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\begin{document}

\title{Global synthesis of the effectiveness of flower strips and hedgerows on
pest control, pollination services and crop yield}



\author[1]{Matthias Albrecht}%
\author[2]{David Kleijn}%
\author[3]{Neal Williams}%
\author[4]{Matthias Tschumi}%
\author[5]{Brett Blaauw}%
\author[6]{Riccardo Bommarco}%
\author[7]{Alistair Campbell}%
\author[8]{Matteo Dainese}%
\author[9]{Frank Drummond}%
\author[10]{Martin Entling}%
\author[11]{Dominik Ganser}%
\author[12]{Arjen De Groot}%
\author[13]{David Goulson}%
\author[14]{Heather Grab}%
\author[13]{Hannah Hamilton}%
\author[15]{Felix Herzog}%
\author[16]{Rufus Isaacs}%
\author[4]{Katja Jacot}%
\author[4]{Philippe Jeanneret}%
\author[17]{Mattias Jonsson}%
\author[1]{Eva Knop}%
\author[18]{Claire Kremen}%
\author[16]{Doug Landis}%
\author[19]{Greg Loeb}%
\author[20]{Lorenzo Marini}%
\author[21]{Megan McKerchar}%
\author[22]{Lora Morandin}%
\author[10]{Sonja Pfister}%
\author[23]{Simon Potts}%
\author[24]{Maj Rundlöf}%
\author[25]{Hillary Sardiñas}%
\author[26]{Amber Sciligo}%
\author[27]{Carsten Thies}%
\author[28]{Teja Tscharntke}%
\author[29]{Eric Venturini}%
\author[30]{Eve Veromann}%
\author[31]{Ines Vollhardt}%
\author[32]{Felix Wäckers}%
\author[3]{Kimiora Ward}%
\author[33]{Andrew Wilby}%
\author[16]{Megan Woltz}%
\author[34]{Steve Wratten}%
\author[1]{Louis Sutter}%
\affil[1]{Agroscope}%
\affil[2]{Wageningen University}%
\affil[3]{Harry H. Laidlaw Jr. Honey Bee Research Facility}%
\affil[4]{Agroecology and Environment, Agroscope}%
\affil[5]{University of Georgia}%
\affil[6]{Swedish University of Agricultural Sciences}%
\affil[7]{Embrapa Amazônia Oriental}%
\affil[8]{Accademia Europea}%
\affil[9]{University of Maine}%
\affil[10]{University of Koblenz-Landau}%
\affil[11]{University Bern}%
\affil[12]{Wageningen Universiteit en Researchcentrum Alterra}%
\affil[13]{Stirling University}%
\affil[14]{Cornell University College of Agriculture and Life Sciences}%
\affil[15]{Research Station Agroscope}%
\affil[16]{Michigan State University}%
\affil[17]{Sveriges Lantbruksuniversitet Fakulteten for Naturresurser och Lantbruksvetenskap}%
\affil[18]{University of British Columbia}%
\affil[19]{Cornell University}%
\affil[20]{University of Padova}%
\affil[21]{University of Worcester}%
\affil[22]{Pollinator Partnership}%
\affil[23]{University of Reading }%
\affil[24]{Lund University}%
\affil[25]{University of California}%
\affil[26]{University of California, Berkeley}%
\affil[27]{Georg-August-Universitat Gottingen}%
\affil[28]{University of Goettingen}%
\affil[29]{The Xerces Society for Invertebrate Conservation}%
\affil[30]{Estonian University of Life Sciences}%
\affil[31]{Affiliation not available}%
\affil[32]{Lancaster Environment Centre}%
\affil[33]{University of Lancaster}%
\affil[34]{Lincoln University}%


\vspace{-1em}



  
  \date{March 13, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Floral plantings are promoted to foster ecological intensification of
agriculture through provisioning of ecosystem services. However, a
comprehensive assessment of the effectiveness of different floral
plantings, their characteristics and consequences for crop yield across
global regions is lacking. Here we quantified the impacts of flower
strips and hedgerows on pest control and pollination services in
adjacent crops using a global dataset of 529 sites. Flower strips, but
not hedgerows, enhanced pest control services in adjacent fields by 16\%
on average. However, effects on crop pollination and yield were more
variable. Our synthesis identifies several important drivers of
variability in effectiveness of plantings: pollination services declined
exponentially with distance from plantings, and perennial and older
flower strips with higher flowering plant diversity enhanced pollination
more effectively. These findings provide promising pathways to optimize
floral plantings to more effectively contribute to ecosystem service
delivery and ecological intensification of agriculture in the future.%
\end{abstract}%



