INTRODUCTION

Meeting the increasing demands for agricultural products while minimizing negative impacts on biodiversity and ecosystem health is among the greatest global challenges (Godfray et al. 2010). Intensive agricultural production and the simplification of agroecosystems threaten farmland biodiversity and associated ecosystem services worldwide (Foley et al. 2005; IPBES 2016, 2018, 2019). Concerns over loss of biodiversity and associated impairment of ecosystem services have helped strengthen the implementation of agri-environmental schemes and other measures to mitigate such negative consequences (IPBES 2016). Beyond restoration of farmland biodiversity in general, an implicit or explicit goal of such measures is to foster sustainable agricultural production through ecological intensification by harnessing biodiversity-based ecosystem services, such as crop pollination and natural pest control services (Bommarco et al.2013; Pywell et al. 2015; Kovács-Hostyánszki et al. 2017). In intensively managed agroecosystems, the establishment of strips or other areas of flowering herbaceous plants, hereafter “flower strips”, and hedgerows are among the most commonly applied measures to achieve these goals (Scheper et al. 2015; Tschumi et al. 2015; Williams et al. 2015; Dainese et al . 2017; Kremen et al. 2019). For example, the establishment of flower strips or hedgerows is supported by the Common Agricultural Policy (CAP) in the European Union and by the Farm Bill (e.g., programs of the Natural Resources Conservation Service of the United States Department of Agriculture) in the United States (IPBES 2016; Kovács-Hostyánszki et al. 2017; Venturini et al. 2017a). Typically established along field edges, flower strips and hedgerows provide green infrastructure for farmland biodiversity, offering resources for pollinators and natural enemies of crop pests such as shelter, oviposition sites, overwintering opportunities and food resources (Tschumi et al. 2015; Hollandet al. 2016; Kremen et al. 2019). There are now multiple demonstrations of such floral plantings locally increasing the abundance and diversity of pollinators and natural enemies of crop pests (Haalandet al. 2011; Scheper et al. 2013; M’Gonigle et al.2015; Williams et al. 2015; Tschumi et al. 2016; Sutteret al. 2017, 2018; Kremen et al. 2019). It is less well understood whether and at what spatio-temporal scales the enhanced species diversity translates to ex situ provisioning of pollination, pest control and increased yield. The ‘exporter’ hypothesis (Morandin & Kremen 2013; Kremen et al. 2019) predicts a facilitative effect of floral plantings and enhanced delivery of ecosystem services through functional spillover (sensu Blitzeret al. 2012; see also Albrecht et al. 2007; Morandin & Kremen 2013; Pywell et al. 2015; Tschumi et al. 2015, 2016; Sutter et al. 2017). Enhanced service provisioning may, however, not necessarily lead to increased crop yield, as a multitude of agricultural management practices such as fertilization, level of pesticide use, pest pressures, and soil cultivation may mask positive effects of services on yield (e.g., Gagic et al. 2017; Sutteret al. 2018). However, according the ‘concentrator’ hypothesis (Kremen et al. 2019; also referred to as the ‘aggregation’ hypothesis (Venturini et al. 2017a) or the ‘Circe principle’ (Landeret al. 2011)), resource-rich floral plantings temporarily compete with flowering crops and concentrate pollinators and natural enemies from the surrounding agriculture into the floral plantings, potentially resulting in (transiently) reduced crop pollination and pest control services (Nicholson et al. 2019). This may explain why plantings fail to enhance crop pollination or natural pest control services, even if they successfully promote local pollinator or natural enemy abundance in restored habitats (e.g., Phillips & Gardiner 2015; Tscharntkeet al. 2016; Karp et al. 2018).
The lack of clarity about effects of flower plantings on ecosystem service provisioning and crop yield scattered in numerous case studies is a barrier to farmer adoption of such measures (Garbach & Long 2017; Kleijn et al. 2019). A quantitative synthesis of such demonstrated broad evidence may assist farmers in making the decision to adopt these measures (Garbach & Long 2017; Kleijn et al. 2019). Moreover, it is important to gain a general understanding of whether such effects are restricted to the area of the crop near to the adjacent planting (Ganseret al. 2019) or be detectable over larger distances (Tschumiet al. 2015). Such knowledge should be considered when designing schemes with optimal spatial arrangement of plantings across agricultural landscapes (Ricketts et al. 2008; Garibaldi et al. 2011), and to facilitate cost-benefit assessments (Blaauw & Isaacs 2014; Morandin et al. 2016 Dainese et al. 2017; Haanet al. 2020; Williams et al. 2019).
To improve the effectiveness of flower strip and hedgerow plantings in promoting crop pollination, natural pest control, and potentially crop production, we need to better understand what determines their failure or success. We hypothesize that at least three factors influence the effectiveness of floral plantings in enhancing crop pollination and pest control services: plant diversity, time since establishment and landscape context. First, theory predicts that higher plant species richness, and associated trait diversity, promotes diverse pollinator and natural enemy communities due to positive selection and complementarity effects across space and time (e.g., Campbell et al. 2012; Scheper et al. 2013; Sutter et al. 2017; M’Gonigle et al. 2017). However, the role of plant diversity for driving effects of floral plantings on pollination and natural pest control services benefits to nearby crops is poorly understood. Second, time since the establishment of floral plantings is likely to play a key role for the local delivery of crop pollination and pest control services (Thies & Tscharntke 1999). This is of particular relevance for sown flower strips that may range from short-lived annual plantings to longer-lived perennial plantings. Perennial plantings should offer better overwintering and nesting opportunities for pollinators and natural enemies (Ganser et al. 2019; Kremen et al. 2019). Thus, the potential contribution of floral plantings to local population growth of wild pollinators and natural enemies might increase over time (e.g., Blaauw & Isaacs 2014; Venturini et al. 2017b). Third, the effectiveness of floral plantings could depend on the agricultural landscape context. At intermediate simplification levels source populations should be available and the ecological contrast (Scheperet al. 2013) of a local measure great enough to be effective (intermediate landscape complexity theory; Tscharntke et al. 2005; Kleijn et al. 2011). While support for this hypothesis has been found with respect to biodiversity restoration (e.g., Bátary et al. 2011; Scheper et al. 2013, 2015; but see e.g. Hoffmannet al. 2020), its validity for ecological intensification and the local delivery of crop pollination and pest control services has only just begun to be explored (Jonsson et al. 2015; Grab et al. 2018; Rundlöf et al. 2018).
Here we use data from 35 studies including 868 service-site-year combinations across 529 sites in North American, European and New Zealand agroecosystems to quantitatively assess the effectiveness of two of the most commonly implemented ecological intensification measures, flower strips and hedgerows, in promoting crop pollination, pest control services and crop production. Moreover, we aim to better understand the key factors driving failure or success of these measures to suggest improvement of their design and implementation. Specifically, we address: (1) the extent to which flower strips and hedgerows enhance pollination and pest control services in adjacent crops; (2) how service provisioning changes with distance from floral plantings; (3) the role of plant diversity and time since establishment of floral plantings in promoting pollination and pest control services; (4) whether simplification of the surrounding landscape modifies the responses; and (5) whether floral plantings enhance crop yield in adjacent fields.
Our synthesis reveals general positive effects of flower strips but not hedgerows on pest control services in adjacent crop fields. Effects on crop pollination, however, depended on flowering plant diversity and age since establishment, with more species-rich and older plantings being more effective. However, no consistent impacts of flower strips on crop yield could be detected, highlighting the need for further optimizations of plantings as measures for ecological intensification.