Invasive species are highly detrimental to economies and to ecosystems. To counter the negative effects of invasive plant species, a better understanding of the mechanisms driving their dominance and success is necessary. The factors of competition, herbivory, and seed source were examined using two plant species invasive to California- Brassica nigra and Foeniculum vulgare. It was found that B. nigra is affected by each factor individually but not by any interactions between the factors. F. vulgare was influenced by an interaction between competition and herbivory. Neither species appears to have evolved since its original invasive attempts