Resource availability versus evolutionary change as drivers of growth, defense, and invasiveness in Alliaria petiolata

The ecological impact of invasive species and their utility as model systems for investigating evolutionary dynamics make understanding invasion of great importance. In this dissertation, I investigated geographic patterns of resource allocation to defense, growth, and reproduction in an invasive biennial, Alliaria petiolata, to test the hypothesis that evolutionary change in allocation patterns has contributed to this species' invasiveness. I utilized a combination of approaches to address this question. I studied field populations in the native and invasive ranges of this species to investigate variation in growth, reproduction, herbivory damage, and investment in the putative defense compound, sinigrin. I also performed two common garden experiments, in the native and invasive ranges, respectively, in which I grew plants from each range together in common environments. In these experiments, I assessed local resource availability and distinguished between constitutive and inducible defenses and herbivory-related competitive ability through herbivore exclusion treatments. Lastly, I investigated the role of current and maternal light environment on sinigrin production, as A. petiolata occurs in a wide range of light habitats, and light availability has been shown to affect resource allocation in plants. The data presented in this dissertation refute previously proposed mechanisms for producing invasiveness in introduced plant species through reallocation of resources in response to release from herbivory. The combination of experiments I performed showed that although differences between native and invasive populations can be observed with regard to allocation to defenses and competitive ability, these patterns are due to variable environmental factors that differ between the native and introduced ranges. My work also showed that both multiple years and multiple life stages should be investigated in studies of geographic patterns of defense chemistry. My work suggests that A. petiolata does not show any consistent, evolutionarily-produced change in defense chemistry against herbivory, nor does it show greater overall competitive ability in its introduced range compared to its home range. The success of this species where introduced appears to be a result of environmental conditions, and perhaps to the relatively low herbivore load it experiences compared to native species, but not to evolutionary responses to that low herbivory.