The Ecology and Evolution of an Invasive Perennial Plant (Lythrum salicaria) in the Context of Biological Control by Specialist Herbivores (Galerucella spp.)

The introduction of non-native species to novel ranges has provided biologists the opportunity to study organisms experiencing sudden and sustained shifts in community composition and selection pressure. Management programs for invasive species can result in similar shifts. In classical biological control programs, non-native species are introduced to control invasive species. The short-term impacts of these introductions on invaded communities are often well documented, but the long-term impacts on the ecology and evolution of target invasive species are not well understood. Using a combination of field surveys, common garden techniques, and quantitative genetic models, I examined the effect of biological control by specialist herbivores (Galerucella spp.) on purple loosestrife (Lythrum salicaria L.) in southern Minnesota (MN), USA. Chapter 1 describes field surveys completed to determine the extent to which biological control directly reduces plant vigor, competitive performance, and reproduction. In Chapter 2, I quantify the extent to which population level variation in vigor, competitive performance, herbivore defense, and herbivore tolerance of L. salicaria reflects an evolutionary divergence following the introduction of biological agents. Finally, in Chapter 3 I assess the potential for L. salicaria to evolve in response to continued selection pressure. The results of these studies show that Galerucella spp. biocontrol agents remain established and continue to feed on L. salicaria in southern MN, but do not universally reduce the vigor, competitive performance, or reproduction of the plant in field populations as compared to populations lacking established populations of biocontrol agents. When grown in a common environment, an experimental garden, populations of L. salicaria having experienced herbivory by Galerucella spp. for 16 years are more vigorous, marginally more competitive, and marginally more tolerant of herbivory compared to populations lacking a historical association with the herbivore. In MN, L. salicaria is currently under selection pressure for increased vigor, and the plant has the genetic capacity to adapt in response to this selection pressure. Further evolution of L. salicaria could decrease the effectiveness of biological control by Galerucella spp.