|D Antonio, Carla|
Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2003
Publication Date: 5/1/2004
Citation: Corbin, J.D., D Antonio, C.M. 2004. Competition between native perennial and exotic annual grasses: implications for a historic species invasion. Ecology. 85(5):1273-1283. Interpretive Summary: California grasslands underwent a largescale conversion from native perennial grasses to exotic annual species during the late 1800s. Debate has since ensued as to the causes of this conversion with some people advocating severe disturbances such as inappropriate levels of livestock grazing and/or severe drought while others believe that it was solely the presence of exotic annual grasses that led to this change. Using a competition experiment with exotic annual grasses and native perennial grasses planted alone and together, we found that native perennial grasses were suppressed by exotic annuals the first year when they were small. However, after this time, those native perennial grasses that survived were themselves very good at suppressing the exotic annuals. Regardless of putting thousands of annual seeds into the native perennial plots each year, we saw very little growth and establishment of exotic annuals after that first year. Our data suggest that native grasses, once established, are good competitors against annuals so something must have had to disrupt the incumbency of the native perennials in order for exotic annuals to have become so abundant.
Technical Abstract: Although established populations of invasive species can exert substantial competitive effects on native populations, exotic propagules may require disturbances that decrease competitive interference by resident species in order to become established. We compared the relative competitiveness of native perennial and exotic annual grasses in a California coastal prairie grassland to test whether the introduction of exotic propagules to California grasslands is the 19th century was likely to have been sufficient to shift community composition from native perennial to exotic annual grasses. Under experimental conditions, we compared the aboveground productivity of native species alone to native species competing with exotics, and exotic species alone to exotic species competing with natives. Over the course of the four year study, native grasses became increasingly dominant in the mixed assemblage plots containing natives and exotics. Although the competitive interactions in the first growing season favored the exotics, over time, the native grasses became well established and significantly reduced the productivity of the exotic grasses. The number of exotic seedlings emerging and the biomass of dicot seedlings removed during weeding were also significantly lower in plots containing natives as compared to plots that did not contain natives. We found evidence that the ability of established native perennial species to limit space available for exotic annual seeds to germinate and light available to exotic seedlings reduced exotic productivity and shifted the competitive interactions in favor of the natives. Given the demonstrated competitiveness of natives, the introduction of exotic grass propagules alone, without changes in land use and/or climatic conditions, was likely insufficient to convert the region's grasslands.