Spatial and Temporal Trade-Offs by Bluegills in Floodplain River Ecosystems

Ecological trade-offs by organisms to minimize mortality and maximize growth is a foundational theme in ecology. Yet, these trade-offs are rarely examined within spatially complex, temporally variable ecosystems, such as floodplain rivers. Here, we evaluate ecological trade-offs across space and time for the bluegill (Lepomis macrochirus) in two unregulated river ecosystems in southeastern USA. Life-history differences among spatially segregated main channel and floodplain lake populations were used to assess effects of habitat type on bluegill fitness. Growth, condition, and gonadal somatic index were all significantly enhanced in floodplain lakes relative to the main channel. Furthermore, stomach fullness was significantly higher, and predator densities significantly lower in floodplain lakes thereby providing an ecological explanation for the life-history plasticity observed across the riverscape. However, historical observations suggested that although floodplain lakes are highly productive for bluegills, they are also prone to complete desiccation by drought approximately every 5 years, revealing the ultimate value of channel habitat, which does not dry, as desiccation refugia. Bluegills are faced with a balancing act associated with variation in foraging opportunities, and risks to predation and desiccation, that change in both the temporal and the spatial dimensions of floodplain rivers. The differential responses to these opportunities and risks help to explain why both habitats remain actively populated by bluegills, as well as many other organisms, in these and many other natural rivers.