Increasing hydroelectric development in the tropics is beginning to place a large percentage of global freshwater diversity at risk. A prime example is the recently completed Belo Monte Hydroelectric Complex (BMHC) on the Xingu River in Brazil, which will have severe impacts on a highly endemic assemblage of rapids-adapted fishes. This dissertation uses community assembly theory as a lens to explore ecological dynamics within rapids of the Xingu prior to hydrologic alteration, with the goal of furthering empirical understanding of the assembly process, as well as providing necessary baseline data for conservation efforts. Focusing on both functional and taxonomic community structure, I use null model comparisons and multivariate statistical approaches to explore: 1) temporal dynamics between wet and dry-season assemblages, 2) variation across different functional traits, and 3) spatial variation along the impacted reach. Significant differences in functional diversity between wet and dry-season assemblages suggest that the relative influence of community assembly mechanisms vary seasonally in response to changing abiotic conditions, with expanded habitat and decreased density of aquatic organisms during the wet season likely limiting the influence of biotic assembly mechanisms, including competitive exclusion. Significant relationships between a trait's deviation from null expectations and its correlation with trophic structure indicates that traits strongly associated with trophic ecology display greater dispersion from the mean and more even spacing of trait values. This suggests that traits associated with trophic ecology are more influential in niche differentiation affecting species coexistence. The dominant compositional pattern observed was the high number of rapids-adapted species found upstream of the main powerhouse, further highlighting this region of the Xingu as a hotspot of aquatic diversity and identifying the area that will be dewatered by the BMHC as critically important for the conservation of this unique fauna. These results emphasize the dynamic nature of the assembly process, but suggest that traits may respond to assembly mechanisms in predictable ways. Maintenance of a dynamic flow regime that contains key components of the historic hydrograph will be critical for the conservation of this globally unique habitat and its associated diversity.
