Organic agricultural landscapes promote the conservation and diversity of cavity-nesting solitary bees

Agricultural landscapes are characterized by close interactions between annually ploughed crop fields and relatively little disturbed noncrop areas. Many species have adapted to these mosaic landscapes and species-rich agroecosystems developed as a result of moderate anthropogenic changes. During the last decades, agricultural intensification has caused a rapid decline of biodiversity in agroecosystems. Intensive farming practices with high agrochemical inputs displaced low-intensity farming, crop fields expanded at the cost of noncrop habitats, and hedgerows and fallow strips were removed in the course of field enlargement. These substantial changes in land-use intensity threat biodiversity and may disrupt trophic interactions and ecosystem services.In this study, we investigated how organic farming, landscape complexity and dispersal corridors may contribute to the diversity of bees and wasps in agroecosystems. Further, we examined how interactions with natural enemies were influenced. Our investigations were conducted in agricultural landscapes in the vicinity of the city of Göttingen, the Soester Börde and the Lahn-Dill-Bergland. Landscapes were selected to encompass a gradient from homogeneous crop-dominated landscapes to heterogeneous landscapes dominated by a diversity of noncrop habitats. Bees, wasps and their natural enemies were recorded in altogether 74 organic and conventional wheat fields, 74 fallow strips, 32 grass strip corridors, 24 hedges and 12 forest edges.Diversity of flower-visiting bees in wheat fields was greatly enhanced by organic farming due to a higher availability of flowering noncrop plants in organic compared to conventional fields. Differences in bee diversity between organic and conventional fields increased with the proportion of crop fields in the surrounding landscape (1 km radius), thereby indicating that processes at the landscape level have the potential to modify the effectiveness of agri-environment schemes. Organic farming in homogeneous landscapes with few remaining flower-rich habitats may reach highest relative effectiveness.Bee communities in fallow strips were influenced by organic crop fields at a local and a landscape scale. At the local scale, species richness of bees and abundance of solitary and bumble bees were higher in fallow strips adjacent to organic than to conventional fields. At the landscape scale, species richness and abundances of bees were enhanced by a high proportion of organic crop fields in the surrounding landscape. An increase of the proportion of organic crop fields from now 4.4 % on average in Germany to 20 % as aimed by the government may enhance species richness of bees in fallow strips by 54 %, density of solitary bees by 66 % and bumble bee density by 156 %.Nest colonization of cavity-nesting bees and wasps was enhanced by heterogeneous landscapes, organic farming and fallow strips. Wasps benefited more from fallow strips than from organic compared to conventional farming. Wasps in fallow strips were not influenced by the farming system of the adjacent field. This suggests that wasps preferred dispersing along fallow strips and that wasps nesting in fallow strips did not substantially utilize resources provided by the adjacent field. The positive effect of fallow strips at the local scale was consistent with a positive effect of high edge densities providing dispersal structures at a landscape scale. In contrast, bee colonization was enhanced by organic farming in both field centres and adjacent fallow strips, which did not significantly differ in bee abundance. This suggests that bees nesting in noncrop habitats benefited from resources in neighbouring organic wheat fields and did not depend on noncrop habitats for foraging or dispersal. A positive effect of high proportions of noncrop habitats in the surrounding landscape underlined that nest colonization of bees was influenced by source habitats.Grass strip corridors connecting forest edges (source habitats) and standardized nesting sites in cropland landscapes enhanced the abundance of solitary wasps in nest patches by 400 % compared to isolated nest patches. Natural enemies largely reflected the patterns found for their hosts, and mortality due to natural enemies did not depend on the presence of a corridor. In agricultural landscapes, where nesting sites are limited and food availability changes frequently, rapid colonization of nest patches may be linked to high population viability.

