Invasive plant species have many negative impacts on native ecosystems; however, the mechanisms by which non-native species are able to successfully invade are not fully understood.  The hypothesis tested here is that annual invasive species have distinct life history and physiological traits that are linked to their success as invaders.  The traits of an invasive non-native plant species (Erodium cicutarium) was compared with two classes of co-occurring native species, a weedy (plants that readily colonize and dominate over a landscape) and a non-weedy native (Amsinckia menziesii and Phacelia cicutaria respectively).  This study was conducted at Wind Wolves Preserve in the southern Central Valley, California, which is a semi-arid winter rain fall region, i.e. it has a Mediterranean-type climate.  Measurements began in the fall 2008 and will end the fall 2009.  We predicted the photosynthetic rates, population growth rates, maximum population density, and reproductive output will be greatest for the non-native invader (NNI) followed by the weedy native (WN) and last, the non-weedy native (NWN).

Results/Conclusions

The results for population growth rate and density support the prediction, i.e. the NNI>WN>NWN.  The differences in maximum population densities were dramatic (Means ± 1SE were NNI= 387.11± 92.8, WN= 96.0± 27.6, and NWN= 17.1± 5.9 individuals/m²).  Wintertime photosynthetic rates did not support our predictions and the pattern for maximum net carbon assimilation rates (A_net) was WN>NWN>NNI (Means ± 1SE wereWN= 26.85± 2.17, NWN= 22.35± 2.59, and NNI= 8.05± 2.07).  The low rates of A_net may have been partially due to stomatal limitation (stomatal conductance means ± 1 SE were WN= 0.456± 0.04, NWN= 0.411± 0.03, and NNI= 0.285± 0.03 mol m^-2s^-1).  The strategy of the NNI E. cicutarium appears to be one of early establishment followed by conservative carbon assimilation and water use during the cool winter wet season.  This may lead to a slower depletion of soil moisture, a protracted growing season, and greater reproductive output in sites that are low in competitors, such as disturbed areas.