Assemblage-Level Diversity of Fish Life-History Strategies Across Riverscapes

Population responses to environmental conditions are mediated by life-history strategies, which represent an adaptive suite of inter-correlated reproductive and demographic traits.  For many, if not most, communities, a diversity of life-history strategies may be simultaneously present.  We examined assemblage-level patterns of fish life-history diversity across Great Plains riverscapes using standardized field surveys and 14 quantitative life-history traits.  Three metrics of life-history diversity (richness, evenness, and divergence) were calculated based on traits and relative abundances in each assemblage. Stream size, network structure and watershed characteristics were summarized using GIS.  Species richness and the three functional diversity metrics exhibited different (yet complementary) relationships with hydrology and stream characteristics. For example, life-history richness was highest in intermediate reaches, and declined in both the largest and smallest streams due to reduced abundance of more specialized strategies, whereas life-history divergence increased linearly with stream size.  Additionally, the specific composition of life-history strategies along river gradients represented a transition to more periodic and equilibrium species in larger streams. Although assemblages may be comprised by species representing a diversity of life-history strategies, strong gradients in the relative composition and abundance of life-histories were tied to primary attributes of river systems representing a gradient of stochasticity to predictability/stability.