Pollinators are sensitive to agricultural intensification at both local and landscape scales. High configurational landscape heterogeneity due to small fields and a high amount of field boundaries is hypothesized to enhance farmland biodiversity. Here, we investigated the effects of organic vs. conventional farming in large- vs. small-scale agricultural landscapes on wild bee communities and their floral resources to improve conservation schemes for pollinators. We sampled bees in Central Germany at the field boundaries of 18 pairs of conventionally and organically managed winter wheat fields along the former iron curtain using pan traps and trap nests. Around traps, we surveyed species richness and flower cover of insect-pollinated forbs. Compared to conventional farming, organic farming was related to higher insect-pollinated forb species richness and flower cover in the field boundaries, presumably due to the lack of herbicide use. Interestingly, small-scale agriculture did not counteract the loss of floral resources under conventional management, as the difference to organic management was even larger in Western small-scale agriculture. Organic farming, but not small-scale agriculture, enhanced species richness of solitary bees, which is in line with their small home ranges. In contrast, bumblebees benefitted only from small-scale agriculture, which matches with their high dispersal ability. Despite similar levels of abundance and diversity of trap-nesting bees in the two landscape types, brood cell parasitism was also higher in small-scale agriculture. Both organic farming and small-scale agriculture directly and indirectly supported different groups of wild bees, suggesting long-term benefits for conservation. Agri-environment schemes should acknowledge the so far neglected benefits of small-scale agriculture for biodiversity and its potential services.

Interacting effects of farming practice and landscape context on bumble bees

The loss of biodiversity in agricultural landscapes has been dramatic over the past few decades with negative trends persisting. Organic farming has received widespread recognition in the scientific and politic fields for its environmental benefits, although the proportion of land cultivated organically is still small and the extent to which organic farming contributes to the promotion of biodiversity is viewed controversially. We present a critical, quantitative review of 98 mainly peer-reviewed papers selected from 801 studies in temperate climate zones published over the period 1990–2017. We quantified differences in the species richness and abundance of selected flora and fauna groups. In total, 474 pairwise comparisons that compared organic and conventional farming systems were considered. Overall, organic farming showed higher species richness or abundance in 58% of the pairs. No differences were found for 38%, 4% indicated negative effects from organic farming. The average (median) species numbers of flora on arable land were 95% higher under organic management as well as 61% higher for seedbank and 21% higher for field margin vegetation. For field birds, the species richness was 35%, and the abundance was 24% higher in organic farming; for insects, the corresponding values are 22% and 36% and for spiders 15% and 55%. Our study underlines that organic farming can play an effective role in acting against the loss of biodiversity. Future research should focus on the combined effects of landscape structures and organic farming, the effect of large-scale organic farming, as well as on the correlation of species diversity and production parameters. To meet the systems’ representativeness, even more strict selection criteria need to be applied in further analysis.

SummaryThe efficiency of agricultural subsidy programmes for preserving biodiversity and improving the environment has been questioned in recent years. Organic farming operates without pesticides, herbicides and inorganic fertilizers, and usually with a more diverse crop rotation. It has been suggested that this system enhances biodiversity in agricultural landscapes. We analysed the effects of organic farming on species richness and abundance using meta‐analysis of literature published before December 2002.Organic farming usually increases species richness, having on average 30% higher species richness than conventional farming systems. However, the results were variable among studies, and 16% of them actually showed a negative effect of organic farming on species richness. We therefore divided the data into different organism groups and according to the spatial scale of the study.Birds, insects and plants usually showed an increased species richness in organic farming systems. However, the number of studies was low in most organism groups (range 2–19) and there was significant heterogeneity between studies. The effect of organic farming was largest in studies performed at the plot scale. In studies at the farm scale, when organic and conventional farms were matched according to landscape structure, the effect was significant but highly heterogeneous.On average, organisms were 50% more abundant in organic farming systems, but the results were highly variable between studies and organism groups. Birds, predatory insects, soil organisms and plants responded positively to organic farming, while non‐predatory insects and pests did not. The positive effects of organic farming on abundance were prominent at the plot and field scales, but not for farms in matched landscapes.Synthesis and applications.Our results show that organic farming often has positive effects on species richness and abundance, but that its effects are likely to differ between organism groups and landscapes. We suggest that positive effects of organic farming on species richness can be expected in intensively managed agricultural landscapes, but not in small‐scale landscapes comprising many other biotopes as well as agricultural fields. Measures to preserve and enhance biodiversity should be more landscape‐ and farm‐specific than is presently the case.

