DISCUSSION

Our quantitative synthesis demonstrates a generally positive effect of flower strips on pest control services but these effects did not consistently translate into higher yields. Although in most cases beneficial effects of plantings were also found for crop pollination services, effects on crop pollination and final crop yield were variable and overall not significant. Effects of wildflower strips on pollination services increased with age and species-richness and declined with increasing distance to hedgerows and flower strips suggesting that the quality of plantings plays a pivotal role in effective service provision. Our results indicate that floral plantings have great potential to benefit ecosystem service provision, but to do so will need to be carefully tailored for functioning at specific spatial scales. Flower diversity and strip age are important drivers through which this can be achieved and they should be considered integrally before floral plantings can make a significant contribution to the ecological intensification of agricultural production.
We found positive effects of flower strips on ecosystem service provisioning in support of the ‘exporter’ hypothesis, although effects were generally variable and only significant for flower strips enhancing pest control services by 16% on average. This is an important finding as it provides general empirical evidence that flower strips can reduce crop pest pressures across various crops, landscape contexts, and geographical regions. One explanation for the more consistent positive effects on pest control services of flower strips compared to hedgerows may be that in many of the studied flower strips the selection of flowering plants was tailored to the requirements of the target natural enemy taxa (Tschumi et al. 2015, 2016) while this was generally less the case in the studied hedgerow plantings.
Wildflower plantings have been heralded as one of the most effective measures to enhance the provision of ecosystem service to crops (Kleijnet al. 2019) with many studies showing positive effects on service provisioning (e.g., Blaauw & Isaacs 2014; Tschumi et al.2015, 2016; included in this quantitative synthesis). Our synthesis shows, however, that although general significant effects of flower strips were found for pest control service provisioning, effects of plantings on crop pollination services were highly variable. This highlights the need to better understand these conditions and drivers of success or failure of floral plantings to promote pollination services. Our synthesis identifies several drivers that explain variability in delivered services and therefore offers pathways to enhance the effectiveness of these measures in the future.
First, the success of flower strips to promote crop pollination services in adjacent fields increased with their age. The strongest increase was detected up to roughly three years since the planting date. Pollination services also appeared to continue to increase with establishment time beyond three years. This trend needs to be interpreted with caution as only three studies assessed four years old or older flower strips highlighting that scarcity of long-term data on the effects of floral plantings on services provisioning and yield, which represents as an important current knowledge gap. We found no evidence that this increase in effectiveness with age is driven by an increase in floral abundance with flower strip age, corroborating results of case studies of Central and Northwestern European regions that suggest relative abundance and species richness of flowering herbaceous plants in sown flower strips on the highly fertilized soils in these agroecosystems peak in the second or third year and then decline again as grasses take over (Steffan-Dewenter & Tscharntke 2001; Ganser et al. 2019). Rather, these findings are in agreement with the expectation that the build-up and restoration of local crop pollinator populations need time (Blaauw & Isaacs 2014; Buhk et al. 2018; Kremen et al.2018). They may also be explained by greater provision of nesting and overwintering opportunities in older floral plantings (Kremen et al. 2019). Nesting and overwintering opportunities are likely scarce in short-lived annual flower strips, which could even be ecological traps for overwintering arthropods (Ganser et al. 2019). In fact, Kremen & M’Gonigle (2015) found higher incidence of above-ground cavity nesting bees compared to ground-nesting bees with hedgerow maturation, and Ganser et al. (2019) reported increased overwintering of arthropod predators and pollinators of perennial compared to annual flower strips.
