipitation the year prior actually reducing its abundance. Combined with our results, this suggests that the hydrological management implemented in Bargerveen is effective in maintaining favourable soil hydrological conditions, even during extended periods of unfavourable weather.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec24\" class=\"Section2\"\u003e \u003ch2\u003eStudy Limitations and Implications\u003c/h2\u003e \u003cp\u003eOur results are in line with our hypothesis that the nature conservation efforts within Bargerveen would aid in sustaining pollinator abundance and richness long-term. They align with short-term studies showing that system-level conservation efforts, e.g. via improving local conditions for vegetation, can lead to pollinator increases (Sexton and Emery \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Larkin and Stanley \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Uhl et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Where these studies focus on dry grassland or forest systems, our results indicate that this positive effect is also seen in raised bog habitats. In addition, most studies on pollinator restoration outcomes focus on short-term effects, whereas our analysis reveals that system-level restoration can have long lasting, stabilising effects for pollinator populations.\u003c/p\u003e \u003cp\u003eAlthough the benefits of both species-specific and system-level conservation approaches are a long-standing debate in ecology (Tews et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Harvey et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Kremen and Merenlender \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), this distinction has received less attention in pollinator conservation, where most measures remain group-focused (e.g., flower strips) (Albrecht et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; P\u0026eacute;rez-S\u0026aacute;nchez et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Our results contribute to this debate by showing that pollinator communities, including characteristic species, can benefit from restoring system-wide abiotic conditions, particularly over longer time scales. This likely reflects the fact that most pollinators are partial habitat users that rely on a mosaic of resources such as different foraging and nesting sites. While targeted interventions can alleviate specific limitations, more recent work describes that they rarely recreate the full habitat complexity required to support diverse pollinator assemblages (Wood et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Requier and Leonhardt \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). In contrast, bottom-up restoration of entire systems appears to support this complexity. Our findings therefore suggest that landscape-level restoration may yield broader benefits for pollinators. Nevertheless, specifically for restoring hydrological conditions, our study provides only an initial indication that rewetting positively affects pollinator communities. Further research linking hydrological data to pollinator trends is needed to confirm this finding, and to better understand the mechanisms through which hydrology influences pollinators, such as through host plant recovery, changes in vegetation structure, or moisture buffering during dry periods.\u003c/p\u003e \u003cp\u003eOur results indicate a positive long-term impact of hydrological interventions. However, nature managers should be aware that such interventions may also lead to an increase in (temporary) disturbances as well as substantial changes within the landscape. Large-scale hydrological interventions may therefore also result in habitat loss in certain locations, with possible adverse effects on characteristic species (Verberk et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; van Duinen et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). For instance, the increased water table of Bargerveen is likely to lead to the disappearance of \u003cem\u003eNardus\u003c/em\u003e grasslands in the center of the nature area, a conscious decision which was made to favor the development of active peat-forming raised bog habitats (Bijkerk et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; van Guldener et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Therefore, in order to preserve the characteristic species which reside in these \u003cem\u003eNardus\u003c/em\u003e grasslands, it is important to compensate for local habitat losses by developing and supporting suitable alternative sites elsewhere within the nature reserve.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eAlthough declines in pollinator populations have been reported in North-West Europe, even within protected natural areas (Biesmeijer et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Hallmann et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Warren et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Barendregt et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; van Swaay and Borkent \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), we show that pollinator communities in Bargerveen are relatively stable. This suggests that landscape-level conservation efforts, and in this case hydrological buffering specifically, are able to act as a buffer against drivers of insect decline. As we observe positive and improving trends, both at the community and species-level, we conclude that management in Bargerveen is effective in supporting pollinator communities. Though much of the management in Bargerveen is focussed on restoring the natural hydrological regime to promote typical raised bog vegetation, we argue that this also benefits pollinator communities.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eTia l\u0026rsquo;Amie : Conceptualization, Data Curation, Formal analysis, Investigation, Visualization, Writing - Original draft preparation. Jan Kuper : Data Curation, Funding acquisition, Writing - Review \u0026amp; Editing. Marijn Nijssen : Data curation, Funding acquisition, Writing - Review \u0026amp; Editing. David Scarse: Investigation Eelke Jongejans: Supervision, Writing - Review \u0026amp; Editing. Constant Swinkels : Conceptualization, Supervision, Writing - Review \u0026amp; Editing\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe are grateful to Klaas van den Berg and Piet Ursem from Staatsbosbeheer for granting access to the sites. A sincere thanks to all the volunteers, especially Jan Rocks, who have collected transect data over the years. We are also grateful to Jos Mensen for sharing his expertise on hydrological data. Thanks go to Marten Geertsma and Lars Willighagen for field assistance. The butterfly transect data used in this study was collected as a part of a larger monitoring scheme, namely the Dutch Butterfly Monitoring Scheme. The Dutch Butterfly Monitoring Scheme is a co-operation between Dutch Butterfly Conservation (De Vlinderstichting) and Statistics Netherlands (CBS), in the context of the Network Ecological Monitoring (NEM), and financed by the Ministry of Agriculture, Nature and Food Quality.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data that support the findings of this study are openly available in Dryad Digital Repository.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAdema E, Molenaar W, Krap S, Stroo A, Rossenaar A-J, Veninga J, Logemann D (2017) PAS-Gebiedsanalyse Natura 2000-gebied Bargerveen (033). 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Biol Conserv 187:120\u0026ndash;126. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.biocon.2015.04.022\u003c/span\u003e\u003cspan address=\"10.1016/j.biocon.2015.04.022\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Long-term pollinator trends, hoverflies, wild bees, butterflies, wet heathland, Nardus grassland","lastPublishedDoi":"10.21203/rs.3.rs-8233210/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8233210/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eRecent studies report declines in abundance and species richness of pollinator communities. In response, nature managers aim to promote pollinators primarily through vegetation manipulation. However, long-term studies that evaluate pollinator community development in restored areas under active management are scarce. To assess whether landscape-level conservation efforts are able to sustain a characteristic pollinator community, we study abundance and richness trends of hoverflies (Diptera: Syrphidae), bees (Hymenoptera: Apidae s.l.), and butterflies/diurnal moths (Lepidoptera), as well as their characteristic species over 30 years within the restored and intensively managed Dutch raised bog system Bargerveen. We describe positive and stable trends in the overall abundance and species richness of pollinators, primarily Apidae and Syrphidae. These trends contrast with nation-wide declines. The effects of recent dry years were evident in declining butterfly abundance trends, although not as pronounced as in national trends. In addition, abundance of species characteristic of raised bog systems remained stable, suggesting that benefits also applied to these generally more sensitive species.\u003c/p\u003e \u003cp\u003e \u003cb\u003eImplications for insect conservation\u003c/b\u003e: Results from this case study show that conservation efforts in raised bog systems can support and improve pollinator abundance and richness, as well as populations of characteristic and vulnerable species. We argue that the positive and stable population trends of pollinators in Bargerveen are primarily a result of large-scale water management, and that landscape-level hydrological management has the potential to act as a buffer against drivers of insect decline even within predominantly agricultural landscapes.\u003c/p\u003e","manuscriptTitle":"Keeping Raised Bog Remnants Wet Stabilizes Characteristic Pollinator Communities","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-19 09:11:51","doi":"10.21203/rs.3.rs-8233210/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"3bc3444a-def6-4b6e-8b89-b814769bc45b","owner":[],"postedDate":"January 19th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-04-24T20:24:17+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-19 09:11:51","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8233210","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8233210","identity":"rs-8233210","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}