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The Mechanism of Competition for Pollination between Two Forest Herbs

Campbell, Diane R., Motten, Alexander F.
Ecology 1985 v.66 no.2 pp. 554-563
Apoidea, Claytonia, Stellaria, flowers, forage, forests, herbs, insects, piedmont, pollen, pollination, pollinators, seed set, seeds, stigma, North Carolina
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.