\sloppy


\textbf{}

\section*{INTRODUCTION}

{\label{agroecology-agri-environment-schemes-bee-pollinators-conservation-biological-control-ecological-intensification-farmland-biodiversity-floral-enhancements-natural-pest-regulation-pollination-reservoirs-sustainable-agriculture-wildflower-strips-introduction}}

Meeting the increasing demands for agricultural products while
minimizing negative impacts on biodiversity and ecosystem health is
among the greatest global challenges (Godfray \emph{et al.} 2010).
Intensive agricultural production and the simplification of
agroecosystems threaten farmland biodiversity and associated ecosystem
services worldwide (Foley \emph{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 \emph{et
al.}2013; Pywell \emph{et al.} 2015; Kov\selectlanguage{ngerman}ács-Hostyánszki \emph{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 \emph{et al.} 2015; Tschumi \emph{et al.}
2015; Williams \emph{et al.} 2015; Dainese \emph{et al} . 2017; Kremen
\emph{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
\emph{et al.} 2017; Venturini \emph{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 \emph{et al.} 2015; Holland\emph{et al.} 2016; Kremen \emph{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 (Haaland\emph{et al.} 2011; Scheper
\emph{et al.} 2013; M'Gonigle \emph{et al.}2015; Williams \emph{et al.}
2015; Tschumi \emph{et al.} 2016; Sutter\emph{et al.} 2017, 2018; Kremen
\emph{et al.} 2019). It is less well understood whether and at what
spatio-temporal scales the enhanced species diversity translates to
\emph{ex situ} provisioning of pollination, pest control and increased
yield. The `exporter' hypothesis (Morandin \& Kremen 2013; Kremen
\emph{et al.} 2019) predicts a facilitative effect of floral plantings
and enhanced delivery of ecosystem services through functional spillover
(\emph{sensu} Blitzer\emph{et al.} 2012; see also Albrecht \emph{et al.}
2007; Morandin \& Kremen 2013; Pywell et al. 2015; Tschumi \emph{et al.}
2015, 2016; Sutter \emph{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 \emph{et al.} 2017;
Sutter\emph{et al.} 2018). However, according the `concentrator'
hypothesis (Kremen et al. 2019; also referred to as the `aggregation'
hypothesis (Venturini \emph{et al.} 2017a) or the `Circe principle'
(Lander\emph{et 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 \emph{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;
Tscharntke\emph{et al.} 2016; Karp \emph{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 \emph{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
(Ganser\emph{et al.} 2019) or be detectable over larger distances
(Tschumi\emph{et al.} 2015). Such knowledge should be considered when
designing schemes with optimal spatial arrangement of plantings across
agricultural landscapes (Ricketts \emph{et al.} 2008; Garibaldi \emph{et
al.} 2011), and to facilitate cost-benefit assessments (Blaauw \& Isaacs
2014; Morandin \emph{et al.} 2016 Dainese \emph{et al.} 2017;
Haan\emph{et al.} 2020; Williams \emph{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 \emph{et
al.} 2012; Scheper \emph{et al.} 2013; Sutter \emph{et al.} 2017;
M'Gonigle \emph{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 \emph{et al.} 2019; Kremen \emph{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 \emph{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
(Scheper\emph{et al.} 2013) of a local measure great enough to be
effective (intermediate landscape complexity theory; Tscharntke et al.
2005; Kleijn \emph{et al.} 2011). While support for this hypothesis has
been found with respect to biodiversity restoration (e.g., Bátary
\emph{et al.} 2011; Scheper \emph{et al.} 2013, 2015; but see e.g.
Hoffmann\emph{et 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 \emph{et al.} 2015; Grab
\emph{et al.} 2018; Rundlöf \emph{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.

