Anthropogenic habitat disturbances are of growing concern due to their impacts on native biota, especially in freshwater ecosystems. Damming, channelization, urbanization, wetland draining, and non-native fish introductions all play large roles in habitat homogeneity, fragmentation, and species competition. This has negative effects on native fish and invertebrate species. In the Midwestern United States, the plains topminnow (Fundulus sciadicus) has been declining across its range, to the point of becoming a species of special concern in Missouri. This is possibly due to a combination of the above anthropogenic habitat disturbances. To better understand the ecology of the plains topminnow in Missouri, this study examined its distribution within the state from 1930-2010 and its diet, size distribution, and habitat preferences in two Ozark populations. I also examined their competitive interactions with the non-native western mosquitofish (Gambusia affinis) in a laboratory experiment. My findings indicate little to no change in plains topminnow populations in Missouri since 1930, while western mosquitofish distribution have grown by over 520% on a county scale. Plains topminnow diets from two Ozark sites indicated a broad range of prey items, including Diptera (larvae and adult), Coleoptera, Gastropoda, and a number of other invertebrate taxa. Size distribution in one study site indicated a healthy population with high recruitment of sexually mature adults and 4 age classes. Habitat data showed a significant correlation of topminnow presence with moderate to dense algae and macrophytes and no significant correlation with dissolved oxygen or temperature. My competition experiment revealed high adult topminnow mortality in the presence of G. affinis through intraspecific competition. G. affinis was also often killed by a territorial topminnow male --Abstract, page iii
