Natural disturbances affect invasion resistance in a California grassland

Background/Question/Methods   California grasslands have experienced a widespread invasion of non-native annual grasses; however, in spite of this invasion remnant patches of native grassland communities still persist. These native communities may resist invasion due to increased species diversity; however, natural disturbances such as fire and drought may alter the ability of native communities to resist invasion. In this study, we used naturally occurring patches of native and non-native dominated grassland communities to evaluate how disturbance factors (drought and fire) may affect the invasion resistance of both native and non-native dominant communities. We hypothesized that 1) non-native grasslands, regardless of diversity, would resist invasion, 2) high diversity native grasslands would resist invasion after a disturbance and 3) low diversity native grasslands would not resist invasion after a disturbance and convert to non-native grasslands. We followed paired communities of native and non-native dominated grasslands at 8 sites in Orange County, California, from 2007-2010. During this time, the communities experienced a severe drought in 2007 and a natural wildfire in 2008. We compared how the different communities were able to recover and resist invasion after these natural disturbances by measuring species abundance and productivity.

Results/Conclusions   We found that drought negatively affected productivity for both non-native and native grasslands; however, fire only marginally decreased productivity for both communities. Regardless of disturbance, non-native communities converged in species composition and were dominated by Avena fatua, as hypothesized. However, native dominated communities responded differently to the disturbances and did not uniformly converge in species composition. After the drought, native communities all converged to a community dominated by Nassella pulchra; however, after the fire, native community composition depended on the abundance of Nassella pulchra, where communities high in its abundance had lower non-native grass abundance. While high diversity communities maintained their diversity, diversity did not play a role in the native community's resistance to invasion, contrary to our hypotheses. These results illustrate the high resistance of non-native dominated California grasslands and the variable invasion resistance of remnant native dominated grasslands. Additionally, these results suggest that the resilience of native grasslands may be buffered by the abundance of one dominant species rather than species diversity. Furthermore, multiple disturbances create complex dynamics which may make land management challenging; however long-term monitoring of areas may elucidate factors that can help predict community responses and maintain native dominant communities.