UNC Ecology Seminar: John Sabo

Floods, droughts and the scaling of food chain length with drainage area in rivers

Floods, droughts and other extreme hydrologic events play key roles in coupling human and natural systems. Floods can cause tremendous damage and mortality and cripple local economies. By contrast, floods are critical to freshwater fisheries, some of which supply the majority of the protein to people in developing nations. Prolonged droughts and human dewatering of rivers for municipal, industrial and agricultural uses pit farmers against fisherman for limiting water resources forcing us to ask tough questions like: "Which rivers (and their fisheries) are we willing to let run dry in order to put produce on the table?" Or, "How will fish, and the food webs that support them, respond to prolonged or intensified extreme events?" Food chain length (FCL) is a fundamental component of food web structure, and large fish--often at the top of aquatic food chains--are the pièce de résistance across the world. In this talk I will describe the results from my recent work documenting how floods and droughts impact FCL in river ecosystems of North America. Studies in a variety of ecosystems suggest that FCL is determined by energy supply, environmental stability, and/or ecosystem size. However, the nature of the relationship between environmental stability and FCL, and the mechanism linking ecosystem size to FCL, remain unclear. In rivers FCL increases with drainage area and decreases with hydrologic variability (floods) and intermittency (severe drought). Both floods and droughts appear to shorten river food chains but do so in different ways. Droughts eliminate top predators whereas floods take out the middle men in food webs forcing top predators to forage lower on the food chain. Hence, the effects of droughts are more severe and appear to be more long lasting. The dewatering of rivers nationwide and globally is having measureable effects on fisheries in freshwater and in coastal ecosystems. These effects will intensify in the Southwest unless we adopt a 7-state, regional water conservation plan aimed at reducing surface and groundwater withdrawals by 16-34% before the next population doubling in the region. Similar actions are called for in the Southeast US where rivers, farms and cities face a similar water shortage fate.