Yellow starthistle represents one of the most spectacular examples of biological invasion in the western United States. However, the mechanisms leading to its success have not been clearly elucidated. Although its success has been attributed to superior competitive ability, few competition studies have been performed with yellow starthistle to test this assertion. Yellow starthistle and wild oat (a dominant component of California annual grasslands) were grown in monocultures and mixtures to assess the strength of competitive interactions between them. For either species, intraspecific competition exerted a greater influence over mean plant weight than did interspecific competition. A companion study revealed temporal separation in the phenology of these plants, explaining the weak role of interspecific competition. Additional measurements of growth and soil moisture dynamics in large 270-cm-tall by 50-cm-diam polyvinyl chloride columns also showed a lack of interspecific competition and confirmed that water use patterns differed between these species, indicating niche partitioning. Wild oat reduced soil moisture to 5% but only to a depth of approximately 150 cm. Yellow starthistle depleted soil moisture to less than 5% throughout the column to a depth of at least 270 cm. These patterns were present when wild oat and yellow starthistle were grown individually or together in the columns, indicating that yellow starthistle had a greater impact on soil moisture and to greater depths. Yellow starthistle's invasion of grasslands in California does not appear to be due to superior competitive ability, but may be due to its ability to access deeper soil moisture. These results support the empty niche hypothesis that implies that invasive species are successful in new habitats because they access resources not available to resident species.
Nomenclature: Yellow starthistle; Centaurea solstitialis L. CENSO; wild oat; Avena fatua L.
Interpretive Summary: Yellow starthistle has invaded California grasslands within the last 100 yr. The mechanism for this successful invasion is not known for certain, even though it would be useful in managing this troublesome weed. We grew yellow starthistle in competition experiments with wild oat. The results indicate that yellow starthistle is not a strong competitor in relation to wild oat. Soil water measurements indicate that yellow starthistle used water from deeper in the soil profile than wild oat, suggesting that these species avoid competition in this way. Results from this study support the “empty niche hypothesis,” which implies that yellow starthistle has successfully invaded California grasslands because there are unused resources, i.e., deep soil water, available to it in these systems. These results support the idea that controlling yellow starthistle without establishing a plant community that also utilizes this deep soil water will not provide a long-term solution to its invasion.