Thesis

Are non-native invasive species more phenotypically plastic than co-occurring native species?

It is essential to understand the conditions that enable invasive plant species to be one of the greatest threats to biodiversity. Phenotypic plasticity can be an important contributor to invasiveness, but evidence in the literature is not consistent. Few studies have examined whether invasive species with higher capacities for plasticity than native species contribute to invaders possessing fitness advantages over natives. Furthermore, environmental factors including elevated soil nitrogen levels due to deposition from air pollution have been found to facilitate invasion success. A common garden experiment was performed to compare plasticities in performance and fitness related traits of two non-native invasive species (Hirschfeldia incana and Erodium cicutarium) and six co-occurring native California species (Eschscholzia californica, Camissoniopsis bistorta, Amsinckia intermedia, Calandrinia ciliata, Plantago erecta, and Lupinus microcarpus) in a range of soil nitrogen conditions (control 0 g/L, low 0.4 g/L, medium 1.4 g/L, and high 2.3 g/L; n=30 for each species in each nitrogen treatment). Both invaders had greater phenotypic plasticity than the six native species. However, all of the native species, except L. microcarpus, were also relatively plastic. Interestingly, H. incana and E. cicutarium demonstrated greater plasticity in traits associated with plant size. Excluding L. microcarpus, all species exhibited high capacities for plasticity in the number of flowers produced in the nitrogen treatment gradient. My results indicate that the invasiveness of H. incana and E. cicutarium is related to a high capacity for phenotypic plasticity found in performance traits that lead to increases in plant size, and that this gives them an ecological advantage over co-occurring plant species. However, this edge may be short-lived. A great decline of survival in the medium and high nitrogen treatments, among a majority of the species, suggests that even species with a plastic advantage will experience reductions in survival when nitrogen levels become too high. With nitrogen levels continuing to increase, this could have a detrimental effect on plant communities with changes in plant compositions including declines in species richness. Even the newly established competitive species will decline in numbers if the nitrogen levels become so high that they are unable to survive in such extreme conditions.

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