Collaborative Research: The role of branching complexity in driving food chain length in rivers

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The origin of variation in food chain length - the number of feeding links from producers to apex consumers in a food web - has intrigued ecologists for decades. Understanding controls of food chain length is important because it influences energy flow through ecosystems and contaminant concentrations in top predators that humans often consume. The prevailing thought has held that food chain length is determined by the size of an ecosystem. Large ecosystems can provide greater amounts of basal resources, support larger population sizes of constituent species, and/or mitigate the impact of environmental changes. However, while ample empirical evidence for the ecosystem size hypothesis exists in spatially simple systems (e.g., oceanic islands and lakes), there is less clear support in more spatially complex systems, such as branching river networks. The mixed results for the ecosystem size hypothesis in spatially complex systems imply the existence of overlooked factors that control food chain length. The project will explore the role of ecosystem complexity that is emerging as a key factor in ecosystem dynamics. Specifically, researchers will focus on the branching complexity of river networks and investigate its importance in driving food chain length. The broader impacts of this project include the following components. First, researchers will participate in outreach events through formal partnerships with K-12 schools. Second, the project will provide research training opportunities for underrepresented minorities and a postdoctoral scientist. Lastly, researchers will distribute open-source software through online repositories to facilitate the application of research products.The project will combine theoretical and empirical approaches to test the following hypotheses: (1) complex river networks, with greater levels of branching, support long food chains because having more branches (i.e., tributary systems) with distinct geological and/or climatic backgrounds may provide diverse habitats that buffer the impact of environmental fluctuations; (2) ecosystem size in rivers (watershed area) has weak effects on food chain length due to the context-dependent association between environmental heterogeneity and ecosystem size. Researchers will develop a series of mathematical models to generate predictions for the association between food chain length and branching complexity/ecosystem size. Theoretical predictions will be validated by meta-analyses of existing datasets of food chain length and environmental covariates across the globe. Overall, the project will provide important insights into how ecological communities are structured in spatially complex systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.