Accordingly, a synthesis of 600 fields from 41 crop systems showed that only two of the 68 most frequent pollinators globally were specialist species: the weevil Elaeidobius kamerunicus pollinating oil palm and the squash-bee Peponapis pruinosa pollinating pumpkin .Because of differences in species functional traits, greater pollinator richness can lead to foraging complementarity or synergy, improving the quantity and quality of pollination and therefore increasing both the proportion of flowers setting fruits and product quality . Across crop species, insects with contrasting mouth part lengths may be needed for the pollination of flowers not only with easily accessible rewards but also with rewards hidden at the bottom of a tubular corolla . Within a crop species, social and solitary bees visited flowering radish plants at different times of day, suggesting temporal complementarity among these pollinator groups . Flower visiting behavior also differs among pollinators of different body sizes, and visits by a range of differently sized pollinator species increase pumpkin pollination. In addition to functional traits, square pot plastic interspecific differences in response traits to climate and land-use change can increase resilience of pollination services.
The role of diverse assemblages of wild insects in crop pollination is also evident from recent global analyses. Worldwide, incomplete and variable animal pollen delivery decreases the growth and stability of yields for pollinator-dependent crops . This lower yield growth has been compensated for by greater land cultivation to sustain production growth . The consequent reduction in natural areas within agricultural landscapes decreases the richness and abundance of wild pollinators, including bees, syrphid flies, and butterflies , further diminishing crop pollination . A possible solution to this “vicious cycle” is to increase pollinator abundance through single-species management, most commonly European honey bees , which are not greatly affected by isolation from natural areas . However, increasing the abundance of one species may complement but not replace the pollination services provided by diverse assemblages of wild insects, and wild insects pollinate some crops more efficiently than honey bees . Moreover, during the past 50 years, the fraction of animal-pollinator dependent agriculture and the number of managed honey bee hives have increased 300% and 45%, respectively, and honey bees have suffered from major health problems such as colony collapse disorder . All of these factors point to the potential benefit of practices that boost the species richness and abundance of wild pollinators.
Indeed, richness and visitation rate of wild pollinators are strongly correlated across agricultural fields globally . Therefore, practices that enhance habitats to promote species richness are also expected to improve the aggregate abundance of pollinators, and vice versa .Below we describe practices that diversify and improve the abundance of resources for wild insects outside the crop field, without affecting crop management. Practices are ranked from less-to-more required area, with practices covering less area likely to be less costly . Nesting resources – such as reed internodes and muddy spots for cavity nesters, and bare ground for soil nesters – can be enhanced at crop field edges without affecting much of the crop area. Although providing such resources can promote the recruitment of certain bee species , evidence of its effects on crop yield is lacking . Hedgerows and flower strips are woody or herbaceous vegetation, respectively, planted at the edge of a crop field, and generally covering only a small area. If appropriate plant species are chosen and adequately managed through time , hedgerows and flower strips can provide suitable food and nesting resources for, and enhance species richness and abundance of, bees and syrphid flies . These practices also enhance pollinators in adjacent fields – rather than simply concentrating pollinators at dense flower-rich regions – and therefore increase crop yield .
Regional programs that augment the quality and availability of seeds from native flowering plants are important for the success of these practices . Conserving or restoring natural areas within landscapes dominated by crops often provides habitat for wild pollinator populations . In addition, pollinators depend on various types of resources , which are difficult to provide in ways other than by enhancing natural areas. Consequently, these areas also enhance pollination services for nearby crops . Enhancing farmland heterogeneity increases pollinator richness because plant species provide complementary resources over time and space, and insect species use different resource combinations . Also, insects usually require resources for periods longer than crop flowering . In fact, a synthesis of 605 fields from 39 crop systems in different biomes found that diversity of habitats within 4 ha enhanced bee abundance by 76% as compared with bee abundance in monoculture fields . Smaller crop fields increase land-use heterogeneity, and also benefit pollinators because most species forage at distances less than 1 km from their nests . Thus, crops in small fields are more likely to benefit from pollinator enhancements such as nearby field margins and hedgerows . Indeed, pollinator richness, visitation rate, and the proportion of flowers setting fruits decreased by 34%, 27%, and 16%, respectively, at 1 km from natural areas across 29 studies worldwide .In contrast to off-field methods that can be ordered from smaller to larger scale , on-field practices are all applied at a similar spatial scale, ie that of the crop field. Here we discuss practices that reduce the use of insecticides and machinery, enhance the richness of flowering plants, and require greater effort because of changes in the crop species or system . Reducing the use of synthetic insecticides that are toxic to pollinating insects should provide an important benefit . For example, in South Africa, insecticides adversely affected pollinators, impairing rather than enhancing mango yield . Insecticides with low toxicity to pollinators, with non-dust formulations, applied locally through integrated pest management practices, and applied during the non-flowering season are less likely to be detrimental to pollinators than highly toxic, systemic insecticides that are broadly sprayed from airplanes . No-tillage farming may enhance populations of ground nesting bees given that many species place their brood cells <30 cm below the surface . Tillage timing, depth, and method probably have differential impacts on pollinators and pollination, but further studies are required to verify this expectation . Similarly, flood irrigation may be detrimental in comparison to drip irrigation because of the increased likelihood of flooding pollinator nests but, particularly in arid systems, irrigation in general can promote wild-insect abundance through higher productivity of flowering plants or by making the soil easier to excavate . Enhancing flowering plant richness within crop fields can benefit pollinator richness and crop pollination, as demonstrated for mango and sunflower in South Africa. Similar results were found for wild plants within watermelon and muskmelon fields in the US . In Ghana, banana intercropping with cocoa boosted pollinator abundance and cocoa pod set . A diverse set of flower species with different phenologies is likely to increase resource stability for pollinators and thus the resilience of pollination services. Herbicides and mowing can negatively affect pollinators by reducing floral resources provided by weeds , square pots for plants but can be useful for reducing the abundance of invasive grasses that could otherwise displace native flowering plants . Organic farming combines some of the practices described above and can enhance wild pollinator populations in comparison to conventional farming , probably because of the absence of synthetic insecticides and/or greater non-crop floral resources.
