Southern California is home to one of the most dramatic ecological invasions worldwide:, the replacement of native perennial grasses by exotic annual grasses and forbs in 9.2 million hectares of grasslands. Understanding the mechanisms driving this widespread invasion is essential to the restoration of native ecosystems. One hypothesis is that invasive populations evolve in their new habitats, allowing them to achieve greater size and competitive ability. Bromus rubens is an exotic annual grass native to Spain that has become a problematic invader in Southern California since its introduction less than 300 years ago. We conducted a 10 week greenhouse experiment to evaluate the hypothesis that B. rubensevolved greater competitive ability since its arrival in California. We grew individuals from seed collected from two populations, California (invasive) and Spain (native), and measured for traits (above and below-ground resource allocation).
Results/Conclusions
Above ground biomass of B. rubens was significantly higher in the invasive California population. There was no significant difference in root biomass between the two populations. However, the invasive California population had a significantly lower root:shoot ratio compared to the native Spanish population. Because B. rubens is already highly invasive in Southern California, these results support our hypothesis that invasive populations are able to achieve larger size and competitive ability in their invaded range. These results show an evidence of evolutionary change over a short timescale. Further studies potentially investigating the underlying selective pressures that differ between the native and invasive populations will contribute to further understanding of correlations between rapid evolution and invasive success.