AbstractSuccessful colonization of newly introduced species is driven by a multitude of factors and is highly dependent on the species. It has long been hypothesized that preadaptation and postestablishment natural selection of introduced species can facilitate their invasion; however, to date, limited research has been dedicated to these theories. In addition, although the correlation between establishment of invasive species and disturbance has been noted and widely studied, the susceptibility of undisturbed habitats to invasion remains unclear. In California, serpentine habitats are severe edaphic environments that have been relatively free of anthropogenic disturbance and nonindigenous species invasions. In this study, we documented the occurrence of the nonindigenous barb goatgrass on serpentine and nonserpentine grasslands in the California Northern Interior Coast Range and the western foothills of the Sierra Nevada Mountains and conducted greenhouse and field experiments to assess the species' degree of adaptation to serpentine soils. Reconnaissance of serpentine intrusions and yearly monitoring suggest that barb goatgrass may grow preferentially on serpentine soil, particularly disturbed serpentine sites. In the greenhouse, for most measures of performance, serpentine populations performed better than nonserpentine populations when grown on serpentine soil. Particularly noteworthy was that serpentine populations had higher root-mass ratios than nonserpentine populations when grown on serpentine soil. In contrast to the greenhouse study, field-grown populations from serpentine and nonserpentine sources performed equally well on nonserpentine; alluvial, disturbed serpentine; and shallow, undisturbed serpentine, although the overall species' performance was diminished on severe serpentine soils. Alarmingly, even in the absence of previous exposure to serpentine, barb goatgrass was capable of establishing and spreading into minimally disturbed sites with strong serpentinitic characteristics.
