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 z -scores
(e.g., Garibaldi et al. 2013; Dainese et al. 2019). The
use of 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 i ) average z -scores follow a normal
distribution, and ii ) the variability present in the raw data is
not constrained as in other indices that are bound between 0 and 1
(Garibaldi 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 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., Karpet 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).