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.