POLLINATORS POLLINATION AND FOOD PRODUCTION

THE ASSESSMENT REPORT ON POLLINATORS, POLLINATION AND FOOD PRODUCTION
186
3. THE STATUS AND TRENDS IN POLLINATORS
AND POLLINATION
temperate regions of the world, particularly of the Northern
Hemisphere. In Sweden, a drastic decrease in bumble
bee diversity has occurred during the last century and
present assemblages are dominated by two short-tongued
species, Bombus terrestris and B. lapidarius (see section
3.2.3). Associated with this shift in bumble-bee community
composition, Bommarco et al. (2012) found declines in
average seed yield of red clover (Trifolium pratense) in recent
years and a long-term trend of yield that was twice as
variable after 90 years. Also, cultivation of some traditional
crops, like mustard (Brassica rapa), has been decreasing in
northern India and Nepal because of declining yields. In this
case, local farmers blame high levels of pesticide application
to crops, and their impact on bees, as the main cause of
reduced yields (Vaissière et al., 2011).
Based on this evidence, large-scale agriculture expansion
should be expected to be associated with decreasing crop
productivity. An analysis of the 54 major crops cultivated
in France over the past two decades found that potential
benefits of agricultural intensification were offset by
increasing pollination deficits, as the mean and predictability
of the yield of pollinator-dependent crops decreased with
increasing intensification (Deguines et al., 2014). However,
at the global scale Ghazoul and Koh (2010) did not find
any consistent relationship between changes in yield in
pollinator-dependent crops and agricultural intensification.
Also, the long-term FAO dataset provides no evidence
so far of a decrease or deceleration in the growth in the
average yield of pollinator-dependent crops compared to
non-dependent crops during the last five decades (Figure
3.13; Aizen et al., 2008). A more detailed analysis confirmed
FIGURE 3.13
no deceleration of yield growth with increasing pollinator
dependency (Garibaldi et al., 2011b), although introduction
of managed pollinators might be masking to some extent
any effect of pollinator decline on crop yield (Potts et al.,
2010). However, the absence of evidence of decreasing or
decelerating agriculture yield over time among pollinationdependent
crops worldwide (Aizen et al., 2008) must not be
taken as proof of an absence of risk of global agriculture to
an on-going or future large-scale pollinator decline.
Further analyses of the FAO dataset revealed that increasing
pollinator dependency is associated with lower and more
variable rates of yield growth (Garibaldi et al., 2011b). These
findings are consistent with the hypothesis that animal
pollination limits the productivity of many crops worldwide,
and stress the vulnerability in the productivity of many crops
to pollination disruption.
Although introduced pollinators can substitute partially
for the pollination provided by declining wild pollinators,
as potentially invasive organisms they can become
superabundant, overexploiting both wild and crop flowers,
and thus reducing reproductive output and agricultural
yield. For instance, the extremely high densities reached
by European B. terrestris in the Patagonia region of South
America (Morales et al., 2013) can be associated with a
marked decrease in raspberry fruit quality via increasing
style damage (Sáez et al., 2014). Also, because the honey
bee, Apis mellifera, harvests large amounts of pollen, the
balance between pollination and pollen theft could shift
with its abundance (Hargreaves et al., 2009). However,
benefits might still be higher than the costs, as in the case
Temporal (1961-2006) trends in mean crop yield. The depicted means (+ or – 1 standard error) in relative yield were estimated from
the subset of 10 pollinator-dependent and 10 non-dependent crops widely cultivated in both the Developed and Developing World
(modified from Aizen et al., 2008).
Percentage change in mean crop yield
Developed world
180
160
140
120
100
80
60
40
20
0
-20
1960 1970 1980 1990 2000 2010
Developing world
180
160
140
120
100
80
60
40
20
0
-20
1960 1970 1980 1990 2000 2010
1 Standard Error
Mean
Dependent
Nondependent