Organic agriculture is increasingly promoted as a more environmentally friendly alternative to conventional agriculture, as it restricts the use of fertilisers and synthetic pesticides. However, the impact of both farming systems on aquatic biodiversity is strongly debated. Ponds are abundant in agricultural landscapes and strongly contribute to biodiversity. They also respond strongly to land use on a very local scale. The present study assessed the effects of conventional and organic agriculture on the taxonomic diversity of multiple groups of aquatic organisms at local and regional spatial scales. We conducted a broad scale field survey to quantify the differential impact of conventional and organic agriculture on local environmental conditions in ponds, on community composition and on local, among site and regional diversity of macrophytes, cladoceran zooplankton and selected macroinvertebrates in Belgian farmland ponds (Flanders). We observed that organic agriculture was moderately positively associated with higher local species richness for shoreline vegetation, but not for other organism groups. Only minor differences were observed for among‐pond variation, and these were mostly related to rare species of cladocerans and heteropterans. At the regional scale, ponds in organic showed higher species richness than in conventional farmland for shoreline vegetation, emergent vegetation, and cladoceran zooplankton, but lower for coleopterans and gastropods. There was no significant effect of agricultural type on water quality. We conclude that organic farming is associated with moderate positive effects on pond biodiversity and regional species richness of plants and zooplankton. We observed no systematic differences between the two types of agriculture in local environmental conditions in ponds. The absence of large differences in biodiversity and water quality between ponds located in organic and conventional farmland might be related to the dominance of intensive conventional farming in our study region and the history of conventional farming around ponds that are now surrounded by organic farming. Future studies should include other factors such as the structure of the landscape and the role of natural elements such as buffer strips surrounding the ponds.

This study was conducted in 2006 in central Uganda to provide baseline data on relationships between bee community variables and local, climatic, landscape and regional drivers affecting bee community abundance and diversity in agricultural landscapes. Bee abundance and species richness increased significantly () with increase in percent cover of semi-natural habitats and the abundance of wild and cultivated floral resources in the landscape. There were strong linear declines () in bee species richness and abundance with cultivation intensity. Bee species richness declined very steeply with forest distance. Bee species richness and abundance were negatively affected by land-use intensity (). Bee species richness and abundance were strongly negatively correlated () with increase in mean annual temperatures in the previous years than in current years indicating potential vulnerability of local bee species to future climate changes. The percent cover of semi-natural habitats and natural in the farmland predicted best the occurrence and distribution in central Uganda. It is therefore recommended to policy-makers and to farmers to invest in the protection of forest fragments (and related semi-natural habitats) acting as buffer in the mitigation of negative effects of climate change on bee biodiversity and pollination services delivery.

Bees are pollinators of both wild and crop plants with significant ecological and economic value. However, recent studies report declines in their populations, particularly in agricultural landscapes. Compared to conventional farming, organic agricultural management is considered less harmful to biodiversity. Despite this, studies addressing wild bees in organic systems remain limited. In this study, we selected organic and conventional farms to examine the influence of farming management on meadow flora. We also accessed landscape-level management and its influence on newly colonized nest of mason bees and their pollen diet. Our results show that organic farming promotes higher plants diversity in meadows. Analyses of pollen inside mason bee nests revealed a more diverse diet in landscapes with a greater proportion of organic fields. Additionally, the number of newly colonized nests increased with the extent of organically managed areas. These findings indicate that organic farming can enhance meadow plant diversity, positively influence the pollen diet of wild bees, and support larger populations of solitary bees. Environmental management aimed at sustainability helps to safeguard the diversity of both plants and mason bees. Overall, sustainable environmental management that increases organic farming coverage in agricultural landscapes holds substantial potential for protecting solitary bee populations. • Bee populations are declining, especially in farmlands. • Organic farming were found to safeguard biodiversity, but studies on wild bees are limited. • Study compares landscapes and farming management, analyzing plants, solitary bees, and pollen. • Organic farms boost plant diversity, solitary bee diet, and nest numbers. • More organic fields support an important european pollinator.

