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).