Lewis, Samuel, authorKanno, Yoichiro, advisorSalerno, Jonathan D., committee memberSanderson, John S., committee memberPreston, Daniel L., committee member2023-01-212023-01-212022https://hdl.handle.net/10217/2359691. Invasive species are a major threat to freshwater conservation. Species coexistence in invaded habitats depends on the relative strength of intra- versus inter-specific competition, where inter-specific competition from invasive species to native species is often stronger than intra-specific competition, jeopardizing their coexistence.2. In this study, I conducted a laboratory experiment to test for the relative competitive strength between native plains topminnow (Fundulus Sciadicus) and invasive western mosquitofish (Gambusia affinis) at 16, 22, and 28 °C. Data were analyzed using the isodar theory, which assumes that animals are ideally distributed to maximize their fitness and thus their distributions measure the quality and quantity of habitat patches. This was supplemented by behavioral observations of intra- and inter-specific competition. 3. Contrary to my predictions, I did not find evidence that competition was asymmetrical from the invasive mosquitofish to the native plains topminnow. Instead, more individuals occupied their shared preferred habitat (a slow-moving pool) in sympatry compared to allopatry, and the isodar analysis demonstrated that intra-specific competition was significantly stronger than inter-specific competition at all temperature levels. This analysis of habitat selection was corroborated by behavioral observations that aggression was most frequent between plains topminnow in sympatry. 4. This study indicates that aggression might not always be the key mechanism of invasion, even for one of the most successful aquatic invasive species widely known for their aggressive behavior. In mosquitofish, other ecological traits such as fast reproductive cycle and tolerance to a wide range of environmental conditions might be responsible for their invasion success globally. This suggests that mosquitofish may not be a driver in native species declines, but rather an opportunistic invader in degrading ecosystems.born digitalmasters thesesengCopyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.Text