Positive & Negative Feedback in Plant-Pollinator Balance
What is intriguing in the scenario above is that the competition causes
positive feedback. That is, existing imbalances, whether it is plants
chasing scarce pollinators by providing more nectar or vice
versa , will be exacerbated (Fig. 3). The behaviour of pollinators
should also facilitate and encourage positive feedback thereby working
against balance. Pollinators such as bees are very mobile, can rapidly
assess nectar rewards, and are able to preferentially visit more
rewarding feeding sites (Balfour et al. 2015; Seeley 1995). As such,
pollinator behaviour will advantage flowers that produce more nectar.
What about times when selection favours plants producing smaller amounts
of nectar leading to relative resource scarcity for pollinators?
Although pollinators should not visit flowers to collect nectar unless
they make a net energy profit in doing so, the energy gains can be small
(Balfour et al. 2015). Pollinators would seem to be exploitable to work
for “low wages” at a time of nectar scarcity, provided these wages are
above the minimum needed to make an energy profit. Bees are able to cope
with small energy gains per flower because they can visit flowers at a
great rate (Couvillon et al. 2015), thereby accumulating many small
rewards. Most female bees and some wasps are nest builders and need to
forage not just for their own needs but for the needs of their nest or
colony. However, most other pollinators, such as butterflies and hover
flies, do not provision a nest and so are only foraging for their
personal energy needs. Here, small amounts of nectar may be sufficient
and a rapid foraging rate not needed. Therefore, producing small
quantities of nectar when pollinators are abundant may be a viable
reproductive strategy for summer-blooming plants.
What about negative feedback to reduce imbalance? Via natural selection,
plants that do not receive adequate pollination may prolong their
flowering period (Udovic & Aker, 1981), decrease their need for
pollinators in various ways such as by producing fewer but larger seeds
(Huang et al., 2017), make better use of pollinator visits (Ashman and
Morgan, 2004.), rely less on outcrossing (Harder and Aizen, 2010) or
even adopt asexual reproduction (Lloyd, 1992). Furthermore, plants may
also increase the display of signals that attract pollinators (e.g.
visual or olfactory; Raguso, 2004). Indeed, there is evidence that scent
advertisement is higher in early blooming species, when pollinators may
be relatively scarce, than late flowering species (Filella et al.,
2013). However, such phenomenon are unlikely to increase floral resource
availability or, in turn, pollinator population growth rates (Ogilvie &
Forrest 2017). Therefore, these adaptations are unlikely to
significantly alter the balance between nectar supply or demand.
There are also several evolutionary and biological constraints within
plant-pollinator communities which may prevent phenological matching
between nectar demand and supply. For example, plant flowering phenology
is thought to be a conservative character (Ollerton & Lack, 1992) and
is partially determined by a number of factors including taxonomic
membership (Kochmer, and Handel, 1986). Likewise, many groups of
pollinators are constrained, for example, by their thermal windows
(Lefebvre et al., 2018) and trait characteristics (Junker et al., 2013).
Moreover, pollinator populations are may be strongly limited by
resources other than pollen and nectar during times of nectar abundance,
for example nesting sites and larval food resource (Benadi, 2015).
Similarly, plant population are constrained by resources availability
beyond pollen limitation, e.g. water and minerals (Kalske et al., 2012).