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Carduus coexistence

My dissertation research investigated the interactions between two congeneric invasive species, Carduus nutans and C. acanthoides. Although many invasive species studies look at a native-invasive pair, invaders frequently interact with other invaders. This can be a negative effect (invasional interference), a neutral effect or a synergistic effect, leading to invasional meltdown. Given limited budgets for management, it is critical to understand which areas are likely to suffer invasional meltdown, or where a new invasion may have little impact due to invasional interference. In the case of Carduus thistles in Pennsylvania, it appears that the two species experience competitive equivalence leading to invasional interference. Areas already invaded by one are unlikely to suffer additional harm from the invasion of the second (Rauschert and Shea, in review). In terms of the spatial patterns their interactions create, we found that this is scale dependent, with negative regional association, positive field-scale association, and negative plot-scale associations, underscoring the importance of explicitly considering scale in ecological studies (Rauschert et al. Biological Invasions in press)