Make flowers, save fruits — A new hypothesis for the adaptive significance of excessive production and abortion of flowers
Many flowering plants are known to produce excessive flowers and abort most of them. To date, several hypotheses have been proposed to explain the significance of this adaption. I developed a simple mathematical model to evaluate whether excessive flower production is a strategy to mitigate pre-dispersal seed predation. I assumed that each individual plant allocates a given amount of resources for flower and fruit production. A fraction of the flowers are oviposited by pre-dispersal seed predators so that all the seeds are consumed. If the expected egg load per flower decreases with the number of flowers on the plant, the plant that produces more flowers experiences a decreased seed predation rate and produces less mature seeds because of the trade-off in resource allocation. I compared the three strategies of a plant: (1) no abortion—all pollinated flowers of the plant develop into mature fruits, regardless of whether they are parasitized; (2) random abortion—a given fraction of the pollinated flowers are aborted and the rest develop, regardless of whether they are parasitized; and (3) selective abortion—all parasitized flowers are selectively aborted and the rest develop.
I observed that the three above-mentioned strategies of plants were similar with respect to reproductive efficiency ((expected number of sound seeds that the plant can produce)/(total resource for reproduction)) when plants have abundant resources or the density of seed predators is low. However, when plants have limited resources and the density of seed predators is high, the plant that exhibits the no-abortion strategy experiences a high seed predation rate because it produces few flowers, which are targeted by the seed predators. In contrast, plants that exhibit the other two strategies produce more flowers and the number of predators per flower is smaller, resulting in a lower seed predation rate. The best strategy is selective abortion, and random abortion is better than no abortion. I have reported that this mechanism, which I call the "seed predator dilution hypothesis," is different from the existing hypotheses for excessive flower production.