A test of the EICA hypothesis using French broom (Genista monspessulana)

Several hypotheses have been proposed to account for the success of non-indigenous plant species in their introduced range.  The EICA hypothesis proposes that the low abundance or absence of herbivores in new environments may drive selection for genotypes with increased allocation to growth and reproduction compared to those in the native range.  We tested this hypothesis using French broom (Genista monspessulana), a woody leguminous shrub native to Europe, which is currently spreading throughout California.  Seeds of G. monspessulana were collected from five populations in both its native and introduced (California) range and were used to conduct two comparative studies.  In the first study, seeds were weighed to assess variation in seed mass at the plant, population, and range-levels.  In the second study, plants were grown in a common-greenhouse experiment to determine whether populations differed in growth response.  Results of both studies suggest that resource allocation towards seeds and early growth in G. monspessulana is greater in individuals from the introduced compared to the native range.  Population-level variation in several growth parameters was also lower in the introduced versus native range.  A better understanding of the factors that drive population-level variability across the native and introduced range of G. monspessulana may improve our ability to predict impacts associated with its spread and aid the development of more effective management strategies.