Species Composition and Diversity Affect Grassland Susceptibility and Response to Invasion

In a microcosm experiment, I tested how species composition, species richness, and community age affect the susceptibility of grassland communities to invasion by a noxious weed (Centaurea solstitialis L.). I also examined how these factors influenced Centaurea’s impact on the rest of the plant community. When grown in monoculture, eight species found in California’s grasslands differed widely in their ability to suppress Centaurea growth. The most effective competitor in monoculture was Hemizonia congesta ssp. luzulifolia, which, like Centaurea, is a summeractive annual forb. On average, Centaurea growth decreased as the species richness of communities increased. However, no polyculture suppressed Centaurea growth more than the monoculture of Hemizonia. Centaurea generally made up a smaller proportion of community biomass in newly created (‘‘new’’) microcosms than in older (‘‘established’’) microcosms, largely because Centaurea’s competitors were more productive in the new treatment. Measures of complementarity suggest that Centaurea partitioned resources with annual grasses in the new microcosms. This resource partitioning may help to explain Centaurea’s great success in western North American grasslands. Centaurea strongly suppressed growth of some species but hardly affected others. Annual grasses were the least affected species in the new monocultures, and perennial grasses were among the least affected species in the established monocultures. In the new microcosms, Centaurea’s suppression of competing species marginally abated with increasing species richness. This trend was a consequence of the declining success of Centaurea in species-rich communities, rather than a change in the vulnerability of these communities to suppression by a given amount of the invader. The impact of the invader was not related to species richness in the established microcosms. The results of this study suggest that, at the neighborhood level, diversity can limit invasibility and may reduce the impact of an invader.