The expansion of global croplands, plantations and pastures in the last decades, which led to a dramatic change in the spatial structuring of agricultural landscapes is one of the main drivers of biodiversity loss. Intensified agriculture, e.g. increased use of agrochemicals, on a local scale and simplification of landscapes on a regional scale were accompanied by a dramatic loss of biodiversity in European agricultural landscapes. However, the relative importance of local and regional drivers differs between taxa and habitats.The first part of the thesis deals with effects of agricultural intensification on both above- and belowground communities. We analysed the diversity of vascular plants, carabid beetles and birds in agricultural landscapes at the scale of plots in cereal fields (n= 1350), farms (n= 270), and European regions (n= 9). We partitioned diversity into its additive components α, β, and γ, and assessed the relative contribution of β-diversity to total species richness at each spatial scale. Agricultural intensification negatively affected the species richness of plants and birds, but not carabid beetles, at all spatial scales. Local agricultural intensification was closely related to β-diversity on larger scales up to the farm and region level, and thereby an indicator of farm and region wide biodiversity losses. β-diversity at the scale of farms (13 - 21%) and regions (68% - 80%) accounted for the major part of the total species richness for all three taxa. For plants, relative importance of α-diversity decreased with agricultural intensification, while relative importance of β-diversity on the farm scale increased with agricultural intensification for carabids and birds. Hence, agricultural intensification does not necessarily homogenize local communities, presumably due to the heterogeneity of farming practices. We sampled diversity and abundance of earthworms and collembolans, soil respiration rate and microbial biomass in 12 pairs of organically and conventionally managed winter wheat fields in landscapes differing in structural complexity to investigate interacting effects of agricultural intensification on local and landscape scales on belowground biota. Results show that only the in-field abundance of collembolans was slightly enhanced under organic farming, in contrast, soil respiration rate was higher under conventional management. Landscape simplification increased microbial biomass carbon in organically managed fields and decreased it in conventionally managed fields. As the same pattern was found for earthworm species richness, earthworms may have enhanced microbial biomass. In general, organic farming appears to be more efficient in conserving aboveground than belowground diversity.In the second part of the thesis we studied the relative importance of spatial and environmental factors for the community composition of belowground biota. The relative importance of environmental or spatial processes determines whether communities are mainly formed by species-environment interactions or dispersal events. In soils as very patchy, complex and poorly connected habitats, dispersal might shape the community composition more strongly than in less complex and better connected habitats. We used a variance partitioning approach to investigate the relative importance of environmental conditions and spatial distances for soil bacterial, collembolan, carabid beetle and earthworm community composition in agricultural fields and old set-aside fallows, comparing samples from a very small scale of 40 m with samples from a regional scale of up to 13 km. We found that environmental conditions are important drivers of soil bacterial community composition. In addition, the spatial distribution of our sampling sites also influenced bacterial community composition, indicating a certain degree of provincialism, probably caused by dispersal limitation even on very small scales. The macroorganism taxa studied showed different patterns depending on their average body size. The larger macroorganisms did not show any spatial signals on the local scales. In contrast, their spatial signal is most prominent on the regional scale, while smaller macroorganisms, i.e. collembola, showed a spatial signal on the local but not on the regional scale. Differences between bacteria and macroorganisms are likely to be due to different dispersal modes for bacteria and macroorganisms in soils.We conclude that agricultural intensification had negative effects on most of the studied aboveground taxa, with detrimental effects acting over multiple spatial scales. In contrast, organic farming appears to be more efficient in conserving aboveground than belowground diversity, which could be due to the enormous functional redundancy of the belowground community and therefore high resilience and resistance to anthropogenic disturbances. However, landscape context played a significant role in interaction with local management, emphasizing the importance of agri-environmental schemes designed for local and regional scales alike. The relative importance of environmental and spatial factors for soil communities appeared to depend on the dispersal mode of the taxa considered.