\section*{MATERIALS AND METHODS}

{\label{materials-and-methods}}

\emph{Data collection}

To identify datasets suitable to address our research questions, we
performed a search in the ISI Web of Science and SCOPUS (using the
search string provided in Appendix S1; records published until
31.12.2017 were considered). To minimise potential publication bias
(i.e., the file drawer problem, Rosenthal 1979) and to maximise the
number of relevant datasets we also searched for unpublished data by
contacting potential data holders through researcher networks. Datasets
had to meet the following requirements to be included in the analysis:
(i) pollination and/or pest control services in crops were measured in
both crop fields adjacent to floral plantings and control fields without
planting; (ii) the replication at the field level was \selectlanguage{english}[?] six fields per
study (three fields with plantings and three without; i.e.,
disqualifying small-scaled plot treatment comparisons within fields). We
contacted data holders fulfilling these requirements and requested
primary data on plant species richness of plantings, time since
establishment, landscape context and crop yield (see below) in addition
to measured pollination and pest control services. Overall, we analysed
data from 35 studies. We here define a study as a dataset collected by
the same group of researchers for a particular crop species and
ecosystem service (pest control or pollination) in a particular region
during one or several sampling years. We collected 18 pest control
service and 17 pollination service studies, representing a total of 868
service-site-year combinations across 529 sites (fields with or without
adjacent floral planting; see Supporting Table S1 for detailed
information about studies). In eight of these studies (122 sites) both
crop pollination and pest control services were measured (Table S1).

\emph{Pollination services, pest control services and crop yield}

As different studies used different methods and measures to quantify
pollination services, pest control services and crop yield, we
standardised data prior to statistical analysis using \emph{z} -scores
(e.g., Garibaldi \emph{et al.} 2013; Dainese \emph{et al.} 2019). The
use of \emph{z} -scores has clear advantages compared with other
transformations or standardization approaches (such as the division by
the absolute value of the maximum observed level of the measured
response) because \emph{i} ) average \emph{z} -scores follow a normal
distribution, and \emph{ii} ) the variability present in the raw data is
not constrained as in other indices that are bound between 0 and 1
(Garibaldi \emph{et al.} 2013). Pollination services were measured as
seed set (number of seeds per fruit), fruit set (proportion of flowers
setting fruit), pollen deposition rate (number of pollen grains
deposited on stigmas within a certain time period) and, in one study,
flower visitation rate (number of visits per flower within a certain
time period). If available, differences in pollination service measures
of open-pollinated flowers and flowers from which pollinators were
excluded were analysed. Measures of pest control services were
quantified as pest parasitism (proportion of parasitized pests), pest
predation (proportion of predated pests), population growth (see below)
or crop damage by pests or pest densities (see Supporting Table S2 for
an overview of pollination and pest control service measures across
studies). Whenever possible, the pest control index based on population
growth proposed by Gardiner \emph{et al.} (2009) was calculated and
analysed (Supporting Table S2). Note that standardized values of pest
density and crop damage were multiplied by -1 because lower values of
these measures reflect an increased pest control service (e.g.,
Karp\emph{et al} . 2018). Crop yield was only considered for the
analysis if a direct measure of final crop yield was available. Too few
studies assessed crop quality which was therefore not considered
further. Yield was measured as crop mass or number of fruits produced
per unit area. Due to a lack of studies measuring crop yield in fields
with and without adjacent hedgerows, the analysis of crop yield focused
on effects of flower strips. Crop yield measures were available from a
total of 12 flower strip studies and 194 fields (see Supporting Tables
S1 and S2 for a detailed description of study systems, crop yield
measures and methods used across studies).

\emph{Descriptors of floral plantings and landscape context}

Flower strips are here defined as strips or other areas of planted wild
native and/or non-native flowering herbaceous plants. Hedgerows are
defined as areas of linear shape planted with native and/or non-native
at least partly flowering woody plants and typically also herbaceous
flowering plants. For hedgerows, information about the exact time since
establishment and number of plant species was not available for most
studies. The analyses of these drivers (question 3) therefore focus on
flower strip effects on pollination and pest control service
provisioning. Information on plant species richness was available in 12
out of 18 pest control studies and 10 out of 17 pollination studies.
Whenever available, the species richness of flowering plants was used.
Otherwise, for some flower strip studies, the number of sown,
potentially flowering plant species (excluding grasses) was used. Time
since establishment of flower strips, i.e., the time span between
seeding or planting and data sampling, was available for all studies
ranging from 3 to 122 months.

