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.