Demographic structure and genetic variability throughout the distribution of Platte thistle (Cirsium canescens Asteraceae)

Aim Understanding spatial variation in the demographic and genetic structure of populations is central to explaining causes of species range limits and to species conservation. The Abundant Centre Hypothesis (ACH) predicts that as one moves away from the centre of a species’ biogeographical range, populations become less frequent and more isolated, as well as exhibiting decreasing within-population density. This increased isolation may lead to reduced genetic variability in peripheral populations by limiting gene flow. In this study, we asked whether the frequency, within-population density and genetic diversity of Cirsium canescens (Platte thistle) populations decreased from the range centre to the edge, as predicted by the ACH. Location Central United States, including portions of the Great Plains and Rocky Mountains. Methods Frequency of population occurrence at regional and landscape scales, within-population density, and within-population genetic variation were evaluated along eight centre-edge transects within the species’ distribution. Leaf tissue samples were collected from each population to establish genetic variability using six simple sequence repeat loci. Results Consistent with the ACH, peripheral regions of C. canescens’ range were less likely to contain populations than central regions. In regions where C. canescens did occur, however, frequency of populations at a landscape scale peaked at intermediate distances from centre and within-population density was unrelated to distance. Populations exhibited reduced genetic variability towards range edges. Main conclusions The ACH underestimates the complexity of the relationship between variation in abundance and genetic diversity with distance from C. canescens’ range centre. Decreases in Platte thistle population frequency combined with no decreases in within-population density near range edges suggest that quality habitat exists in the peripheral range, but these patches are rarer. Although genetic variability was reduced at the distribution edges, this decline was stronger towards the western edge, associated with increased topographic complexity.