I studied the spring wildflower community of mesic deciduous forests in piedmont North Carolina to determine (a) the extent to which fecundity is pollination—limited in the community, (b) the importance of competition for pollination in affecting seed—set, and (c) the characteristics of plants and their floral visitors that most contribute to full pollination. Although inadequate pollination seems likely in the community, supplemental hand—pollination significantly improved fecundity in just 3 of the 12 species I examined. Pollination—limited reproductive success was evident only in a distinctive subset of the community, species pollinated primarily by queen bumble bees. The majority of wildflower species are pollinated by flies and solitary bees. Measurements of visitation rates and pollinator effectiveness on these plants confirmed that they are usually adequately pollinated in spite of a short blooming season, considerable overlap in flowering times, extensive pollinator sharing by concurrently blooming species, and inclement weather that frequently interrupts insect activity. Many of the flies and solitary bees are inconstant foragers, yet competition for pollination among wildflower species through differential pollinator attraction or interspecific pollinator movements usually does not significantly decrease the seed—set of plants with shared visitors. Competition may act with other causes of insufficient pollination, however, as a selective force to maintain a characteristic set of floral biology traits within the community, including autogamy and self—compatibility, extended receptivity, and pollination by a variety of visitor types. That these floral traits contribute significantly to the successful pollination of vernal herbs was demonstrated by observations of visitor behavior, plant caging experiments that excluded visitors or restricted their access to selected flowers, and measurements of floral lifetimes and seed—set for individual plants. These traits are effective regardless of the source of pollination—limited fecundity, and it is the prevalence of such traits, rather than floral specialization or character displacement, that distinguishes the forest spring wildflower community from other communities with potentially inadequate pollinator service.

The primary mechanism of competition for pollination between the two forest herbs Stellaria pubera and Claytonia virginica in piedmont North Carolina is interspecific pollen movement. The most common visitor, the bee fly Bombylius major, forages indiscriminately among flowers of the two species. In only one of five experiments did the presence of C. virginica reduce the pollinator visit rate per S. pubera flower, and in other experiments addition of C. virginica enhanced visit rate. Thus these plant species exhibit little or no competition through pollinator preference. In some natural populations, however, visits to S. pubera are frequently immediately preceded by a visit to C. virginica, and a flower receives less conspecific pollen and produces fewer seeds following such an interspecific visit than if the visitor has arrived directly from a conspecific flower. Interspecific pollen movement is responsible for most, if not all, of the reductions in seed set of S. pubera due to pollinator sharing.Although insects deposit a substantial amount of S. pubera pollen on stigmas of C. virginica, little C. virginica pollen is found on S. pubera stigmas. Moreover, application of foreign pollen to the stigma does not influence seed production of S. pubera. The effect of interspecific pollen movement is due to loss of conspecific pollen, not stigmatic interference.

Abstract. We measured the pollination effectiveness and visitation rates of major insect visitors of Claytonia virginica, an obligately insect-pollinated spring wildflower, in a North Carolina deciduous forest. Seed set in the population was not pollinator-limited except during rainy weather and very early in the flowering season. The solitary bee Andrena erigeniae and the bee fly Bombylius major were responsible for more than 75% of the visits to C. virginica. Andrena erigeniae is a specialist on C. virginica, while B. major is a common visitor to many plant species. We measured the effectiveness of a pollinator by the probability that a visit resulted in fruit (capsule) formation. For those flowers that were successfully pollinated and thus produced a capsule, number of seeds did not vary with visitor identity or the total number of visits received. Although B. major and female A. erigeniae differ greatly in morphology and foraging behavior, a visit by either insect results in equally high seed set. As B. major is about two-thirds as abundant as A. erigeniae females on C. virginica, both insects contribute substantially to seed set in our population. With the visitation frequency and pollination effectiveness we measured, the generalist B. major alone has the potential to pollinate three-quarters of the flowers.

A polymorphism for anthocyanin production was used as a genetic marker to document the relationship between anther-stigma separation and outcrossing rate in the predominantly self-fertilizing weed Datura stramonium. White-flowered plants that differed in anther-stigma separation were placed into populations consisting exclusively of purple-flowered plants. Self vs. outcross origin of progeny was evident in the hypocotyl color of the seedlings. Outcrossing rates measured for single flowers were significantly positively correlated with anther-stigma separation, albeit with some scatter around the regression line, especially for flowers with exserted stigmas. We also performed an 8 × 8 diallel cross to determine whether anther-stigma separation is genetically determined. Heritability in two field plots was ∼0.3 and in the greenhouse was ∼0.2. Maternal effects, epistasis, and dominance appeared to be relatively unimportant. Genotypes performed consistently across the three environments, although total plant size varied more than fivefold. It appears that the mixed-mating system of D. stramonium has a heritable basis and would be capable of responding to selection.

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