SPATIOTEMPORAL VARIATION IN ABUNDANCE AND VARIATION IN FORAGING BEHAVIOR OF THE POLLINATORS OF GYNODIOECIOUS PHACELIA-LINEARIS (HYDROPHYLLACEAE)

Variation in pollinator assemblages at several levels can affect the likelihood of evolutionary specialization of plants for particular pollinators and patterns of natural selection on floral characters. This paper describes variation in a pollinator assemblage (mainly solitary bees) at three levels - geographic occurrence, abundance, and foraging behavior as a function of floral characters - for the gynodioecious annual Phacelia linearis (Hydrophyllaceae). The set of Hymenoptera that visit P. linearis flowers varies geographically. Phacelia linearis populations in five of eight areas separated by 2-80 km had species of flower visitors that were not collected at the seven other sites. In experimental plant populations in one of these areas, the relative abundances of 12 common groups of pollinators varied significantly over the season within sites, at the same site between consecutive years, and between sites 2 km apart in the same year. Some groups that were very common in one experimental population were rare or absent in the others. The ten most common groups of visitors varied in their behavioral responses to flower gender (hermaphrodite or female), flower size, and flower number. Four groups arrived at hermaphrodite plants at a significantly higher rate than at female plants. Hermaphrodites have larger flowers than do females, and in two insect groups that preferred to visit hermaphrodites, their preference was all or in part a correlated effect of their preference for large-flowered plants. One group, Pseudomasaris spp. (Hymenoptera; Masaridae) preferred to visit female plants. The arrival rate of nine insect groups increased with flower number. The arrival rate of beetles and of four relatively large-bodied bees increased with hermaphrodite flower size, while the arrival rate of a small bee, the Phacelia specialist Chelostoma minutum (Megachilidae) increased with female flower size. Five groups visited significantly more (but proportionately fewer) flowers after arriving at many-flowered plants than at few-flowered plants, and two groups visited significantly more flowers per hermaphrodite than per female. Variation in the occurrence and abundance of its pollinators makes evolutionary specialization of P. linearis for particular pollinators unlikely. Variation in pollinator abundance and behavior may cause small-scale spatio-temporal variation in selection for floral characters in this species. A specific hypothesis is that selection favors large-flowered hermaphrodites in populations and years in which the pollinator assemblage is dominated by large-bodied bees.