Demographic and genetic invasion history of a 9-year-old roadside population of Bunias orientalis L. (Brassicaceae).

The population history of a 9-year-old roadside population of the invasive plant Bunias orientalis was reconstructed by demographic analysis including size, position, age (determined by herbchronology) and RAPD-PCR patterns of individual plants. We evaluated emerging patterns of population growth and genetic structure during a full period of population development under typical site conditions (anthropogenic disturbance) and their possible consequences for the invasion potential of the species. The population has grown rapidly and continuously (though with slowing geometric population increase) during the 9 years since its foundation, filling the space available in the study area. Genetic variation (RAPD markers) was already high in the founder cohorts and remained at the same level throughout population development (variance fluctuations <15%). Both results may be related to the mowing management at the site which seems to promote population growth of B. orientalis relative to other co-occuring species and to prevent the genetic drift and the development of spatial genetic structure that would be expected under isolation-by-distance models. Large founder plants had comparatively low genetic variance and were more closely related to younger cohorts than were small founder plants, indicating that selection acted during population development. Overall, the current anthropogenic disturbance regimes may contribute to high genetic variability by artificially increasing gene flow and thereby promoting the adaptability of invasive species to the often unpredictable conditions at disturbed sites. Our approach using retrospective demographic investigation allows the detection of spatio-temporal microscale patterns in genetic and phenotypic variation. Thus it allows a thorough understanding of local invasions of perennial herbaceous plants.