Demographic structure and genetic variability of Platte thistle (Cirsium canescens asteraceae) in central and peripheral populations
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 (whether the species is present at a site) and density of individuals in populations. The Abundant Center Hypothesis (ACH) predicts that as one moves away from the center of a species’ biogeographic range, populations become less frequent, more isolated, and less dense within populations. Isolation is likely to cause reduced genetic variability in peripheral populations. I tested predictions of the ACH in Cirsium canescens (Platte thistle), a monocarpic perennial. Occurrence was measured along eight center-edge transects within the species’ distribution. 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 indicate that C. canescens populations occur 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. Results from SSR genotyping show reduced genetic variability at the distribution edges. The decline in genetic diversity with increasing distance from the distributional center was stronger toward the western edge of the species’ range, related to increased topographic complexity. This difference in results between portions of the periphery illustrates the inadequacy of testing ACH predictions at only a single range limit. Decreases in C. canescens population spatial extent with no decreases in within-population density near the range periphery suggest that quality habitat exists in the peripheral range, but these patches are rarer near the edge.