The proportional cover of arable crops was available and analysed as a
proxy for landscape simplification (e.g., Tscharntke \emph{et al.} 2005;
Dainese \emph{et al.} 2019) in 11 pest control and 12 pollination
studies. Proportional cover of arable crops was calculated in circular
sectors of 1 km radius around focal crops, or 750 m or 500 m radius (two
studies for which data on a 1 km radius were not available; see Table
S1; results remained qualitatively identical when only considering the 1
km radius datasets).

\emph{Statistical analysis}

We used a mixed effect-modelling approach to address our research
questions. In all models, study was included as a random intercept to
account for the hierarchical structure of the data with field measures
nested within study. To assess whether flower strips and hedgerows
enhanced pollination and pest control services in adjacent crops
(research question 1) linear mixed-effect models with planting (field
with or without planting) were separately fitted for flower strips and
hedgerows for the response variables pollination service and pest
control service. To test how the effects on service provisioning change
with distance (continuous variable; meters) from plantings (question 2)
and with landscape simplification (question 4) these explanatory
variables and their interactions with the fixed effects described above
were included in the models. Exploratory analyses showed that neither
distance nor landscape simplification effects differed between flower
strips and hedgerows; i.e., no significant interactive effects of
planting type with any of the tested fixed effects. We therefore pooled
flower strip and hedgerow data in the final models, excluding planting
type and its two or three-way interactions as fixed effects. In addition
to linear relationships we tested for an exponential decline of measured
response variables from the border of the field by fitting
log10(distance) in the linear mixed-effect models described above. In
this case, field nested within study was included as a random effect. To
test the intermediate landscape complexity hypothesis, we tested for
linear as well as hump-shaped relationships between landscape context,
and its interaction with local floral plantings by fitting landscape
variables as a quadratic fixed predictor in the models described above
(second degree polynomial functions). To present the ranges covered by
the agricultural landscape gradients, we did not standardize measures of
landscape simplification within studies (e.g., Martin \emph{et
al.}2019). To examine how pollination and pest control service
provisioning relates to flower strip plant diversity and time since
establishment (question 3) plant species richness and log10(number of
months since establishment) were included as fixed effects in models
with study as a random effect. Using log(months since establishment)
predicted the data better than establishment time as linear predictor.
Plant species richness and time since establishment of flower strips
were not correlated (r = 0.22). Only 10 studies measured services in
several years since establishment (Table S1), and we included only data
from the last sampling year. To assess how the presence of plantings
affected the agronomic yield of adjacent crops (question 5), we fitted a
linear mixed-effect model with the same fixed and random structure as
described for question 1, but with crop yield as the response variable.
Statistical analyses for different models and response variables
differed in sample sizes as not all studies measured crop yield in
addition to pollination or pest control services (Tables 1, S1). In all
models we initially included planting area as a co-variate in an
explorative analysis, but removed it in the final models, as it did not
explain variation in any of the models and did not improve model fit
(not shown).

Effect sizes provided in the text and figures are model estimates of
z-transformed response variables. For statistical inference of fixed
effects we used log-likelihood ratio tests (LRT) recommended for testing
significant effects of \emph{a priori} selected parameters relevant to
the hypotheses (Bolker \emph{et al.} 2009). For all models, assumptions
were checked according to the graphical validation procedures
recommended by Zuur \emph{et al.} (2009). All statistical analyses were
performed in \emph{R} version 3.5.2 (R Core Team 2017) using the\emph{R}
-package \emph{lme4} (Bates \emph{et al.} 2015).

