Dynamic responses to habitat fragmentation in a prairie plant

Background/Question/Methods

The tallgrass prairie of central North America is extremely fragmented. It is generally unknown if small and isolated remnant populations will persist. Since 1995, I have investigated the roles of demography, genetics, and their interaction in population persistence, using a prairie plant that serves as a model for advancing basic population biology and applied conservation biology in changing and fragmented habitats. The narrow-leaved purple coneflower, Echinacea angustifolia, was once abundant in the plains and prairies but now is confined to small remnants. My study area is 6400 ha in rural western Minnesota dominated by conventional agriculture. Remnant populations persist on steep hills and other areas that escaped the plow. Echinacea exhibits characteristics typical of many native prairie plants, including self-incompatibility, longevity, reproduction by seed, and pollination by generalist bees. Furthermore, reproduction in Echinacea is limited by receipt of compatible pollen and declines dramatically in isolated plants and in small populations. Three fundamental constraints to reproduction that are hypothesized to be exacerbated in fragmented habitat include pollinator scarcity, scarcity of compatible mates, and flowering asynchrony between potential mates. Using observations, experiments, and modeling approaches, I have quantified spatial patterns of reproduction and tested these three hypothesized mechanisms for reproductive failure.

Results/Conclusions

Of the three mechanisms proposed to cause reproductive failure in fragmented habitat, mate scarcity overshadows the others. Reproduction declines when nearby flowering plants are mating incompatible (a plant rejects pollen that shares a mating-type allele). Over time, models predict that this genetic-based Allee effect will strengthen as genetic drift reduces numbers of mating-type alleles. Remarkably, reproduction in Echinacea is not limited by pollinator visitation. In a two year study, the most isolated plants reproduced the least, yet were visited most frequently by pollinators. Emerging evidence about the role of flowering asynchrony in reproductive failure suggests that its effects may be comparable in magnitude to spatial effects and likely depend on the density of flowering plants. In sum, living in fragmented habitat imposes substantial constraints to reproduction in Echinacea due to a scarcity of nearby, synchronously flowering, and compatible mates. Insights from each of these studies have helped define research questions, such as current questions about how prescribed burns and inbreeding depression influence constraints to reproduction in Echinacea. I will discuss how the duration of these studies impacts their contribution to basic population biology and applied conservation biology.