Faced with multiple types of flowers in a plant community, an individual pollinator typically associates more strongly with a subset of flower types than would be expected based on the frequency of the plants in the community. To the extent that these associations are based on innate or fixed preferences, they should be consistent across individuals within pollinator taxa (Waser 1986). That they often are less than completely consistent within a pollinator species indicates the potential for learning, modification of foraging preferences based on individual experiences, and perhaps different innate preferences among individuals. Not only may different individuals preferentially visit different types of flowers, but the same individual may exhibit different preferences over time. Such "labile preference" is considered adaptive given the dynamic nature of floral resource availability (Heinrich 1976). There also can be further specialization by individuals due to constraints on information processing, particularly memory retrieval. Such specialization results in visitation to fewer flower types than would be predicted if foraging were optimal (Chittka et al. 1999; but see Menzel, this volume). These various types of non-random pollinator behavior all contribute to the widespread phenomenon of flower constancy, broadly defined, in which pollinators tend to restrict their visits to one or a few types of flowers within a foraging bout, i.e., to specialize to some extent, at least in the short term. It is useful to analyze separately these different kinds of behavior that make up flower constancy, as their implications for plant evolution are not the same.
Different pollinator taxa may display different innate preferences for certain flower types, based on the morphological fit between animal and flower, for example, or on differing sensitivity to various stimuli such as floral color or scent (see Grant 1994 for review; Giurfa et al. 1995). The traditional view - that divergent selection by different pollinator taxa drives floral differentiation and promotes reproductive isolation via assortative mating (Grant 1949) - assumes that pollinator preferences are reasonably consistent over time, as innate preferences are likely to be. At least a few cases appear to fit this scenario, notably in the species-rich Cape flora of South Africa, where some pollinators have highly specialized morphology that "fits" with similarly specialized flowers. For example, several plant taxa in the Cape region have very long, narrow corolla tubes accessed by flies with exceedingly long proboscides (Goldblatt et al. 1995; Johnson 1996; see Johnson & Steiner 2000 for review). However, other more common pollinator taxa such as bees and hummingbirds often are quite generalized and opportunistic (Waser et al. 1996), as experience may override innate preferences (Giurfa et al. 1995). Investigation of the extent to which pollinator taxa partition floral resources in the same way over replicate communities from year to year (e.g., Cripps & Rust 1989) and of how the presence or absence of one pollinator taxon influences resource partitioning by the rest (e.g., Inouye 1978; Laverty & Plowright 1985), would help to determine the potential for innate preferences characteristic of pollinator species to impose consistent selection on floral traits.
Within the range of floral types suitable for a given pollinator species, different pollinator individuals may make different choices (Heinrich 1976). Labile preference and constancy due to behavioral constraints both come into play, as individuals learn from experience but often apparently do not make optimal use of the information gained. Potential explanations for flower constancy involving behavioral constraints have been reviewed thoroughly in this volume (Gegear & Laverty; Menzel) and by Chittka et al. (1999).
In addition to varying preferences among individual pollinators, preferences might be expected to vary over the course of a day as reward distributions change or over a season as different plant species become available. Thus, labile preference can be examined by testing for homogeneity of preference among foraging bouts of an individual over time, or among bouts across individuals (see Jones 1997 for methods). Behavioral constraints seem to be especially important in the quick succession of choices within a foraging bout, for which short-term memory may be more important than long-term memory (Chittka et al. 1999; but see Menzel, this volume). For this, analyses of the sequences of plants visited within foraging bouts are appropriate (Bateman 1951; Waser 1986; Jones
While it is useful to distinguish floral resource partitioning between versus within pollinator taxa for comparative purposes (see next section), the two processes certainly are not mutually exclusive. Partitioning between taxa is easy to test by counting visitors to the various floral types, but differences among individuals within pollinator taxa may be superimposed on overall species preferences; thus pollinator-mediated assorta-tive mating may be stronger than counts of visitors might suggest (e.g., Fulton & Hodges 1999; see Thomson & Chittka, this volume). Even in a reasonably clear case of partitioning of pollinator taxa between "hummingbird-pollinated" Mimulus cardinalis and "bee-pollinated" M. lewisii, pollinator preferences were not completely consistent within taxa (Schemske & Bradshaw 1999). Bumble bees collecting pollen were observed most often on red or orange (rather than pink) flowers, whereas increasing carotenoid (yellow pigment) concentration in petals showed a strong negative correlation with bee visitation overall, as bees foraged for nectar on most bouts on the experimental arrays. Assortative mating in the Mimulus complex appears to result from partitioning of floral resources both among and within pollinator taxa, a likely scenario for sympatric plant species pairs in general.
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