To further our understanding of biological invasions, it is important to evaluate hypothesized causal factors facilitating invasions using manipulative field experiments. Disturbance and density are frequently invoked as major factors operating during the process of plant invasions. Soil disturbance by mowing can increase nutrient and light availability while intraspecific competition can reduce potential invader success. Disturbance can therefore reduce the negative effects of density, however the interaction between these effects on invaders have rarely been tested. We studied the combined effect of intraspecific density and soil disturbance on the population dynamics of Centaurea stoebe, an invader in North America. We used an experimental field study of spotted knapweed established in Stony Brook, NY, to parameterize matrix models for four combined disturbance and density treatments. We conducted elasticity analysis to investigate the potential influence of demographic variables on population growth rate. We also used retrospective analysis to identify the actual influence of these variables on the population growth rate of plants in our experimental study. We found higher differences in lambda between disturbance treatments compared to between density treatments. LTRE analysis showed that the increased lambda of spotted knapweed in response to disturbance was due to substantial increases in survival of vegetative plants and reproduction. Prospective elasticity analysis revealed that lambda was most sensitive to small changes in the transition of vegetative plants to reproductive plants, and the production of vegetative plants from reproductive plants. Our results show that the increased growth rate response of spotted knapweed populations to disturbance occurs irrespective of density, and emphasizes the importance of reducing reproduction in the management of spotted knapweed.
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