Pollination is one of the most important processes in terrestrial ecosystems. This free nature service depends on a homogeneous distribution and sufficient density of flower-visiting insects, mainly wild bees and managed honey bees. Habitat destruction and habitat fragmentation and intensive land-use led to species loss on multiple trophic levels.The aim of this work was to answer the question whether pollination is still sufficiently ensured by wild bees and honey bees in intensively managed agricultural landscapes. For this, it was necessary to quantify local honey bee abundance and wild bee species composition and density in relation to landscape structure at different spatial scales. Thus it was tested, how solitary wild bees, social bumble bees and honey bees interact with the surrounding landscape (chapter 4). Furthermore, the influence of possible pollination limitation due to low densities of flower-visiting bees on yield of oilseed rape was studied in dependence of the surrounding landscape (chapter 5). Finally, the effect of different pollination vectors on oilseed rape was measured to quantify the relative importance of bee pollination and wind pollination for fruit set. The spatial distribution and density of managed honey bees was measured using Geographical Information Systems, for the district of Göttingen (Germany). In Germany, in a period of twelve years (1989 2000), the total number of managed honey bees decreased by 26% representing a loss of about 260.000 honey bee locations. Bee diseases and the high percentage of old beekeepers led to this situation. In conclusion, the current situation of beekeeping in the district of Göttingen has reached a critical level. The conservation of wild pollinator and the support of beekeeping must be a central topic to ensure pollination services for agricultural crops. If not, a full-coverage pollination by honey bees cannot be guaranteed.An aim of this study was the analysis of the effects of landscape context and the influence of the percentage of semi-natural habitats on species composition of trap-nesting wild bee communities a gradient of structurally poor to structurally rich landscapes. An increasing proportion of the semi-natural habitats in the surrounding landscape had no positive effect on species diversity at any spatial scale. In contrast, an increasing proportion of rapecrop fields influenced the number of brood cells per nest and the total nest number of Osmia rufa (Hymenoptera: Apidae) at small spatial scales. In conclusion, all studied agricultural landscapes only supported relatively low numbers of solitary bees and wasps.Bee visitation is necessary for pollination. To analyse the value of the degree of flower-visiting bees we analysed, if an increasing diversity and density of flower-visiting bees may have positive effects on the yield of oilseed rape (Brassica napus L. ssp. oleifera (Metzg.), Brassicaceae). Flower visitor observations indicated significantly increased seed set per flower with increasing honey bee densities, whereas the abundance of flower-visiting wild bees had no measurable effect on yields of oilseed rape. Bee visitation had no effect on real yield (g/m²), probably as a result of the high compensatory-growth ability of oilseed rape and variation in cultivation methods or local soil conditions between the landscapes. In contrast, rape yield was positively related to a higher percentage of rape crop fields in the surrounding landscape, possibly due to higher pollen densities in the air and enhanced wind pollination.Summarizing this study, species richness and abundance of solitary wild bees depends on landscape context and habitat composition. Species diversity and abundance of flower-visiting bees increase with higher percentage of semi-natural habitats in the surrounding landscape. Generally, habitat fragmentation and destruction of ecologically valuable habitats can cause decreasing bee diversity (solitary wild bees, social bumble bees and honey bees). Pollination as a free nature service depends on a high pollinator density and may result in a higher fruit set and yield thus confirming the economical importance of pollination. In conclusion, a further loss of pollinators must be avoided to protect most wild plant species and to ensure high yields of insect-pollinated cultivated plants.

