Geographic patterns in the distribution, productivity and population genetic structure of Cirsium species across their UK geographic range
Geographically peripheral populations are believed to be particularly important in a species' response to environmental change and for the conservation of intraspecific genetic diversity. It is expected that as a species' range limits are approached, productivity and reproduction will decline and populations will become less abundant and more isolated. Decreased genetic variability and increased genetic divergence of peripheral populations is predicted based on these patterns. Cirsium heterophyllum reaches a southern geographical limit in the UK, C. acaule and C. eriophorum reach a northern limit and C. arvense occurs throughout the UK. These species have been used to determine whether contemporary patterns of distribution, productivity and reproductive potential across a species' UK latitudinal range are reflected in the predicted patterns of population genetic structure (assessed using microsatellite markers). Population frequency declines approaching the periphery of Cirsium acaule and C. heterophyllum. A decline in abundance was found in C. heterophyllum only. Community surveys suggest that peripheral populations do not occur in atypical habitat. There is no latitudinal variation in morphological characters across the species range, whereas reproductive potential declines approaching the periphery of the species that reach a latitudinal limit in the UK. Population genetic analysis revealed a decline in genetic variation toward the latitudinal limit of C. acaule. This pattern is absent in C. heterophyllum despite a marked decline in seed production and increase in population - . isolation approaching its periphery. C. heterophyllum exhibits almost randomised geographical structure of genetic variation. The lack of agreement between patterns of reproductive potential and population frequency and population genetic structure suggests that contemporary patterns of population distribution and reproduction may be inadequate for indicating patterns of population genetic structure within a species. Interspecific differences in post-glacial history may be important in explaining this disparity.