\section*{RESULTS}

{\label{results}}

\section*{\texorpdfstring{\emph{Effects of floral plantings on pest
control and pollination
services}}{Effects of floral plantings on pest control and pollination services}}

{\label{effects-of-floral-plantings-on-pest-control-and-pollination-services}}

The provisioning of pest control services in crop fields adjacent to
flower strips was enhanced by 16\% on average compared to fields without
flower strips. On average, pest control services were also increased in
crops adjacent to hedgerows, but effects were more variable and overall
not statistically significant (Fig. 1; Table 1). Pest control services
declined exponentially with distance from the field edge, but the slopes
of the distance functions between fields with and without adjacent
floral plantings did not differ (Fig. 2a; Table 1).

Crop pollination effects were more variable across studies and overall
not significantly different between crops with or without adjacent
floral planting across all studies and within-field distances (Fig 1;
Table 1). However, effects of distance to field edge differed for fields
with floral plantings compared with control fields (significant
interaction between presence of planting and distance from field border;
Table 1). Pollination services were increased near floral plantings and
decreased exponentially with increasing distance from plantings, while
no such effect of distance to field edge was detected for control fields
(Fig . 2b). The fitted distance curves for fields with or without floral
plantings intersected at 43 m (Fig. 2b).



\emph{The role of flowering plant diversity and time since establishment
of flower strips}

Crop pollination services, but not pest control services, tended to
increase with flowering plant species richness of the adjacent flower
strip (52\% predicted increase in crop pollination from 1 to 25 plant
species in adjacent flower strip; Fig. 3a; Table 1). Crop pollination
services also tended to increase with time since establishment of the
adjacent flower strip, but showed a positive saturating relationship
(Fig. 3b; Table 1). Pollination services increased by 27\% in two year
old strips compared with the youngest plantings (roughly 3 months old),
while the additional predicted increase from two to four years or older
strips was approximately 5\% on average (Fig. 3b; only few strips were
older than four years, see Fig. 3b and explanations in figure caption).
Pest control services in crops adjacent to flower strips did not
increase with flower strip age (Table 1).

\emph{Effects of landscape simplification}

The model testing for a linear relationship between service provision
and landscape simplification and its interaction with local flower
presence fitted the data better than a model testing for hump-shaped
relationships (Table S3). Pollination, but not pest control services,
decreased linearly with landscape simplification (12\% decrease from 50
to 100\% crops in the surrounding landscape), irrespective of the
presence of a floral planting (no significant floral planting x
landscape simplification interaction; Fig 4; Table 1).

\emph{Effects of flower strips on crop yield}

Overall, no significant effect of flower strips on yield in adjacent
crops was detected (subset of 12 studies and 194 sites for which crop
yield data was available; Fig. 5; Supplementary Table S4). Furthermore,
no effects of within-field distance, plant species richness, time since
establishment or landscape simplification, or their interactions with
flower strip presence on yield, were detected (Table S4).

\section*{DISCUSSION}

{\label{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 \emph{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
(Kleijn\emph{et al.} 2019) with many studies showing positive effects on
service provisioning (e.g., Blaauw \& Isaacs 2014; Tschumi \emph{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 \emph{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 \emph{et al.} 2018; Kremen \emph{et
al.}2018). They may also be explained by greater provision of nesting
and overwintering opportunities in older floral plantings (Kremen
\emph{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 \emph{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 \emph{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 \emph{et al.} 2013; Kremen \& M'Gonigle 2015;
Sutter \emph{et al.} 2017; Kremen \emph{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 \emph{et al.} 2015; M'Gonigle \emph{et al.} 2017;
Lundin \emph{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;
Sardinas \emph{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 \emph{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 \emph{et al.} 2018; Dainese \emph{et al.}2019;
but see Veres \emph{et al.} 2013; Rusch \emph{et al.} 2016;
Martin\emph{et al.} 2019). The effect of adding a flower strip or
hedgerow was, however, independent of landscape context. Although
individual case studies (Jonsson \emph{et al.} 2015; Grab \emph{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 \emph{et
al.}2013; Gagic \emph{et al.} 2017), which can potentially mask benefits
from improved natural pest regulation or pollination services
(Sutter\emph{et al.} 2018). Positive effects of floral plantings have
been shown by some case studies included in this synthesis (e.g.,
Tschumi\emph{et al.} 2016; see also Pywell \emph{et al.} 2015), although
sometimes only several years after the establishment of plantings
(Blaauw \& Isaacs 2014; Morandin \emph{et al.} 2016; Venturini \emph{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 \emph{et al.} 2016), temporal dynamics
(Blaauw \& Isaacs 2014; M'Gonigle \emph{et al.} 2015), optimized ratios
of floral planting (contributing to ecosystem service supply) to crop
area (affecting service demand; Kremen \emph{et al.} 2019;
Williams\emph{et 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 \emph{et al.} 2015). Moreover, floral
plantings contribute to biodiversity conservation (e.g. Haaland \emph{et
al} . 2011; Scheper \emph{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.

