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
pollinators by alien ones can reduce native plant species
richness and abundance and promote processes leading to
inbreeding depression (by enhancing selfing) or hybridization
(by moving pollen across closely related alien and native
plants) and ultimately lower fitness of plants (Dohzono and
Yokoyama, 2010; Kenta et al., 2007; Morales et al., 2013;
Morales and Traveset, 2008). Certain solitary bee species
have been introduced, some possessing similar traits to
invasive social bees, but relatively little is known about their
impact on the ecology of native pollinators; representing a
gap in understanding that could help to forecast impacts of
future invasions (Goulson, 2003; Kenis et al., 2009).
2.5.6 Vulnerability of different
pollinator habitats to invasions
As invasions are primarily a human-mediated process,
anthropogenic and disturbed environments are likely to
be prone to the immigration and establishment of alien
species, for example where human activity creates or makes
accessible new niches (Catford et al., 2012; Mack et al.,
2000). A recent global meta-analysis suggested that the
tendency for alien invasions to reduce pollinator diversity or
abundance was both statistically non-significant and did not
differ among forest, shrubland, and grassland ecosystems
(Montero-Castaño and Vilà, 2012). While these broad
ecosystem classifications were necessary for this metaanalysis
due to data limitations, they were lacking important
contextual information (e.g., level of disturbance or human
activity, carrying capacity of recipient habitat, mainland
vs. island), which may have affected the sensitivity of the
analysis (Mack et al., 2000). Oceanic island ecosystems
may be particularly vulnerable to disruption of pollination
systems, at least where those ecosystems support a
smaller and more specialised plant-pollinator fauna (Abe
et al., 2011; Hansen and Müller, 2009; Mack et al., 2000;
Traveset et al., 2013; Traveset and Richardson, 2006).
Island pollination systems tend to be more robust when
the native pollination system is generalised and thus the
invasive alien species becomes integrated without significant
disruption (Kaiser-Bunbury et al., 2011; Olesen et al., 2002).
Although because of likely different demographic processes,
populations of animal or plant species that are rare or
restricted to oceanic islands or have undergone a strong
recent genetic bottleneck related to habitat fragmentation
are likely to have less genetic diversity than more common
or less spatially restricted species (Darvill et al., 2006;
Eckert et al., 2010; Frankham, 1997; Stuessy et al., 2014).
The impoverished genetic diversity of such species may
thus affect adaptive processes that could contribute to the
success or failure of invasions, depending of the type of
interaction they have with the invasive species. For instance,
modeling approaches indicated that a higher ability to adapt
(higher genetic diversity) in the invasive species generally
leads to establishment, and further, higher genetic diversity
in the resident species can lead to exclusion of the invasive
in predator-prey interactions, and may allow adaptation
to the invasive (and thus favor invasion) and survival of
both species (mutualism or competition) (Jones and
Gomulkiewicz, 2012).
2.5.7 Conclusions
The outcome of an invasion on pollinator populations,
diversity, network structure or pollination processes is likely
to be highly contingent on the ecological and evolutionary
context. For example, invasive plant species are often readily
incorporated into native pollination networks, especially
where generalised plants and pollinators predominate. This
can have major consequences for the function, structure
and stability of pollinator networks, negative impacts on
particular native pollinator species and, less commonly,
reductions in overall pollinator abundance or diversity. The
ramifications of such changes for native plant pollination
can be positive or negative depending on the traits of
the species involved. By altering the plant community,
introduced mammal herbivores can have a profound
effect on pollinator communities and pollination, but the
effects of invasive insect herbivores are unknown. Invasive
predators can directly kill pollinators or disrupt pollinator
communities and associated pollination systems, whilst
invasive pollinators can outcompete or transmit diseases
to native pollinator species or simply be accommodated
in the existing pollinator assemblage. The ecological
complexity and context of different invasions precludes
overall generalisation. Nonetheless, the trophic position
(plant/herbivore/pollinator/predator) of an invasive species
and the degree of specialisation in the invasive and the
recipient pollination system are crucial to understanding
the outcome of alien species invasions. There is also a
risk that the impact of invasive alien species on pollinators
and pollination may be further exacerbated when it occurs
in combination with other threats (section 2.7) such as
diseases, climate or land-use change (González-Varo et al.,
2013; Schweiger et al., 2010; Vanbergen and the Insect
Pollinators Initiative, 2013).
2.6 CLIMATE CHANGE
2.6.1 Vulnerability of biodiversity
and ecosystems to climate
change
Climate change “refers to a change in the state of the
climate that can be identified … by changes in the mean
and/or the variability of its properties, and that persists for
an extended period, typically decades or longer. Climate