Invasive species researchers often ask: Why do some species invade certain habitats while others do not? Ecological theories predict that taxonomically related species may invade similar habitats, but some related species exhibit contrasting invasion patterns. Brassica nigra, Brassica tournefortii, and Hirschfeldia incana are dominant, closely related nonnative species that have overlapping, but dissimilar, distributions. Brassica tournefortii is rapidly spreading in warm deserts of the southwestern United States, whereas B. nigra and H. incana are primarily limited to semiarid and mesic regions. We compared traits of B. tournefortii that might confer invasiveness in deserts with those of related species that have not invaded desert ecosystems. Brassica tournefortii, B. nigra and H. incana were compared in controlled experiments conducted outdoors in a mesic site (Riverside, CA) and a desert site (Blue Diamond, NV), and in greenhouses, over 3 yr. Desert and mesic B. tournefortii populations were also compared to determine whether locally adapted ecotypes contribute to desert invasion. Experimental variables included common garden sites and soil water availability. Response variables included emergence, growth, phenology, and reproduction. There was no evidence for B. tournefortii ecotypes, but B. tournefortii had a more rapid phenology than B. nigra or H. incana. Brassica tournefortii was less affected by site and water availability than B. nigra and H. incana, but was smaller and less fecund regardless of experimental conditions. Rapid phenology allows B. tournefortii to reproduce consistently under variable, stressful conditions such as those found in Southwestern deserts. Although more successful in milder, mesic ecosystems, B. nigra and H. incana may be limited by their ability to reproduce under desert conditions. Rapid phenology and drought response partition invasion patterns of nonnative mustards along a gradient of aridity in the southwestern United States, and may serve as a predictive trait for other potential invaders of arid and highly variable ecosystems.
Nomenclature: Black mustard, Brassica nigra (L.) Koch; Sahara mustard, Brassica tournefortii Gouan; shortpod mustard, Hirschfeldia incana (L.) Lagr.-Foss.
Interpretive Summary: Sahara mustard (Brassica tournefortii), black mustard (Brassica nigra), and shortpod mustard (Hirschfeldia incana) are common invasive mustards throughout parts of the western United States. In particular, all three species are found in southern California, but they have different distributions. Brassica tournefortii is primarily an invader of warm desert ecosystems, black mustard is most common in more mesic regions, and shortpod mustard is most common in the arid Mediterranean-climate regions between the coast and warm deserts. We hypothesized that B. tournefortii's adaptations to arid North Africa might predispose it to successfully invade North America's arid, warm-desert ecos