Second, our findings reveal that higher species richness of flowering plants tends to enhance pollination service delivery in adjacent crops. This is an important finding as it indicates that restoring plant diversity can not only promote rare pollinator species and pollinator diversity (cf. Scheper et al. 2013; Kremen & M’Gonigle 2015; Sutter et al. 2017; Kremen et al. 2018), but also crop pollination services. Flowering plant diversity likely promotes complementary floral resources for a high number of pollinator taxa with different resource needs. Furthermore, it should increase phenological coverage and continuity of floral resource availability throughout the season (Schellhorn et al. 2015; M’Gonigle et al. 2017; Lundin et al. 2019). Our synthesis reveals that floral plantings enhance pollination services, but only in the part of adjacent crops near to plantings, while declining exponentially with distance to plantings (Fig. 2). In fact, the exponential decline function predicts pollination service provisioning of less than 50% at 10 m and slightly more than 20% at 20 m compared to the level of service provisioning directly adjacent to plantings, partially explaining the overall non-significant benefits when considering all measured distances across the entire field (Fig. 2). This may also explain part of the high variability observed across studies and reconcile some of the contrasting findings with respect to pollination service provisioning in studies measuring services relatively near plantings (e.g. up to 15 m; Blaauw & Isaacs (2014), or up to larger distances, e.g. up to 200 m; Sardiñas et al. (2013)). Further possible reasons for the high variability in observed effects of plantings on crop pollination services may include variation in pollination services measures or dependency of crops on insect pollination
Consistent with previous studies (e.g., Dainese et al. 2019), landscape simplification was associated with decreased pollination services, irrespective of the presence of floral plantings. In contrast, no such effects were detected for pest control services, in agreement with recent studies (Karp et al. 2018; Dainese et al.2019; but see Veres et al. 2013; Rusch et al. 2016; Martinet al. 2019). The effect of adding a flower strip or hedgerow was, however, independent of landscape context. Although individual case studies (Jonsson et al. 2015; Grab et al. 2018; included in this synthesis) found support for the intermediate landscape hypothesis, enhanced ecosystem services associated with floral plantings were not generally limited to moderately complex landscape contexts across all studies considered here. The fact that positive impacts of floral plantings occurred regardless of landscape context may encourage farmers to adopt these measures irrespective of the type of landscape in which they are farming.
Crop yield is affected by a complex interplay of a multitude of agricultural management practices such as fertilization, level of pesticide use, pest pressures, soil cultivation and other factors such as local soil and climatic conditions (e.g., Bartomeus et al.2013; Gagic et al. 2017), which can potentially mask benefits from improved natural pest regulation or pollination services (Sutteret al. 2018). Positive effects of floral plantings have been shown by some case studies included in this synthesis (e.g., Tschumiet al. 2016; see also Pywell et al. 2015), although sometimes only several years after the establishment of plantings (Blaauw & Isaacs 2014; Morandin et al. 2016; Venturini et al. 2017b), but we did not detect consistent effects on crop yield associated with adjacent floral plantings. The identified drivers of the effectiveness of floral plantings to enhance crop pollination services, such as age and flowering plant diversity, could provide promising pathways towards optimizing plantings as measures contributing to ecological intensification. Future optimizations should also consider the potential for synergistic interactions of enhanced pollination and pest control services by “multi-service” designs of plantings (Sutter & Albrecht 2016; Morandin et al. 2016), temporal dynamics (Blaauw & Isaacs 2014; M’Gonigle et al. 2015), optimized ratios of floral planting (contributing to ecosystem service supply) to crop area (affecting service demand; Kremen et al. 2019; Williamset al. 2019), and the distance-dependency of services quantified by this synthesis. However, floral plantings are also established for other goals than yield increase. From an environmental and health perspective, keeping yield levels constant despite reductions of insecticide input through replacement by enhanced natural pest regulation services by floral plantings should be considered as a great achievement (e.g., Tschumi et al. 2015). Moreover, floral plantings contribute to biodiversity conservation (e.g. Haaland et al . 2011; Scheper et al . 2013), but farmers are often reluctant to adopts such measures due to concerns of negative effects on crop yield e.g. due to spillover of pests. Our findings of similar crop yield in fields with and without plantings can dispel such concerns.