Population Size And Reproduction In Phlox Pilosa

We analyzed the relationships between population size and reproductive characteristics in the perennial prairie forb Phlox pilosa, an obligate outcrossing butterfly-pollinated species. We examined 27 populations ranging in size from 9 to over 75,000 flowering ramets in two regions of the state Iowa (eastcentral and northwest) in 1993 and 1994. We collected fruiting ramets from each population at the end of the Phlox growing season and scored them for height, biomass and reproductive variables, including the number of flowers initiated correlated with the number of capsules matured per ramet. Differences between populations in capsule production were set primarily at the population stage. In 1993, pollen arrival to stigmas was significantly lower and was correlated with population size in eastcentral Iowa population, indicating that lower reproduction in small populations that year was at least partially due to the inadequate amounts of pollen being moved. In 1993, weather conditions likely depressed pollinator activity, but absolute capsule formation was high because of high flower production per ramet and high population densities. In 1994, when pollen arrival to stigmas was relatively high and unrelated to population size, outcross pollen movement was greater in larger populations. Increased efficacy of outcross pollen movement between ramets or from variation between years in fine-scale spatial genetic substructuring of populations. Our results indicate that the viability of Phlox pilosa can be best ensured by protecting and creating populations of at least 1000-2000 flowering ramets.