POLLINATORS POLLINATION AND FOOD PRODUCTION

THE ASSESSMENT REPORT ON POLLINATORS, POLLINATION AND FOOD PRODUCTION
3.8 TRENDS IN CROP
POLLINATION AND YIELD
3.8.1 Outline of section
probably the largest pollination industry. In this nation, more
than two million honey-bee colonies are rented and even
moved across the country to pollinate crops (Morse and
Calderone, 2000). In fact, approximately 1.7 million hives
are transported to California for almond pollination during a
six-week period (Sumner and Boriss, 2006).
Deficits in pollination quantity and/or quality often limit crop
yield. This section reviews and discusses the relevance
of pollinator diversity to narrow pollination deficits, and
whether crop pollination deficits have increased along spatial
disturbance gradients and over time. It also briefly discusses
the impact on agriculture of bees that were introduced
outside their native range and have become invasive.
3.8.2 Crop pollination deficits
In pollination, pollen can be insufficient in quantity, e.g.
stigmas receive too few pollen grains, or quality, e.g. stigmas
receive pollen with low vigour due to genetic (i.e., self or selfincompatible
pollen) or environmental factors (e.g., pollen
produced by water-stressed or defoliated plants). Both may
restrict wild plant reproduction and crop yield (Knight et al.,
2005; Chapter 1). Technically, a crop pollination deficit refers
to quantitative or qualitative inadequate pollen receipt that
limits agricultural output (Vaissière et al., 2011). Even though
pollination commonly limits seed production, decreases
in pollinator diversity and visitation by effective pollinators
may exacerbate chronic pollination deficits experienced by
many crops.
A recent worldwide meta-analysis including data for
41 crops grown in 600 cultivated fields distributed across
all continents, except Antarctica, reveals that diverse
assemblages of wild bees seem to be important to reduce
pollination deficits and sustain high yields of many pollinatordependent
crops (Garibaldi et al., 2013). Specifically, this
study found that flower visitation by wild bees increases
crop fruit and seed set, on average, twice as much as
visitation by the domesticated honey bee, Apis mellifera, on
a per-visit basis. Furthermore, declining pollination provided
by wild bees might not be substituted by stocking fields with
more honey bee hives, although honey bees can add to the
pollination provided by wild bees (Garibaldi et al., 2013).
Whereas complementary pollinating activity between wild
bees and honey bees can explain this overall additive effect,
diverse pollinator assemblages ensure the inclusion of one
or more species of efficient pollinators (see also Chapter 1).
For instance, yield of marketable French bean production in
the Mt Kenya region was found to be positively correlated
with the abundance of carpenter bees (Xylocopa spp.),
despite high abundance of honey bees (Masiga et al., 2014).
On the other hand, the risks of relying on a single pollinator
species for large-scale crop pollination are exemplified
by almond (Prunus dulcis) in the US, the country with
Besides questioning the efficiency of honey bees in
pollinating almond flowers compared to wild pollinators
(Klein et al., 2012), the continuous drop of the stock of
honey-bee hives in the US during the last decades (National
Research Council, 2007) questions the rationality and
sustainability of such a practice. Furthermore, at a global
scale the growth of the stock of domesticated honey-bees
hives have proceeded at a much lower rate than demands
for pollination (Aizen and Harder, 2009a), stressing the
importance of wild pollinators for the productivity of many
pollinator-dependent crops (Breeze et al., 2011). Similarly,
in some regions of several Asian countries people have
resorted to hand pollination of apple following declines in
native apple pollinators and unavailability of managed honey
bees to perform this function (Partap and Partap, 2007).
A recent global analysis (Kleijn et al., 2015), which
includes data from 20 pollinator-dependent crops in
about 1400 crop fields, proposes that the contribution
of wild bees to crop production is limited to a subset
of bee species that are common in agroecosystems. It
seems likely that (i) crop pollination deficits are common
and (ii) enhanced and sustained yields of many crops can
be better ensured by both promoting specific pollinator
species and the maintenance and restoration of diverse
pollinator communities.
3.8.3 Spatial and temporal trends
in pollination deficits
Remnants of natural and semi-natural habitats, hedgerows,
and field margins, which supply essential flowering and
nesting resources, can become important pollinator sources
in different agro-ecosystems (Winfree et al., 2009; Morandin
and Kremen, 2013; see Chapter 2). Therefore, increasing
distance from field edges into crop fields greatly reduces
flower visitation and the number of visiting species (Ricketts
et al., 2008; Garibaldi et al., 2011a). On average, bee
visitation rates and richness are reduced by half at distances
about 670 and 1500m, respectively, from natural vegetation
(Ricketts et al., 2008). As a consequence, not only does
average crop yield often decrease with distance to field
margins or natural vegetation (albeit at lower rates than
pollinator abundance and richness), but it also becomes
less predictable (Garibaldi et al., 2011a). A long-term
survey conducted in Scandinavia also revealed trends in
the composition and diversity of bumble bee communities
and crop yield. Bumble bees are important pollinators in
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3. THE STATUS AND TRENDS IN POLLINATORS
AND POLLINATION