POLLINATORS POLLINATION AND FOOD PRODUCTION

THE ASSESSMENT REPORT ON POLLINATORS, POLLINATION AND FOOD PRODUCTION
for pollinators adapted to exploit them, there may also be a
potential risk to pollinator health if invasive alien plant pollen
is nutritionally poor compared to that from native plants
(Stout and Morales, 2009).
Invasive plants are expected to affect pollinators adversely
if they either ill-adapted to exploit the alien food resource
or dependent on native plants outcompeted by the invader
(Bjerknes et al., 2007; Palladini and Maron, 2014; Stout
and Morales, 2009). There is, however, little evidence from
meta-analyses or reviews (Bjerknes et al., 2007; Montero-
Castaño and Vilà, 2012; Stout and Morales, 2009), and only
very few individual examples (Lopezaraiza-Mikel et al., 2007;
Moroń et al., 2009; Nienhuis et al., 2009) of alien plant
invasions consistently lowering overall pollinator diversity or
abundance.
There is more evidence, however, that alien plant invasions
can influence the assembly of pollinator communities.
Plant-pollinator community networks are permeable to plant
invaders (Traveset et al., 2013; Traveset and Richardson,
2014), which according to the species involved can rewire
plant-pollinator interactions (e.g., Bartomeus et al., 2008).
Network architecture can often be relatively unaltered by
alien plant invasions, for instance, a pan-European analysis
showed network nestedness, a property thought to confer
a degree of stability on the community, was unaffected
by the integration of alien plants into the network (Vilà et
al., 2009). Although recent global meta-analyses have
demonstrated changes in network properties following
integration of alien invasive plants, the attractiveness of
these invasive plants to native pollinators altered their
behaviour, which led to changes in network properties
(e.g. modularity, interaction strength) that are thought to
enhance community stability (Aizen et al., 2008; Albrecht
et al., 2014). For example, invasive plant species increased
connectivity between network modules (subsets of tightly
connected species) (Albrecht et al., 2014), which potentially
increased the network’s robustness to species losses
arising from future environmental changes. Furthermore,
highly invaded networks are, on average, characterised by
weaker mutualism strength (i.e. weak or asymmetric mutual
dependences between interacting species), a property that
may reduce the probability of secondary extinctions should
a partner species in the network be lost (Aizen et al., 2008).
It should be noted, however, that many of these predictions
around network robustness are derived from simulation
models of empirical network data (frequency of pairwise
species interactions at the community level). The challenges
of collecting such data typically preclude greater biological
realism (temporal network dynamics, species competition)
being built into these simulations. Therefore, due caution
is required in interpreting these insights from simulation
models for community stability. Nonetheless, while invasive
plant species do not generally alter diversity or abundance
(Montero-Castaño and Vilà, 2012) through usurpation and
domination of pollinator interaction networks, they often hold
a key role in community organisation (Aizen et al., 2008;
Albrecht et al., 2014).
This key role of invasive alien plants (and invasive alien
pollinators – see section 2.5.3), once integrated into
pollinator networks, has potential ramifications for individual
native plant species. If the native plant becomes overly
reliant on the invader for facilitation of pollination, then
there is a potential risk to the native species should
those connections become eroded or lost due to further
environmental changes (Aizen et al., 2008).
Invasive alien plants may alter interactions between native
plants and their pollinators either through competition for
pollinator visitation (Bjerknes et al., 2007; Dietzsch et al.,
2011) or by elevation of pollinator activity to the level where
co-flowering native plant pollination is facilitated (Bjerknes
et al., 2007; Cawoy et al., 2012; McKinney and Goodell,
2011). Primary and meta-analyses suggest that pollinator
visitation rates to native plant species tend to decrease with
plant invasion, suggesting that competition for pollinators
may be the prevailing process (Brown et al., 2002; Montero-
Castaño and Vilà, 2012; Morales and Traveset, 2009).
Whether this translates into reduced reproductive output
of native plant species is less certain, potentially because
of plant compensatory mechanisms (i.e., self-reproduction,
recruitment of alternative pollinators) (Bjerknes et al.,
2007; Dietzsch et al., 2011; Morales and Traveset, 2009;
Traveset and Richardson, 2014), but see examples where
negative effects are reported (Brown et al., 2002; Chittka
and Schurkens, 2001; Thijs et al., 2012). Furthermore,
the level of impact on flower visitation may be contingent
on the composition of the pollinator community because
of differential responses of pollinator groups (e.g., flies
versus bees) to the invasive plant (Carvalheiro et al., 2014;
Montero-Castaño and Vilà, 2012). The negative impact that
alien plant invasions can have on native plant pollination and
reproductive success is increased at high relative densities
of alien flowers and/or when alien and native plants are
related or have similar floral traits (i.e., flower anatomy,
color, phenology large floral displays) (Bjerknes et al., 2007;
Brown et al., 2002; Carvalheiro et al., 2014; Morales and
Traveset, 2009; Pysek et al., 2011). Only if some or all of
these conditions are met will the extent of pollinator sharing
between the native and the invasive plant species rise to
the point where there is an impact, positive or negative, on
the native plant (e.g. Thijs et al., 2012). There have been no
studies (to our knowledge at the time of writing) that have
examined the impact of invasive alien wild plants on food
crops, which represents a significant knowledge gap. The
pollinator-mediated impacts of native co-flowering plant
species on flowering invasive plants are not clear and have
been less studied (Carvallo et al., 2013).
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2. DRIVERS OF CHANGE OF POLLINATORS,
POLLINATION NETWORKS AND POLLINATION