Understanding the Exotic Distributions of Invasive Species

Invasive species are organisms whose introduction and spread in exotic ranges result in a multitude of ecological impacts. Understanding the factors that constrain the exotic distributions of invasive species is of considerable interest. Biotic associations formed with taxa in the invaded community may be particularly important in shaping invader distributions. These associations emerge from interactions between the traits of the invasive species and some subset of the traits present in the invaded community. Focusing on how organism traits influence the outcomes of biotic interactions may inform predictions of invader distributions. This kind of trait-based approach may be most easily applied to systems where invaders specialize on particular hosts because such associations imply a close correspondence between the traits of the invader and hosts. This dissertation focuses on the South American cactus moth (Cactoblastis cactorum, Lepidoptera: Pyralidae), an invasive consumer in North America whose larvae infest prickly-pear cacti (Opuntia spp.). Chapter One is a brief introduction providing background and context to the presented research. In Chapter Two, I quantify Opuntia morphological and tissue macronutrient traits hypothesized to correlate with patterns of C. cactorum host use. Tissue macronutrient traits appear important in predicting C. cactorum infestation whereas a model containing Opuntia morphological traits had poor predictive ability. Chapter Three describes a method that uses host Opuntia identity and availability to estimate habitat suitability in order to predict the North American distribution of C. cactorum. I then simulate C. cactorum dispersal relative to scenarios of habitat suitability and Opuntia availability. Chapter Four alters the model in Chapter Three so that habitat suitability for C. cactorum is determined by the availability of trait-based groupings of Opuntia hosts. I then simulate C. cactorum dispersal via a different method from that described in Chapter Three. In Chapters Three and Four, I evaluate the degree of similarity among model predictions and the relative contribution of modeling constraints in generating variation in this similarity. Chapters Three and Four predictions were most affected by estimates of abiotic suitability and dispersal constraints, respectively. Chapter Five is a short summary of my results and a discussion of their more general applicability