Farmland heterogeneity can also be increased by organic management practices, which account for less than 1% of global agriculture . When the extent of organic farming was expanded in a German agrolandscape from 5% to 20%, bee richness rose by 50%, while the density of solitary bees and bumble bees increased by 60% and 150%, respectively . Pollination-related benefits of organic practices were also found for strawberry in Sweden and canola in Canada . Sowing flowering crops, instead of crops that do not offer floral resources for pollinators, may enhance wild pollinators in heterogeneous landscapes . In western France, solitary-bee richness and abundance were higher in margins of canola fields than in fields of other crops . In the UK, bumble bee abundance was higher in areas adjacent to bean fields than to wheat fields but only during crop flowering , suggesting a short-term behavioral response to flower abundance rather than a long-term population enhancement. Similarly, in Germany, canola improved bumble bee early-colony growth but not whole-season sexual reproduction , and greater land cover of mass-flowering crops increased the number of bumble bee workers but not colony numbers . Therefore, although crops can provide abundant resources, the short duration of floral availability, the low diversity of resources, the application of insecticides, and the presence of tillage may limit the capacity of one crop species to support wild pollinator populations on its own . Furthermore, large monocultures of flowering crops can suffer from pollination deficit and trigger indirect negative effects on pollinators . Sowing crops that bloom in different periods may therefore increase wild-insect populations; in Sweden, bumble bee reproduction was improved in landscapes with both late-season flowering red clover and early-season mass-flowering crops . Moreover, managing crop phenology to better match the availability of efficient pollinators should enhance pollination, but we found no studies on this practice .The effectiveness of pollinator-supporting practices is influenced by interactive effects between large and small scale factors. For example, the effects of landscape composition on bee richness are greater on farms with low habitat diversity than on farms with high habitat diversity . Similarly, in Argentina, the importance of wildflower strips as pollinator sources for sunflower increased in the absence of large remnants of natural habitats nearby . In South Africa, the importance of weed richness for enhancing sunflower seed set increased with larger distances from natural areas . Throughout Europe, extensive programs aim to mitigate biodiversity loss on farmland through practices such as organic farming or wildflower strips, thereby offering a unique opportunity to understand interactions among these methods. A meta-analysis showed that these practices enhanced pollinator richness , but their effectiveness varied with the magnitude of increase in flowering plant cover resulting from the practices, farmland type, and landscape context . Because intensively managed croplands are generally devoid of flowering plants, pollinator-supporting practices in these landscapes result in the largest increase in floral resources and thus pollinator richness . On the other hand, conventionally managed grasslands generally contain more flowering plant species than arable fields,making it more difficult to enhance floral resources and pollinators . Finally, local effects were more positive in structurally simple landscapes than in cleared or complex landscapes, presumably because cleared landscapes lack sources of pollinator colonists and complex landscapes have less need of restoration. Recently, researchers have begun to explore the relative effectiveness of different pollinator-supporting practices. In Europe, flower strips were more effective than grass-sown or naturally regenerated strips . Globally, the effect of landscape composition and farm management was more important for improving bee richness than the effect of landscape configuration . Interestingly, conventional farms with high in-field habitat diversity maintained similar pollinator abundance as organic farms with low in-field habitat diversity, across the gradient of heterogeneity in surrounding land use. Thus, different combinations of local and landscape practices can result in similar outcomes in terms of promoting pollinator richness, providing alternative solutions suited to different agricultural settings. The importance of small-scale practices is likely greater for insects with short flight ranges foraging from a fixed nest, such as small- to medium-sized bees, which usually forage within an area of a few hundred meters and comprise the greatest fraction of bee species . Consistent with the idea that small-scale practices alone can have high impact, a study designed to separate the effects of local- versus landscape-scale habitat on pollination services delivered to blueberries found that the local scale had stronger positive effects . Indeed, farmers acting individually are more likely to improve the quality of their own fields and the immediate surroundings than to be able to manage complete landscapes for pollinators. Assuming a foraging range of 200 m from the nest for small bee species , diverse and high quality habitats need to be provided within 13 ha .Understanding the socioeconomic consequences of pollinator-supporting practices is essential to effectively enhancing wild pollinator richness in “real-world” landscapes . Farmers generally face implementation costs, such as those for planting hedgerows, and opportunity costs, such as those for setting aside natural habitats that could otherwise be cultivated .