Doctor of Philosophy (PhD)
Many of the most notorious biological invasions occur at continent-wide or global scales but studies investigating mechanisms enhancing species invasions are often conducted at small spatial scales. Moreover, the contribution of mechanisms facilitating invasion might also vary across a geographical space. I used biogeographical approaches to explore the mechanisms enhancing invasion of introduced genotypes of Phragmites australis along the coastal wetlands of North America. I tested the hypotheses that large-scale disturbance events, such as hurricanes and tropical storms, enhance the invasion success of introduced P. australis in North America. The growth rate of P. australis patches was strongly and positively related to the frequency of hurricane events along the coastal wetlands of the United States. Hurricane frequency alone explained 81% of the variation in the growth rate of P. australis patches over this broad geographical range. I also examined the evolution of latitudinal gradients in native and invasive genotypes of P. australis in relation to plant-herbivore interactions in North America. Common garden experiments revealed that the native and invasive genotypes of P. australis have evolved latitudinal clines on traits associated with herbivory, specifically for aphid Hyalopterus pruni. For a chewing herbivore, Spodoptera frugiperda, only native genotypes exhibited latitudinal clines. The existence of non-parallel latitudinal gradients between native and invasive genotypes creates spatial heterogeneity in the importance of herbivory on P. australis invasion and suggests the greater susceptibility of high-latitude communities. These latitudinal and genotypic variations on plant defenses, palatability, and herbivory suggest a possibility of an asymmetric apparent competition between native and invasive genotypes. A replicated field study in four sites along the Atlantic Coast showed that invasive genotypes could suppress the fitness of native genotypes by herbivore-mediated apparent competition. Moreover, the intensity of apparent competition declined with increasing latitude. These biogeographical studies suggest that multiple processes might be contributing to the spread of the introduced genotypes of P. australis in North America and that the importance of these processes might vary along an environmental gradient. These studies have broader implications for understanding species invasions at the continental scale and for managing natural habitats that are threatened by plant invasions.
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Bhattarai, Ganesh Prasad, "Biogeographical Approaches for Studying Species Invasion" (2014). LSU Doctoral Dissertations. 771.
Cronin, James T.