POLLINATORS POLLINATION AND FOOD PRODUCTION

THE ASSESSMENT REPORT ON POLLINATORS, POLLINATION AND FOOD PRODUCTION
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2. DRIVERS OF CHANGE OF POLLINATORS,
POLLINATION NETWORKS AND POLLINATION
survivorship (Boggs and Freeman, 2005), and pollinators
will likely be forced to seek alternative habitat. Skórka et al.
(2013) found that butterfly roadkill in Poland increased as
mowing frequency increased; adult butterflies that dispersed
to find new habitat after roadsides were mowed were more
likely to collide with vehicles.
The frequency and timing of mowing influence the
composition of vegetation over time (Forrester et al., 2005),
thus indirectly influencing pollinator diversity and abundance.
Frequent mowing during a growing season reduces native
plant growth and the ability of forbs to compete with
grasses. Excessive roadside mowing may have led to a
decrease in flowers and a subsequent decrease in bumble
bees in Belgium (Rasmont et al., 2006). Intensively-mowed
roadsides generally have the shortest vegetation and lowest
amount of nectar, which together result in decreased
butterfly abundance (Gerell 1997; Saarinen et al., 2005).
However, carefully timed roadside mowing can have positive
effects on plant diversity (Parr and Way, 1988) that in turn
benefit pollinators (e.g., Noordijk et al., 2009).
Mowing technique can have a great influence on the effects
on pollinators. Frick and Flury (2001) estimated losses
from rotary mowers as between 9,000 and 24,000 bees
per hectare in flowering white clover fields and 90,000 per
hectare in flowering Phacelia. Mowing without a conditioner,
which processes hay so it dries more quickly, reduced the
mortality by a factor of seven. In order to avoid significant
bee losses, the researchers recommend refraining from
mowing in periods of increased flight activity. Humbert et
al. (2010) analysed the direct impact on invertebrates of
different hay harvesting processes. The use of a conditioner
reduced the survival rate of orthopterans from 32% to
18%. Leaving uncut refuges and delaying mowing mitigate
the impact on pollinators (Buri et al., 2012; Humbert et al.,
2012). Although there is no evidence about the effect of
mowing mortality on local pollinator population dynamics
and its impact on pollination, studies suggest mowing can
have a negative impact.
2.2.2.2.2 Logging
Tree removal leads to alteration in the albedo (fraction of
solar energy reflected back from earth), light regime, soil
dynamics, hydrology, soil chemistry and plant composition
(Foley et al., 2005), with profound effects on ecosystem
structure. It is therefore expected that pollinators will also
be affected by logging. Studies on logging indicate that the
pollinator group and the biome play a role in the response
of pollinators to logging disturbances. In tropical forests,
forest fragmentation associated with logging leads to a
rapid reduction in butterfly diversity and abundance (Daily
and Ehrlich, 1995). In contrast, while selective logging
negatively affects stingless bees (Eltz et al., 2002; Samejima
et al., 2004), it can maintain the presence of some butterfly
groups, at least if logging is associated with maintenance
of land cover heterogeneity within the logged patch
(Hamer et al., 2003; Lewis 2001). In the Western Amazon
pollen deposition rate of some hardwood tree species
was reduced, others were increased, while some species
were unaffected at logged sites compared to non-logged
forest (Maues et al., 2007). Moth diversity and abundance
increased with levels of disturbance in montane rainforests
(Axmacher et al., 2004), a result that agrees with works on
several types of insect pollinators in temperate and boreal
forests (Jackson et al., 2014; Pengelly and Cartar 2010;
Romey et al., 2007). In the boreal forest of Canada there
were generally more bumble bees, species of bumble
bee-visited plants, and flowers in moderately (50-75%
of trees remaining) logged sites, but logging affected the
distribution of bumble bees across floral resources, with too
many bumble bees in the flower-poor compartments and
too few in the flower-rich ones than merited based on the
quantity of flower resources (Cartar, 2005). Controlling for
flower density, bumble bee density was significantly greater
in clearcuts than in the highly (10-20% of remaining trees)
or moderately logged (50-75% of trees remaining) plots.
By disproportionately visiting plants in clearcuts (relative to
flower density) bumble bees in clearcuts should experience
higher levels of competition. Forests experiencing different
levels of disturbance were also shown to harbour different
plant and insect species, thus plant-pollinator networks also
show different characteristics (Nielsen and Totland, 2014).
2.2.2.2.3 Fire
Fire is often used as a management tool for agricultural
conversion and prescribed burning is used as a forest
management strategy to suppress fires and improve land
cover types in many regions of the world. These burnings
have been shown to benefit the diversity of Lepidoptera
in the Western US coniferous forests (Huntzinger, 2003),
species richness of Hymenoptera and Lepidoptera in forest
from the Southern Alps (Moretti et al., 2004), and species
richness in central European forests (Bogusch et al., 2014).
Fires in Mediterranean oak-pine forests lead to an initial
strong reduction of bee diversity in recently burnt areas, with
a recovery in the following years, which has been shown to
be highly correlated to floral diversity (Potts et al., 2003).
Fire considerably changes vegetation and land cover
conditions, and therefore can have an important effect on
pollinators and plant pollination, which may be detrimental
(e.g., Ne’eman et al., 2000; Panzer, 2002). Burns during
the growing season remove floral resources, host plants,
and nesting materials, and can be detrimental to species
with life stages that cannot fly to safety at the time of the
burn (Hopwood et al., 2015). Burns during the dormant
season can kill overwintering pollinators such as butterflies,
moths, syrphid flies, and soldier beetles that overwinter at
the base of plants, in leaf litter, or underneath the surface of