Evolution of growth but not structural or chemical defense in Verbascum thapsus (common mullein) following introduction to North America

Authors: ALBA, CHRISTINA - Colorado State University; BOWERS, DEANE - University Of Colorado; Blumenthal, Dana; HUFBAUER, RUTH - Colorado State University

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/2011
Publication Date: 8/11/2011
Citation: Alba, C., Bowers, D., Blumenthal, D.M., Hufbauer, R. 2011. Evolution of growth but not structural or chemical defense in Verbascum thapsus (common mullein) following introduction to North America. Biological Invasions. 13:2379-2389.

Interpretive Summary: Post-introduction evolution of increased growth or reproduction may contribute to the success and therefore the impact of invasive plant species. However, increased growth and reproduction are not consistently associated with a loss of defense, as predicted by the influential evolution of increased competitive ability (EICA) hypothesis. Inconsistent support for the EICA hypothesis likely reflects the fact that, although invasive plants are released from attack by some enemies, typically specialists, they often do not escape attack from generalists. We found that introduced populations of Verbascum thapsus (common mullein) had significantly greater shoot biomass than natives, but remarkably similar investment in both chemical defenses (effective against generalists) and structural defenses (effective against specialists). Furthermore, despite significant population-level variability for all defenses, there was little evidence of trade-offs between growth and defense or among different types of defense. These results suggest that evolution of increased growth in V. thapsus is not fueled by decreased allocation to defense, and that selection on defense may vary more at the population scale than the continental scale.

Technical Abstract: Post-introduction evolution of increased growth or reproduction has been observed in many species of invasive plants; however, it is not consistently associated with a loss of defense, as predicted by the influential evolution of increased competitive ability (EICA) hypothesis. Inconsistent support for the EICA hypothesis likely reflects the fact that, although invasive plants are released from attack by some enemies, typically specialists, they often do not escape attack from generalists. Thus, different types of defense (e.g., structural versus chemical) may evolve in different directions following introduction. We used a common garden experiment to test whether a shift in allocation among defenses (as opposed to a simple increase or decrease in a single defense) is associated with increased growth in introduced Verbascum thapsus populations. Introduced populations had significantly greater shoot biomass than natives. However, root biomass was similar between ranges, and highly variable, resulting in only marginal differences in total biomass. Mean investment in all three defenses was remarkably similar between the native and introduced populations, providing no evidence for post-introduction evolution of defense. This finding was consistent with the fact that, despite significant population-level variability for all defenses, there was little evidence of trade-offs between growth and defense or among different types of defense. These results suggest that evolution of increased growth in V. thapsus is not fueled by decreased allocation to defense, and that selection on defense may vary more at the population scale than the continental scale.