Wild salmon ecosystems: Watershed complexity and its importance to fisheries and wildlife

Background/Question/Methods

River systems have been vastly simplified in places where their watersheds have been developed by humans whose activities tend to reduce the structural complexity of terrestrial and aquatic habitats. Loss of structural complexity in developed watersheds is often correlated with more erratic population dynamics of species of commercial interest and reduced viability in species of conservation concern. How loss of system complexity has contributed to reduced ecosystem functioning remains poorly understood. Studies of intact river systems in wilderness areas provide the opportunity to quantify system complexity, as expressed across spatial, temporal and taxonomic dimensions of ecosystems. We will provide an overview of research on watersheds of western Alaska conducted across spatial scales ranging from meters to 100s of kilometers, and temporal scales from hours to centuries, to highlight the continuous expression of complexity across enormous ranges of space and time scales.

Results/Conclusions

Results from contemporary and paleo studies show that watersheds in western Alaska are characterized by substantial complexity expressed continuously across broad ranges in spatial and temporal scales. Mobile organisms, ranging from juvenile fishes to terrestrial carnivores and humans, actively exploit the variation in key resources, and their scale-dependencies, to satisfy important life-history requirements. The critical lesson from our work in these watersheds is that complexity provides organisms with a rich array of habitat options whose productivity varies across space and time in response to variation in regional and global changes in the environment. Many of these options are simply not appreciated at the space and time scales at which ecologists typically study ecosystems and we suspect that many habitat options that mobile organisms rely upon are simply not observable in advance of when they become critical. Such observations offer pessimistic views to restoration efforts that are unable to reestablish the disturbance regimes that maintain complexity of most watersheds. These results also emphasize that policy designed to protect ecosystem services must move forward in the absence of deep understanding about ecosystem functions and drivers of population dynamics.