POLLINATORS POLLINATION AND FOOD PRODUCTION

THE ASSESSMENT REPORT ON POLLINATORS, POLLINATION AND FOOD PRODUCTION
to temperature between plants and their pollinators can
also affect butterflies; flowering time of butterfly nectar food
plants is more sensitive to temperature than the timing of
butterfly adult flight (Kharouba and Vellend, 2015).
Bedford et al. (2012) focused on evidence for geographical
range shifts among butterflies in Canada. They collected
data for 81 species and measured their latitudinal
displacement between 1960 and 1975 (a period prior to
contemporary climate change) and from 1990 – 2005
(a period of large climate change). They identified an
unexpected trend, given the mobility of butterflies, for
species’ northern borders to shift progressively less relative
to increasing minimum winter temperatures, suggesting
that even these mobile pollinators have been unable to
extend their ranges as quickly as would be required to keep
pace with climate change; this might be because of their
dependence on larval host plants, which may not be shifting
quickly either (Bedford et al., 2012).
grounds after flowering begins, which could reduce their
nesting success (McKinney et al., 2012).
3.2.3 Shifts in pollinator
abundance
All animal populations fluctuate in abundance and pollinator
populations are no exception. That said, there is evidence
that some pollinator populations are now changing in
abundance to such a degree that they have exceeded
the range of variation previously recorded (Cameron et al.,
2011); a few have suffered local or even global extinctions
(Cox and Elmqvist, 2000; Maes and Van Dyck, 2001;
Grixti et al., 2008; Mortensen et al., 2008; Ollerton et al.,
2014). Although there is some evidence for changes (see
references cited in previous section), this is a topic for which
much additional work is needed before we have a clear
picture for trends on a global scale.
A similar study of 48 butterfly species in Finland found that
they shifted their range margins northward on average by
59.9km between 1992-1996 and 2000-2004, with nonthreatened
species showing a larger change than the more
stationary threatened species (Pöyry, 2009). Such poleward
shifts (Parmesan, 1999) are probably a common feature
of many pollinator species geographical distributions in
recent years (although not much is known about southern
hemisphere species), and are likely being matched by
altitudinal shifts as well, as seen for both butterflies and
bumble bees (Forister et al., 2011; Pyke et al., 2012; Wilson
et al., 2007). However, Kerr et al. (2015) found in a survey of
historical data for bumble bee distributions in both Europe
and North America that there were consistent trends in
failures to track warming through time at species’ northern
range limits, range losses from southern range limits, and
shifts to higher elevations among southern species. So this
important group of pollinators is being affected negatively
by this response to climate change. Responses to climate
change are also compounded by changes in habitat. For
example, Warren et al. (2001) found that 75% of 46 butterfly
species expected to be expanding their range north are
declining in abundance, and attributed this to negative
responses to habitat loss that have outweighed positive
responses to climate warming. Adverse effects of nitrogen
deposition on butterfly host plants may also be taking a toll
on that group of pollinators (Feest et al., 2014).
The changing climate may also pose challenges for avian
pollinators. One study of the potential changes in distribution
that will result considered South Africa, where some
of the migratory pollinator species may be at particular
risk (Simmons et al., 2004; Huntley and Barnard, 2012),
and a study of hummingbird migration in North America
found that if phenological shifts continue at current rates,
hummingbirds will eventually arrive at northern breeding
Insect populations are notoriously variable in abundance
(Andrewartha, 1954), and with few exceptions we do not
fully understand the underlying causes for this variation in
insect pollinator populations. Despite our ignorance of the
exact causes of variation in most pollinator populations,
we do know that diseases (Colla et al., 2006; Koch and
Strange, 2012; Fürst et al., 2014; Manley, 2015), parasites
(Antonovics and Edwards, 2011; Arbetman et al., 2013),
pesticides (Gill et al., 2012; Stokstad, 2013; Johansen,
1977; Canada, 1981), a lack of diverse food sources (Alaux
et al., 2010), and habitat loss (not always separated from
fragmentation; Hadley and Betts, 2012) (Schüepp et al.,
2014), which reduces both nest sites and floral resources
(Kearns et al., 1998), can all potentially affect pollinators
negatively, including species of particular concern for
crop pollination (Stephen, 1955). (See Chapter 2 for
additional information.)
Bumble bees (Hymenoptera)
Very few studies assess shifts in pollinator abundance,
mainly because historic population counts are not available.
A remarkable exception is that of clover pollination by
bumble bees in Scandinavian countries (Bommarco et al.,
2012; Dupont et al., 2011). Drastic decreases in bumble
bee community evenness (relative abundance of species),
with potential consequences for the level and stability of
red clover (Trifolium pratense) seed yield, were observed
in Swedish clover fields over the last 90 years (Bommarco
et al., 2012; Figure 3.2). Two short-tongued bumble bees
(Bombus terrestris and Bombus lapidarius) increased
in relative abundance from 40 to 89 per cent and now
dominate the communities. Several long-tongued bumble
bees declined strikingly over the same period. The mean
number of bumble bees collected per field was typically
an order of magnitude higher in the 1940s and 1960s
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3. THE STATUS AND TRENDS IN POLLINATORS
AND POLLINATION