Submitted to: Biological Invasions
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/22/2006
Publication Date: 4/1/2007
Citation: Jongejans, E., Skarpaas, O., Tipping, P.W., Shea, K. 2007. Establishment and spread of founding populations of an invasive thistle: the role of competition and seed limitation.. Biological Invasions. 9:317-325. Interpretive Summary: The ability of plumeless thistle, Carduus acanthoides, to invade and spread into a new area was examined over a 7 year period. This thistle species is a pest throughout North American, particularly in overgrazed pastures. Although the number of seeds produced by the planted founder plants was equivalent, less than 50% of the plots were permanently invaded. The spread was slow within the plots with the majority of seedlings recruited in the shadow of the flowering parent plant. Long distance dispersal was not examined in this study. More seedlings were recruited in the spring compared with the fall but the greatest mortality occurred during the summer, especially with seedlings recruited that spring. Drought posed the most serious obstacle to survival along with competition from other plant species. Despite the invasive tendencies of this species, establishment is not assured in new areas, which may provide addition incentive to move rapidly to control new infestations before they become established.
Technical Abstract: Successful plant invasions require both the founding and local spread of new populations. High plant densities occur only when founding plants are able to disperse their seeds well locally to quickly colonize and fill the new patch. We test this ability in a seven-year field experiment with Carduus acanthoides, an invasive weed in several North American ecosystems. Founder plants were planted in the center of 64m2 plots and we monitored the recruitment, distribution pattern, mortality and seed production of the seedlings that originated from these founding plants. Competing vegetation was clipped not at all, once, or twice each year to evaluate the importance of interspecific competition. More seedlings recruited in the intermediate once-clipped plots, and these seedlings also survived better. The control plots had fewer microsites for seedling recruitment; clipping a second time in September stimulated grasses to fill up the gaps. The number of C. acanthoides recruits and their median distances from the founder plants were also explained by the initial seed production of the founding plants. Overall, the experiment shows that the success of founder plants can fluctuate strongly, as 55% of the plots were empty by the sixth year. Our study suggests that the local invasion speed following initial establishment depends strongly on both the propagule pressure and availability of suitable microsites for seedling recruitment and growth.