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The ecological niche is key to our understanding of trophic interactions, species distributions, trait evolution, and competitive dynamics. The wide array of applications of the niche is matched by the diversity of its meanings, and it has been variously defined to be the ecological opportunities available in a community, the functional roles that species play in their community, the environmental conditions necessary for species persistence, the resource-use distribution of species, or combinations of these. Despite this heterogeneity all the niche definitions have assumed that species differ in their relation to the environment, and that understanding the ways in which these relations differ is essential for responding to questions in ecology and evolution.My thesis uses historical, modelling, and experimental methods to explore how the niche has informed ecological thought, and how its current use can guide ecological and evolutionary research, especially as it regards competition. I used a citation network analysis to explore the structure of the niche literature from 1917-1999 and the pattern of its disciplinary spread from its origins among animal researchers. Surprisingly, this analysis revealed that while the niche had spread to a number of sub-disciplines by the close of the 20th century, integration between key sub-disciplines in the network was not strong, suggesting that a common niche literature had not yet emerged. Neutral theory has recently challenged niche theory by suggesting that in some cases competitors may actually be equivalent, and thus that competition may be driven by stochastic processes and not inherent differences between competitors. I applied a modelling approach to assess the predictability of competitive outcome along gradients of fitness inequality (the difference in competitive ability between competitors) and demographic stochasticity, and found that both gradients interacted to affect competition. Outcomes not predicted by niche theory were common in some conditions, which suggests that competitive dynamics may be niche or neutrally structured dependent on how environmental conditions affect fitness inequality and stochasticity. Finally, I experimentally approached the question of how ecological niches might change in response to the imposition of environmental stressors, and found that while the niches of some populations were relatively unaffected by some forms of stress, other populations saw contraction in the size of their niches, and declines in their fitness within the niche after selection in some environments. While my thesis demonstrates that the use and predictions of niche theory are context dependent, niche theory has productively inspired ecological thought in many areas, and is perennially changing in response to its own limitations. As the corner-stone of ecological thought for how organisms relate to their environments it will no doubt continue to be at the forefront of responses to emerging questions in ecology and evolution. La niche écologique est au coeur… Advisors/Committee Members: Andrew Gonzalez (Supervisor2), Gregor Fussmann (Supervisor1).