Evolutionary & Ecological Logic
Pollination is central to plant reproduction yet pollen limitation is widespread (Ashman et al. 2004; Burd 1994; Knight et al. 2005). Fig. 1 shows a standard theoretical cost-benefit model that determines optimal levels of nectar secretion in terms of plant reproduction for an individual plant at times of relative scarcity or abundance of pollinators. Optimal nectar production is higher when pollinators are scarce. The assumptions underlying the model are biologically realistic: (i) greater nectar production results in more pollinator visits (Wyatt and Shannon, 1986; Klinkhamer and de Jong, 1990) and generally (however, see Fisogni et al., 2011) increases plant reproductive success (e.g. Neiland & Wilcock, 1998; Larson, & Barrett, 2000), (ii) nectar has a non-zero cost of production (Southwick, 1984, Pyke, 1991); (iii) plant reproduction increases with pollinator visits and approaches the maximum in an asymptotic manner (Silander and Primack, 1978; Snow, 1982; Ashman et al. 2004; Morris et al., 2010).
How would these individual-level evolutionary responses affect nectar availability in the wider ecosystem? If pollinators are scarce, an individual plant can increase its reproductive success by producing more nectar and thereby attracting more of the available pollinators. That is, it becomes a superior competitor. However, the same logic also applies to other plants competing for the same limited number of pollinators. Overall, and via the action of natural selection at the individual level, this should result in increased nectar availability in the ecosystem. The same logic applies in reverse when pollinators are abundant and leads to an overall decrease in nectar availability.