The application of competition theory to invaders and biological control: A test case with purple loosestrife (Lythrum salicaria), broad-leaved cattail (Typha latifolia), and a leaf-feeding beetle (Galerucella calmariensis)

Invasive species pose an enormous threat to native species and imposes substantial costs on the US economy. Although the threat of exotic species is well recognized, the general ecological mechanisms that underlie these invasions remain elusive. Predictions both for invasion success and the success of biological control remain poor. In this dissertation, I harness plant competition theory to predict the success of invasions and biocontrol, using a model system composed of invasive purple loosestrife (Lythrum salicaria), native broad-leaved cattail (Typha latifolia), and Galerucella calmariensis, a leaf-feeding beetle widely released to control loosestrife. In chapter 1, I introduce the problem and summarize my results. In chapter 2, I extend resource competition theory to competition for light: species coexistence is possible if one species is sufficiently taller and with less dense foliage than its competitor. This integrated model of competition for light is easily parameterized through measurements of light availability in monoculture and thus can easily be tested in the field. In chapter 3, I test the ability pf three models of plant competition (response to resource availability, plant size, and resource reduction) to predict competitive outcomes between loosestrife and cattail. My experimental design included monoculture mesocosms in which to measure plant traits, and mixture mesocosms in which to determine competitive outcomes. Surprisingly, while loosestrife was, on average, negatively affected by the presence of cattail, cattail was not, on average, negatively affected by loosestrife. Indeed, at high fertility, cattail was strongly negatively affected by loosestrife in the absence of insect herbivores of both species, yet was strongly facilitated when herbivores of both species were present. The facilitation of cattail by loosestrife was likely due to density dependent predation by cattail's natural enemies. Cattail abundance in mixture was not predicted by any of the three models, which is not surprising considering the lack of an overall competitive effect of loosestrife on cattail. In contrast, loosestrife abundance in mixture was well predicted by species height, as predicted by the plant size model. These results suggest that competitive traits may predict invasion success and biocontrol, but only when species interact only through competition for resources