Invasive species represent one of the most important components of global change. Two introduced woody legumes originally from Europe, Genista monspessulana L. and Cytisus scoparius L., are among the most invasive plants in the western U.S. Invasive plants are often more vigorous in introduced than in native regions. I compared the population density and performance characteristics of field populations of G. monspessulana finding that introduced populations in California were more vigorous, produced more seeds per pod, and had higher population densities than native populations in Europe. Seed rain and seed bank size were also higher in California.

The increased vigor of invasive plants may be explained by the Evolution of Increased Competitive Ability (EICA) hypothesis, which predicts that release from herbivory will select for individuals that reallocate resources from defense to growth and reproduction. I tested the EICA predictions that G. monspessulana in California has (1) evolved reduced resistance or tolerance to herbivory, and (2) evolved to grow more vigorously than European plants. I found no difference in the performance of the herbivorous psyllid Arytinnis hakani between Californian and European plants, suggesting no reduction in resistance to herbivory in Californian plants. However, Californian plants showed evidence of possible overcompensation to A. hakani herbivory, unlike European plants, suggesting a reduction in tolerance to herbivory and partial support for the EICA hypothesis in explaining the success of G. monspessulana in California. I detected no difference in plant growth between Californian and European plants grown under identical conditions, suggesting that the increased vigor of Californian field populations was not due to post-invasion evolution, but to a phenotypic response to environmental conditions.

Integrated management strategies may be more effective in controlling invasive brooms than individual control techniques. I evaluated whether biological control-alone (seed predation by Exapion fuscirostre) or in combination with either fire or mowing enhanced control of C. scoparius. While both integrated management strategies outperformed biological control-alone in reducing seed production and the seed bank of C. scoparius, fire in combination with biological control may prove to be the more effective strategy due to its slightly greater potential to deplete the seed bank.