Patterns of Ant-mediated Pollen Dispersal in Diamorpha smallii (Crassulaceae)

Abstract

Gene flow distances in Diamorpha smallii (Crassulaceae) effected through pollen dispersal by ants (Formica schaufussi and F. subsericea) are extremely short and their frequency distributions are strongly leptokurtic in form. Increased plant density is associated with decreased pollen dispersal distances. This pattern of pollen dispersal may explain earlier electrophoretic data indicating a highly subdi- vided population structure in D. smallii. Only recently, with increased emphasis on plant-animal interactions (Percival 1965; Free 1970; Faegri and van der Pijl 1971; Proctor and Yeo 1972; Heinrich and Raven 1972; Gilbert and Raven 1975), has the phe- nomenon of pollination by ants, dismissed by classical pollination ecol- ogists (Kerner 1878; Muiller 1883; Knuth 1906), been described and documented. Hickman (1974) was the first to report effective pollination of the flowers of Polygonum cascadense Baker (Polygonaceae) by the ant Formica argentea Wheeler (Formicidae). That study did not, however, include an assessment of the quantity and quality of pollen dispersal effected by the ants. Recent studies of a diversity of flowering plants indicate that gene flow in the form of pollen or seeds is highly leptokurtic and restricted in range (Levin and Kerster 1974; Levin 1978, 1979a). We report here the pattern of pollen dispersal within populations of Diamorpha smaliji Britt. (Crassulaceae), a succulent annual endemic to granite outcrops in the southeastern United States (McCormick and Platt