This paper reviews the biology of two closely related Mediterranean annuals, yellow starthistle and common crupina, which have invaded grassland, shrub steppe, and open woodland habitats in western North America. Despite the similarity of their winter annual life cycle, the two species differ significantly in population dynamics. Common crupina has traits that favor persistence rather than rapid population growth: large, heavy achenes with an after-ripening requirement; lower fecundity but higher germination success; and reproduction regulated by vernalization and photoperiod in addition to thermal time. Persistence traits also foster invasion of undisturbed or less degraded steppe habitats. Yellow starthistle has more ruderal traits: small, light, rapidly germinating achenes; higher fecundity, with greater seedling mortality; and reproduction that is less sensitive to photoperiod and vernalization. These characteristics confer a greater adaptability for rapid spread and colonization of disturbance by yellow starthistle than by common crupina. An understanding of the relative differences in biological characters of each species and in their function in invaded environments is relevant to ecological management of these pest species.
Nomenclature: Common crupina, Crupina vulgaris Cass., CJNVU; yellow starthistle, Centaurea solstitialis L., CENSO.