\emph{Conclusions and implications}

Our synthesis demonstrates enhanced natural pest control services to
crops adjacent flower strips plantings, across a broad suite of regions,
cropping systems and types of flower strips studied. However, it also
reveals inconsistent and highly variable effects of flower strips and
hedgerows on crop pollination services and yield. This highlights a
strong need to identify the key factors driving this variability and the
effectiveness of different types of floral plantings in contributing to
ecosystem service delivery. Informed by such improved understanding, the
design, implementation and management of floral plantings can increase
their effectiveness as measures for ecological intensification. This
synthesis identifies several promising pathways towards more effective
floral plantings for the provision of ecosystem services and ecological
intensification: the modelled exponential distance-decay function of
pollination service provisioning by floral plantings into crop field
helps to predict service provision in crop fields; together with the
lack of a strong planting area effect, our findings suggest that a dense
spatial network of relatively small plantings will be more effective
than a few large ones to optimize pollination service provisioning.
Moreover, it identifies important drivers of the effectiveness related
to type and composition of floral plantings for delivery of crop
pollination services: flowering plant diversity and age. Based on these
findings we strongly encourage the promotion of perennial floral
plantings that ensure the availability of high floral diversity across
several years as promising pathways towards optimized measures for
ecological intensification.

\textbf{ACKNOWLEDGEMENTS}

We thank all farmers, field and technical assistants, researchers and
funders who contributed to the studies made available for this
synthesis. We thank Matthias Suter, Lukas Pfiffner, Henryk Luka, Mario
Balzan, Michael Garratt and Emily Martin for statistical advice and
inspiring and valuable discussions. Furthermore, we are grateful for
funding by the EU COST-Action FA1307 ``SUPER-B'' that enable fruitful
discussions with members of the SUPER-B network during the conceptual
and analysis phase of this work. AJC was funded by the Biotechnology and
Biological Sciences Research Council (BBSRC) and Syngenta UK as part of
a case award PhD (grant no. 1518739). DAL was supported by the Great
Lakes Bioenergy Research Center, U.S. Department of Energy, Office of
Science, Office of Biological and Environmental Research (Award
DE-SC0018409), by the National Science Foundation Long-term Ecological
Research Program (DEB 1832042) at the Kellogg Biological Station, and by
Michigan State University AgBioResearch. RI and BB were funded by the
USDA Sustainable Agriculture Research and Education program and by
AgBioResearch. EV acknowledges funding from Estonian Research Council
Institutional Research Funding project IUT36-2 and the QuESSA project
funded by the European Union (FB7, grant agreement no. 311879. EV and FD
were funded by The United States Department of Agriculture -- National
Institute of Food and Agriculture Specialty Crops Research Initiative
Grant 2011-51181-30673, and by the University of Maine School of Biology
and Ecology. MT was funded by the Hauser and Sur-La-Croix foundations.
SCP was supported through the QuESSA project funded by the European
Union (FB7, grant agreement no. 311879). SGP was supported through the
Insect Pollinators Initiative UK Crop pollination project funded by
BBSRC, Defra, NERC, the Scottish Government and the Wellcome Trust
(BB/1000348/1), and SMOOPS project funded by BBSRC, Worldwide Fruit
Limited, Avalon and Syngenta (BB/P003664/1). MJ acknowledges funding
from the Tertiary Education Commission through the Bio-Protection
Research Centre at Lincoln University, New Zealand and Centre for
Biological Control at Swedish University of Agricultural Sciences.

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\textbf{SUPPORTING INFORMATION}

Additional Supporting Information may be downloaded via the online
version of this article at Wiley Online Library
(www.ecologyletters.com).

