Performance of a prairie mating system in fragmented habitat: Self-incompatibility and limited pollen dispersal in Echinacea angustifolia

In the prairie and plains of North America many plants species grew in large, continuous populations. Recently, European agricultural practices and human activities have destroyed most of the habitat of native plants, fragmenting large populations into smaller and more isolated local populations. I studied the consequences of habitat fragmentation on reproduction in native prairie plants by focusing on Echinacea angustifolia (Asteraceae) which has a typical prairie-plant mating system: self-incompatibility and limited pollen dispersal. I focused on the local abundance of flowering conspecific plants and its effect on reproduction. Habitat fragmentation increases isolation of individual plants and reduces the size of local populations.; This dissertation reports on three major findings based on observations in 25 square miles of fragmented prairie in Western Minnesota. First, isolated Echinacea plants produce fewer seeds than non-isolated plants because they are not pollinated. To determine this, I mapped 2000 plants and their nearby flowering conspecifics and then characterized their pollen limitation over three years using a novel, non-destructive measure of pollination that is independent of seed set. I then related seed yield in individual plants to their pollination and isolation. Second, offspring fitness in small local populations is lower than offspring fitness in large local populations. To test this hypothesis, I compared the vigor of seedlings derived as seed from small and large local populations. Third, small Echinacea populations have lower genetic diversity and greater fine-scale spatial genetic structure than large populations. I tested this hypothesis by characterizing the population-genetic structure using allozyme electrophoresis. This third finding helps explain inbreeding depression in progeny from small local populations and suggests that pollen limitation of seed set in isolated plants may be due to in part to mating incompatibility. These results show how Echinacea reproduction fails in small local populations compared to large local populations.