The intensification of agriculture in western Europe over recent decades has led to declines in the populations of many wild plant and animal species formerly characteristic of farmland (Robinson & Sutherland 2002). In order to reverse these declines, it is mandatory for European Union (EU) member states to operate agri-environment schemes as part of the Common Agricultural Policy. The objectives of the schemes differ, depending on country and region, but all include measures whereby farmers are paid to manage their land for the benefit of particular habitats and species (Ovenden, Swash & Smallshire 1998). In England, Environmental Stewardship (ES) is a new agri-environment scheme that operates at two levels: the Entry Level (ELS) (Defra 2005a) is open to all farmers, while the Higher Level (HLS) (Defra 2005b) offers greater rewards to land managers for the delivery of a wider range of biodiversity benefits on targeted sites, such as those with existing high-priority environmental features.
Bumble bees (Bombus spp.) are considered important as pollinators because of their roles in enhancing the yields of entomophilous crops (Corbet, Williams & Osborne 1991; Free 1993), particularly fruit crops (Willmer, Bataw & Hughes 1994), and in maintaining populations of native plant species that have been fragmented within the agricultural landscape (Steffan-Dewenter & Tscharntke 1999). However, many bumble bee species have shown declines in abundance and contractions in range across Europe and North America since the mid-20th century (Williams 1982; Rasmont 1988; Buchmann & Nabhan 1996). In the UK, three species have been declared extinct and up to half the remaining 22 species are under threat (Edwards & Jenner 2005).
Their requirements for a season-long supply of pollen and nectar sources and undisturbed nesting, mating and hibernation sites make bumble bees susceptible to the effects of intensive farming. Changes in management practice, such as the conversion of species-rich hay meadows for silage production and the degradation of perennial vegetation in field margins and hedgerows, are likely to have had detrimental effects on all Bombus spp. (Osborne & Corbet 1994). Some species, including Bombus sylvarum and Bombus ruderatus, are thought to have been particularly affected by the loss of unimproved grassland in the UK (Fuller 1987) and are listed as priority species on the UK Biodiversity Action Plan (BAP) (Anonymous 1999). Suitable management of semi-natural areas where their populations persist is a conservation priority (Carvell 2002), but the potential to provide resources for these rarer species on farmland requires further investigation. While the more common species may benefit from mass flowering crops, such as oilseed rape Brassica napus ssp. oleifera (Westphal, Steffan-Dewenter & Tscharntke 2003), these temporary forage resources alone are unlikely to be sufficient to sustain their colonies throughout the season, or to support the full species assemblage in agricultural landscapes. Agri-environment schemes therefore offer an important opportunity to restore habitats of value to bumble bees in intensively farmed areas. It is, however, essential that management options within such schemes are both based on sound scientific evidence and subject to scientific evaluation to ensure that they are successful in attracting the desired species (Kleijn & Sutherland 2003; Knop et al. 2006).
One objective of the UK agri-environment schemes is to enhance the abundance and diversity of flowering plant species within arable systems through changes in management within or at the margins of fields. Field margins are a key feature of agricultural landscapes and there are well-documented agronomic and ecological reasons why they have become the focus of management options within the schemes (Marshall & Moonen 2002; Defra 2005a, 2005b). Margins act as buffers to protect hedgerows against pesticide and fertilizer drift, prevent the spread of pernicious weeds into crops, and provide important refuge habitats for wildlife (Marshall & Moonen 2002; Meek et al. 2002; Critchley et al. 2004). Initial assessments of these management options suggested that the potential benefits for bumble bees were mixed (Kells, Holland & Goulson 2001; Kleijn et al. 2001; Goulson et al. 2002), despite positive effects being recognized for other taxa. Conservation headlands, where pesticide and herbicide applications at the crop edge are reduced, are more likely to encourage annual plants than perennials and biennials, which are the preferred forage species for most bumble bees (Fussell & Corbet 1992; Dramstad & Fry 1995; Critchley et al. 2004). Uncropped margins left to regenerate naturally may provide suitable forage species on some sites but can encourage pernicious weeds such as Cirsium spp. and can take several years to develop suitable mid-successional communities (Corbet 1995; Carvell et al. 2004).
Sowing a mixture of annual or perennial grassland species on arable field margins has been shown to overcome some of the above restrictions and significantly enhance the abundance and diversity of bumble bees and their forage plants (Carreck & Williams 2002; Meek et al. 2002; Carvell et al. 2004; Pywell et al. 2005, 2006). However, these studies have either been conducted at a single location or during a single year, where factors such as soil geology, the local Bombus spp. assemblage, climatic conditions and timing in relation to the establishment of field margin habitats may influence the outcome. Furthermore, many agri-environment scheme assessments have been compromised by a lack of standardized management practices or seed mixtures across study sites, caused by variation in farmer expertise and understanding of the desired plant communities (Kleijn et al. 2001). Options for field margins and arable land within the new ES scheme in England are accompanied by clear management guidelines involving standard agricultural techniques (Defra 2005a, 2005b). They may require greater intervention in the early stages to achieve successful establishment (Marshall & Nowakowski 1995), but the outcome is likely to better resemble the intended vegetation community and habitat quality for target species, and thus achieve the objectives of the scheme. To our knowledge, there have so far been no comprehensive assessments of the effects of these new ES options with standardized management prescriptions on any taxon.
In this study we assessed the effects of ES options for arable land on bumble bees and their forage plants over 3 years using a multisite experiment. We tested the following hypotheses. H1: field margin management according to different ES options has significant effects on the abundance and diversity of flowering resources and foraging bumble bees. H2: the effects of margin management on bumble bees and their forage resources change over time, between years. H3: the effects of seed mixture composition on flowering resources and foraging bumble bees change during the season.
The results are discussed in terms of the efficacy of different ES options in attracting foraging bumble bees, and the potential role of agri-environment schemes in enhancing and sustaining bumble bee populations on arable farmland.