Re-established grasslands on farmland promote pollinators more than predators

Seasonal population abundance of the assembly of solitary wasps and bees (Hymenoptera) according to land-use in Maranhão state, Brazil

Agricultural intensification is driving declines in pollinator diversity and the degradation of pollination services worldwide. Organic management, which prohibits the use of synthetic inputs, has considerable potential for combatting these declines by enhancing biodiversity in agricultural landscapes. However, the efficacy of organic farming for promoting pollinator diversity has recently been questioned. We conducted a meta‐analysis to determine the effect of organic management on pollinator species richness and abundance. We compiled 42 studies across four continents and calculated a Hedges' g effect size for each of 76 species richness and 57 abundance observations. We then estimated average effect sizes overall and across five moderators. Species richness and abundance of pollinating insects tended to be higher in organic farms than conventional ones, with average effect sizes of 0.68 and 0.74, respectively. Landscape context, crop type, pollinator group, sampling location, and sampling method were factors impacting the response of pollinator diversity/abundance to organic farming. We found benefits for pollinator diversity from organic farming in all landscape types, with the strongest response in simple compared with more complex landscapes. Pollinator diversity benefited the most in habitats within organic cereal systems and the least in organic pastures. Among pollinator groups, bumblebee diversity benefitted the most, while moths and hoverflies showed positive but non‐significant responses. Higher pollinator diversity/abundance was detected in organic farms at various sampling locations. Transect sampling showed the greatest diversity gains among sampling methods. Synthesis and applications. Our results affirm that organic farming has positive effects for pollinator species richness and abundance, but benefits are variable and not guaranteed. We recommend that future studies adopt a standard sampling protocol and cover a larger geographic range to understand the global potential of organic farming to promote pollinator biodiversity. Efforts to increase pollinator abundance and diversity through organic farming should focus on organic systems involving cereal phases, especially within simple landscapes, for the most positive outcomes. However, due to high variability in pollinator responses alongside the challenges to achieve economic profitability from organic farming, crafting organic farming systems to specific farm‐scale opportunities and needs may be necessary.

Wild bees are the most important group of pollinators among insects. In agroecosystems, they promote successful pollination and thus ensure crop yields. In order to enhance species diversity and abundance of wild bees in farmland, it is essential to study the impact of the surrounding landscape as well as local habitat factors on the occurrence of wild bees, and identify and implement biodiversity enhancing measures. We established a national-wide monitoring program for wild bees in Austria’s main agricultural regions for the first time (project BINATS 2). This was achieved through a stratified random sampling design comprising 1,000 transects across 100 test areas. In this paper, we investigate the direct and indirect effects of local habitat and landscape factors on the richness of wild bee species, their abundance and selected functional traits (ground nesting, above-ground nesting, solitary, social and polylectic bees, Fabaceae specialists, bumblebees). Additonally, we analyse whether these effects vary between the early and late seasons using structural equation modeling (SEM). Our results show that wild bee species richness and abundance are directly positively related to flower cover and semi-natural open landscapes. Grasslands and insect pollinated crops contribute to increased local flower cover, and thereby indirectly enhance wild bee species richness and abundance. Although insect pollinated crops have a stronger positive effect on flower cover in spring compared to summer, the effect on wild bee species richness and abundance does not vary significantly throughout the season. Solitary and above-ground nesting bees are also positively influenced by the percentage of organic farming in the different test areas, while bumblebees – beside above-ground nesting wild bees – are specifically enhanced by plant diversity. In addition, bumblebees are the only group directly positively affected by insect pollinated crops. These results demonstrate the strong and complex dependency of wild bees on flowering fodder plants and semi-natural open landscapes. This highlights the risks of their decline within agricultural landscapes caused by a reduction of various landscape structures and diverse floral richness present over long periods of time. We also confirm that organic farming is a crucial factor for promoting certain wild bee traits.

On the restoration of hedgerow ground vegetation: Local and landscape drivers of plant diversity and weed colonization