Effects of interspecific competition on early life history of the invasive thistles carduus nutans and C. acanthoides

Establishment of many invasive species, including the Eurasian thistles Carduus nutans and C. acanthoides, is believed to be promoted by disturbance. Prior research suggests that emergence and early survival of these two weeds is far higher on bare ground than in areas with interspecific competition, but this relationship has not been well quantified. We studied the effects of microsite size and watering regime on the emergence and early survival of these invasive thistles. Microsite competition was found to significantly negatively affect successful emergence and first-month survivorship of both species, while additional water was not found to significantly increase seedling emergence. These findings suggest that adequate bare ground is an important predictor of successful thistle establishment, and thus that conservation practices aimed at reducing available microsites should help to limit the invasion of these two species.