Print PageMany invasive species exhibit more vigorous growth habits in introduced relative to native ranges. The classic explanation for such divergence in growth habits between ranges is the Enemy Release Hypothesis (ERH), which posits that introduced species benefit from an absence of coevolved enemies. A corollary of the ERH is the Evolution of Increased Competitive Ability (EICA) hypothesis, which proposes that upon release from enemies introduced species will shift resources from defense to increased growth – a trade-off that decreases enemy resistance in return for a competitive advantage. If selection for such a trade-off has occurred in introduced ranges, then these genotypes should outperform native genotypes in growth rate and allocation patterns. To test this prediction of the EICA, we compared the rate of C-acquisition (photosynthetic performance) and biomass allocation patterns of Bromus rubens(red brome) populations from Spain (native range) and California (introduced range) grown under common conditions.
Results/Conclusions
Bromus rubens from the introduced population were significantly larger than those of the native range. The total biomass and shoot height of the Californian population were 45% and 26%, respectively, larger than the Spanish population (total biomass: F = 15.23, P < 0.0001, shoot height: F = 39.23, P < 0.0001). However, the Spanish population showed 38% greater photosynthetic performance (F = 7.01, P < 0.01), and 12% greater root:shoot biomass ratios. This divergence in allocation patterns may reflect adaptations to the introduced range. For example, carbon sequestration in roots may be unnecessary in the absence of enemies, whereas greater photosynthetic performance and storage may be traits vital to survival in their presence. These results suggest that introduced genotypes of B. rubens experience selective pressures in novel environments that could contribute to differences in habitat distribution, abundance, and invasiveness.
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