Submitted to: The American Naturalist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/13/2007
Publication Date: 5/15/2008
Citation: Franks, S.J., Pratt, P.D., Dray Jr, F.A., Simms, E.L. 2008. Selection on herbivory resistance and competitive ability traits in an invasive plant. The American Naturalist, Vol. 171, No. 5, pp. 678-691. Interpretive Summary: We tested a hypothesis (the evolution of increased competitive ability, or EICA, hypothesis) relating to plants that are introduced into areas they haven’t previously occupied. The hypothesis argues that such plants are released from natural enemies found in the plants’ native ranges. Because such plants no longer have to defend themselves from many/most natural enemies, they are free to redirect energy from defense into growth. We obtained seeds of the Melaleuca tree from Australia (its native range) and Florida (its introduced range) and planted them in the same experimental plot. Some of these seedlings were treated with insecticides and some were not, and biological control insects were allowed to colonize the plot. Comparisons of the patterns of plant growth (a measure of competitive ability) showed some support for this hypothesis. However, comparisons of traits that help these plants resist insect attack failed to support the hypothesis. We also might have expected seedlings from the Florida stock to show evidence of rapid evolutionary changes in traits related to resisting insect attack, but hey did not. Overall, the data from this study suggest that the evolutionary processes we investigated are more complex than proposed by the EICA hypothesis, and that biological control workers should take this into account in their studies.
Technical Abstract: Invasive species face different conditions in their new range, which may lead to evolutionary change. The evolution of increased competitive ability (EICA) hypothesis proposes that invasive species evolve decreased defense and increased growth rate and competitive ability following introduction. We used quantitative genetics and selection analysis on data from a field experiment to test this hypothesis and examine evolutionary potential in the invasive plant Melaleuca quinquenervia. There was selection for increased stem elongation, and the intensity of selection was stronger in the absence of herbivores and stronger for the home range genotypes, as predicted. We found selection for increased resistance for the home range genotypes but no selection on resistance for the introduced range genotypes, contrary to EICA. There was a negative genetic correlation between resistance and competitive ability. There was no genetic variation in plasticity (GxE) for any trait nor difference in plasticity between home and introduced range genotypes, indicating that increased plasticity does not seem to have evolved in Melaleuca since its introduction. Thus, patterns of selection on competitive ability but not resistance match EICA predictions, and plasticity to herbivory does not seem to explain trait expression or evolution in Melaleuca.