Behavioral responses of pollinators to variation in floral display size and their influences on the evolution of floral traits

The number of flowers open at any one time on a plant, i.e., floral display size, varies greatly among plant species. For example, some species flower during a brief period and have many open flowers, while others have extended flowering with only a few open flowers at one time (Gentry 1974; Bawa 1983). Also, floral display size often varies among individuals of the same plant species (e.g., Willson & Price 1977; Pleasants & Zimmerman 1990). The causes of such variations in floral display size are enduring interest to plant ecologists (reviewed by de Jong et al. 1992).

Numerous studies have reported that variation in floral display size produces marked alterations in pollinator behavior. Especially, two types of pollinator response to increased floral display size have been recognized from the perspective of their influences on pollen dispersal. First, larger floral displays attract more pollinators per unit of time (Fig 14.1A; reviewed by Ohashi & Yahara 1998). This will promote cross-pollination in terms of increased pollen receipt, removal, or potential mate diversity (Harder & Barrett 1996). Second, the number of flowers that individual pollinators probe per plant also increases with floral display size (Fig. 14.1B; also reviewed by Ohashi & Yahara 1998). This will increase self-pollination among flowers on the same plant ("geitonogamy"; Richards 1986; de Jong et al. 1993). Thus, variation in floral display size may lead to a substantial difference in pollen dispersal and, in turn, plant fitness.

To understand how plant fitness can be related to floral display size, we have to know the shapes of the functional relationships between floral display size and the two types of pollinator response, i.e., visitation rate per plant and the number of flowers probed per plant visit. One possible approach is to examine the actual pollinator behaviors empirically (e.g., Ohashi & Yahara 1998). Another is to consider how pollinators should

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