A developing concept in ecology is that broad niches predict geographical range size. Plasticity in resource use is often considered to be important for responding to novel environments, and study of species that are established across different ranges provides a model for determining the drivers of species spread and distribution. Contrasting distributions of invasive species are often associated with species-specific feeding ecology, but comparisons are rarely made for multiple taxa, spatial scales or between native and invaded ranges.;Stable isotopes of carbon (delta13C) and nitrogen (delta 15N) are common tracers in food web ecology and provide a quantitative measure of intra and interspecific variation in dietary niche. In this dissertation, I compare the dietary niches (variation in delta13C and delta 15N) of successful and less successful aquatic invasive species of fish and invertebrates established across broad and narrow ranges in invaded regions, fishes in native and invaded ranges, and the role of time since invasion for niche diversification in relation to habitat complexity and genetic diversity.;This dissertation demonstrates the importance of intra and inter-individual variation in resource use for successful widespread distribution of invasive fishes. Inconsistency in this trend for oysters, tunicates and waterflea suggested dietary niche breadth is less important for invasion success in aquatic invertebrates than it is for fishes. Moreover, dietary niche plasticity was associated with a broad body size range in fishes, but not invertebrates. Habitat complexity and environmental heterogeneity were associated with peaks in niche breadth that were exceptions to general trends of increasing niche breadth and genetic diversity with time established. Gain in niche breadth from native to invaded regions for populations of most successful invasive fishes, but loss of niche breadth for less successful species, further indicated the importance of intra-individual variation in resource use for widespread distribution. Loss of dietary niche breadth for less successful species implied that the invasion process could be associated with severe bottlenecks for less successful species, which could be a mechanism for limited spread. These findings highlight the potential for considerable impact from the invasion process for the ecology of the invader, and that establishment in novel environments can drive diversification in the absence of founder effects.
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