Environmental resistance mediates propagule pressure in a novel plant community.

Invasive species success in novel habitats is shaped by the interplay between propagule pressure and environmental resistance. Although high propagule pressure often drives invasion, establishment can fail when strong resistance limits new recruits. Disturbance may reduce resistance by disrupting resident communities and increasing resource availability especially for fast-growing species. To test the generality of these dynamics, Oxalis stricta L. was used as a model invader and turfgrass assemblages as generalized low-diversity systems common in anthropogenic landscapes. In a controlled glasshouse experiment, O. stricta and manipulated environmental resistance were introduced through variation in plant cover, soil moisture, and nutrients. Despite producing more than 400,000 seeds in 17 weeks, O. stricta was almost entirely excluded from vegetated trays (< 2% cover, no reproduction), whereas cover in bare soil often approached 100%. After colonization, seedpod production plateaued at ~ 75% cover, consistent with negative density dependence. Supplemental watering increased O. stricta growth in bare soil, whereas fertilization had little direct effect but shifted turfgrass competition which further suppressed invasion. These results suggest that even extreme propagule pressure is insufficient to overcome strong environmental resistance. At the same time, the ability of O. stricta to self-pollinate and persist in disturbed microsites indicated that minimal founder populations may still establish under fluctuating conditions.