Concluding remarks
Our imbalance hypothesis may shed light on the surprising fact that seed production is often pollen limited (Ashman 2014; Knight et al. 2015; Burd 2004). This is surprising because, according to Bateman’s principle (Bateman 1948; Burd 2004), female reproductive success is usually limited by resources not males. Bateman’s principle appears to work better for animals than plants, and there are animal species with massively female-biased sex ratios in which the few males are still able to fertilize most of the females (Dyson & Hurst 2004; Jiggins et al. 2000). Pollen limitation is probably in part a consequence of plants “mating” via intermediaries in the form of pollinators. Ashman (2014) notes that the leading hypotheses for pollen limitation are that the situation is not at equilibrium, for example due to human interference, or that it is at equilibrium but subject to great stochastic variability (Burd 1994). Our hypothesis suggests that we should not necessarily expect the equilibrium to be a balance in terms of pollination-system needs. Rather it may be an equilibrium of costs and benefits than exacerbates imbalance rather than an equilibrium of needs. The flip side of the coin to pollen limitation, meaning that there is not enough pollinator activity, is nectar dearth, meaning that bees and other pollinators struggle to find nectar. Dearth is frequently observed by beekeepers (e.g. Mogren et al., 2018). Although pollination is a mutualism it would seem that one of the two partner classes is, on average, frequently underserved or exploited by the other (Bronstein, 2001; Nepi et al., 2018).