How Long to Stay on, and How Often to Visit a Flowering Plant?: A Model for Foraging Strategy When Floral Displays Vary in Size

Abstract

Variation in the number of flowers open at any one time on a plant or an inflorescence, i.e. floral display size, is probably the major factor that affects pollen flow and plant fitness through behavioral responses of pollinators on plants. Many authors have reported that pollinators visit large floral displays more frequently than small ones (e.g. Heinrich 1979, Ohara and Higashi 1994), whilst also visiting more flowers on larger displays (e.g. Geber 1985, Harder and Barrett 1995). With regard to these two types of pollinator response to increased display size, it has often been explained that pollinators visit large floral displays more frequently as a consequence of the associated lower flight costs per flower (Harder and Cruzan 1990, Robertson and Macnair 1995). However, it seems that pollinators do not always behave in a way that will minimize flight costs per flower. Rather, it has often been observed that a smaller proportion of open flowers was probed on larger displays (Pleasants and Zimmerman 1990 and references therein). This fact seems to contradict the above explanation. Why do pollinators visit a smaller fraction of open flowers during their visit to a large floral display? And if they do, then why should they visit larger displays more frequently than smaller ones? In this article, we propose a new idea for answering these questions by exploring theoretically how foraging patterns of pollinators may depend on floral display size, from the perspective of pollinators' optimal foraging. We then test whether observed patterns in the literature support the predictions of our model.