NO SIMPLE TRADEOFFS: CENTAUREA PLANTS FROM AMERICA ARE BETTER COMPETITORS AND DEFENDERS THAN PLANTS FROM THE NATIVE RANGE

Two non-mutually exclusive hypotheses for invasive success have important evolutionary implications. The \u27natural enemies\u27 hypothesis posits that exotic invaders explode in abundance because they are not suppressed by specialist herbivore consumers in their invaded range. The \u27novel weapons\u27 hypothesis posits that exotic invaders explode in abundance because they possess biochemicals that are more effective against evolutionarily naïve plants, microbes, and generalist herbivores than against those species that have evolved tolerance in their communities of origin. I explored the potential for novel allelopathic or herbivore defense biochemicals as a potential alternative mechanism to tradeoff-driven evolution of increased competitive ability in invasive plants by comparing growth, reproduction, competitive effect and response, and defense capabilities of invasive North American populations of Centaurea maculosa to populations in Europe, where the species is native. I found that Centaurea from North America were larger, but produced fewer flowers than plants from European populations. North American Centaurea demonstrated much stronger competitive effects and responses than European Centaurea against North American grasses. Importantly, competitive superiority did not appear to come at a cost to herbivore defense. North American Centaurea genotypes were better defended against specialist and generalist consumers, and showed both a stronger inhibitory effect on the consumers (resistance) and a better ability to grow in response to attack by herbivores (tolerance). Better defense by North Americans corresponded with higher constitutive levels of biochemical defense compound precursors, tougher leaves, and more leaf trichomes than Europeans. North American F1 progeny of field collected lines retained the traits of larger size and greater leaf toughness suggesting that genetic differences, rather than maternal effects, caused the intercontinental differences. My results indicate that the evolution of increased competitive ability may not always be driven by simple physiological tradeoffs between the allocation of energy or resources to growth or to defense. Instead, I hypothesize that new plant neighbors and generalist herbivores encountered by Centaurea in its invaded range appear to exert strong directional selection on the weed\u27s competitive and defense traits