Demographic structure and genetic variability of Platte thistle (Cirsium canescens Asteraceae) in central and peripheral populations

Background/Question/Methods

Understanding spatial variation in the structures and dynamics of species’ populations has long been a focus in ecology.  Species abundance is a function of both frequency of occurrence (whether the species is present at a site) and density of individuals within populations.  The Abundant Center Hypothesis (ACH) predicts that as one moves away from the center of a species’ biogeographic range, increasingly peripheral populations become less frequent, more isolated from one another, and less dense within populations.  The ACH also has implications for the population genetics of central vs. edge populations, with isolated peripheral populations likely to experience genetic drift and reduced genetic variability.   We tested certain predictions of the ACH in Cirsium canescens (Platte thistle), a monocarpic perennial.  First, we generated a model of the maximum distribution of C. canescens in the program MAXENT based upon localities from herbarium collections and environmental conditions. We then established eight center-edge transects within this distribution and measured the frequency of C. canescens at 40km intervals along each transect.  Two central and two peripheral populations along each transect were used to quantify population density.   Tissue samples were collected from each population to analyze genetic variability using six simple sequence repeat (SSR) loci.  

Results/Conclusions

Our preliminary analyses indicate that C. canescens populations occur significantly less frequently as one moves away from the center of the species range, consistent with the ACH.  However, within population density was unrelated to distance from the center.  Rather than a single peak of abundance for within population density, our results indicated two areas of highest density.  Results from SSR genotyping show a reduction in genetic variability at the edges of the distribution.  Mountain ranges in the western portion of C. canescens range may lead to greater isolation of populations than do the less topographically complex habitats at the species eastern range limits.  This decline in population genetic diversity with increasing distance for the biogeographic center was stronger toward the western edge of the species distribution than toward the eastern edge.  This difference in results between portions of the periphery further illustrates the inadequacy of testing ACH predictions at only a single range limit.