As a service to our authors and readers, this journal provides
supporting information supplied by the authors. Such materials are
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authors.

\textbf{TABLES AND TABLE LEGENDS}

\textbf{Table 1.} Summary of results of linear and generalized linear
mixed-effects models testing the effects of presence and type of floral
plantings (flower strips and hedgerows) on crop pollination and natural
pest control services, and how effects are influenced by in-field
distance, local planting characteristics and landscape context. Response
variables, explanatory variables, estimates, numerator degrees of
freedom and denominator degrees of freedom (Df), differences in
log-likelihood for chi-squared tests (LRT) and \emph{P} values
(\emph{P}\textless{} 0.05 in bold; P \selectlanguage{english}[?] 0.05 \textless{} 0.10 in bold
italic) are shown for each model. Note that effects of local drivers
(i.e., flowering plant species richness and time since establishment)
considered only crops adjacent to flower strips.

\textbf{Hosted file}

\verb`image1.emf`  available at \url{https://authorea.com/users/309032/articles/440047-global-synthesis-of-the-effectiveness-of-flower-strips-and-hedgerows-on-pest-control-pollination-services-and-crop-yield}

\textbf{FIGURES AND FIGURE LEGENDS}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 1.} Forest plot showing effects of flower strips and
hedgerows on pollination and pest control service provisioning in
adjacent crops compared to control crops without adjacent floral
plantings. Squares illustrate predicted mean effects (z-score
estimates), bars show 95\% confidence intervals (CIs). On average, pest
control services were enhanced by 16\% (z-score: 0.25) in fields with
adjacent flower strip compared to control fields.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 2.} Predicted relationships between \textbf{(a)} mean
natural pest control service and \textbf{(b)} mean crop pollination
service (z-scores (solid lines) \selectlanguage{ngerman}± 95\% CI (dashed lines)) and in-field
distance to field border for field with (red lines; dots) or without
adjacent floral planting (black lines, triangles).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 3.} Predicted relationships between mean crop pollination
service (z-scores (fat solid lines) \selectlanguage{ngerman}± 95\% CI (fine solid lines))
and\textbf{(a)} flowering plant species richness and \textbf{(b)} time
since establishment of adjacent flower strips. Predicted relationship
and results of an analysis without the points representing flower strips
older than four years were qualitatively identical.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 4.} Predicted relationship between mean (a) pest control
and (b) crop pollination service (z-scores (solid lines) \selectlanguage{ngerman}± 95\% CI
(dashed lines)) and landscape simplification (percentage of arable crops
in the landscape) in fields with adjacent floral planting (red line; red
circles) or without planting (black line; black triangles). Pollination
services, but not pest control services, declined with landscape
simplification; the slight differences in slopes for
pollination-landscape simplification relationships of fields with or
without adjacent plantings were statistically not significant.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 5} Mean predicted crop yield (z-scores; \selectlanguage{ngerman}± 95\% CI) of
fields with adjacent flower strips (red circles) and control fields
without adjacent flower strip (black triangles). The dataset includes a
subset of 12 studies and 194 sites.

\textbf{Hosted file}

\verb`ELEalbrechtST1.docx`  available at \url{https://authorea.com/users/309032/articles/440047-global-synthesis-of-the-effectiveness-of-flower-strips-and-hedgerows-on-pest-control-pollination-services-and-crop-yield}

\textbf{Hosted file}

\verb`ELEalbrechtST2.docx`  available at \url{https://authorea.com/users/309032/articles/440047-global-synthesis-of-the-effectiveness-of-flower-strips-and-hedgerows-on-pest-control-pollination-services-and-crop-yield}

\textbf{Hosted file}

\verb`ELEalbrechtST3.docx`  available at \url{https://authorea.com/users/309032/articles/440047-global-synthesis-of-the-effectiveness-of-flower-strips-and-hedgerows-on-pest-control-pollination-services-and-crop-yield}

\textbf{Hosted file}

\verb`ELEalbrechtST4.docx`  available at \url{https://authorea.com/users/309032/articles/440047-global-synthesis-of-the-effectiveness-of-flower-strips-and-hedgerows-on-pest-control-pollination-services-and-crop-yield}

\selectlanguage{english}
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