Can more water make life harder for native shrub seedlings when invasive grass is present?

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Predicting how species respond to climate change requires understanding both the direct effects of environmental change and indirect effects caused by changes in interactions between species. Those indirect effects may be particularly important for native species where highly competitive exotic species have invaded.

Mediterranean climate regions are predicted to experience more frequent drought and are increasingly invaded by exotic annual plants. Native shrub seedlings in these regions establish episodically, in association with high-rainfall years. But exotic annual plants also often become more abundant with high rainfall, suggesting that their competition could change the relationship between rainfall and shrub seedling establishment.

Theories such as the stress gradient hypothesis predict that competition should become more intense as resource supply increases. However, few studies have evaluated competitive interactions across experimentally created gradients of soil moisture availability.

This experiment examined how competition from an exotic annual plant affected native shrub establishment across an experimental soil moisture gradient. Seedlings of two native shrub species, Encelia californica and Eriogonum fasciculatum, were grown with and without the exotic grass competitor Avena fatua at eight water availability levels. Researchers monitored seedling growth and survival. These species are common and abundant in the Mediterranean climate region of coastal Southern California, where climate change projections include long-term drought over the coming decades.

Without competition, shrub seedlings had higher growth and survival at high water availability. With the exotic grass competitor, however, shrub seedlings had higher growth and/or survival under relatively dry conditions. This suggests that competition can modify, and even reverse, species responses to changing rainfall patterns compared with predictions made without competitors.

When planted with the native shrub seedlings, the exotic grass strongly reduced soil nitrate and water availability. The grass itself grew more as water addition increased, but its growth response saturated.

Synthesis: The experiment demonstrates that competition from invasive species can alter native species responses to climate change. The findings are consistent with ecological theories predicting a positive association between the supply of limiting resources and the intensity of competition.