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.