POLLEN EXCHANGE AS A FUNCTION OF SPECIES PROXIMITY IN PHLOX

The isolation of plant species has beei and remains a subject of active interes and research. There have been numerous studies of reproductive and environmenta factors which limit or preclude hybridi zation, and sufficient evidence is at hanc to warrant some generalizations about th4 isolating mechanisms of taxa sharing cer tain habits or habitats (cf. Grant, 1971) One factor which has a profound effect or the isolation of species, but which has re ceived little attention, is the proximity o heterospecific populations. Although refer ence is made to the fact that isolation iL less when populations are confluent rathei than discontinuous, or closely adjacenl rather than far apart, the writer knows oi no analysis relating, in quantitative terms the isolation of species in nature to the spatial association of their populations. The degree to which entomophilous spe cies are isolated by a given distance is E function of the pollen dispersal agent. Some groups of pollinators routinely fly long dis tances between consecutively visited plant. (Janzen, 1971), whereas others typicall3 traverse short distances on single flights (Levin and Kerster, 1968; Free, 1970) Accordingly, the degree of isolation experi enced by two species separated by a giver distance may be quite different from thal achieved by two other species separated b3 the same distance. Like flower constancy foraging flight distance is a behavioral at tribute of pollinators, and should be in cluded with flower constancy as an effector of ethological isolation. The present investigation was undertaker to determine the relationship between th( proximity of Phlox divaricata L. and P bifida L. and the level of pollen exchange These species are especially common in Brown County, Indiana, which was chosen as the study area. Phlox bifida generally is restricted to sites with open canopies prevailing in forest borders, and the more open oak-hickory forests of upper slopes and ridges. On the other hand, P. divaricata occurs under a spectrum of forest canopy conditions and in moister habitats, including wooded depressions, moist slopes, stream flats, and water drainage courses. In spite of these differences, the phloxes often grow within a few meters of each other, and occasionally form contiguous or confluent populations especially where the canopy has been thinned. The species flower for about six weeks and have overlapping flowering periods, P. bifida reaching its flowering peak in late April and P. divaricata in early May. Both have lavender corollas and are suited for lepidopteran pollination. The nectaries are located at the base of a salverform corolla whose tube varies from 9 to 14 mm in length in P. bifida and 11 to 16 mm in P. divaricata. Pollination of both species is accomplished by species of Papilio, Danaus, Colias, Polites, and other lepidopterans (Grant and Grant, 1965; Levin, unpubl.). Although there are no mechanical barriers to interspecific pollination, the flowers of the species are distinctive in two respects which relate to this discussion. First, the corolla lobes of P. bifida are strongly bifid, whereas those of P. divaricata are notched (Fig. 1). This feature may be an important recognition signal for pollinators. The ability of the aforementioned lepidopterans to discriminate between different corolla outlines in Phlox has been established (Levin, 1969). The species also differ in