The food chain - the number of organisms that feed on each other -- in the world's streams and rivers depends more upon the size of the stream and whether the waterways flood or run dry than the amount of available food resources, Yale University and Arizona State University (ASU) researchers report online in the Oct. 14 issue of the journal Science Express.
The findings suggest that large predators in river systems will be threatened by increased variability in water flow induced by climate change. The research also helps settle an old debate among ecologists about what determines the length of nature's food chains, which sustain all life on earth.
"The food web is a regulatory network of ecosystems, and for nearly 100 years ecologists have debated the causes of variation in the length of the food chains, said David Post, professor of ecology and evolutionary biology at Yale and co-author of the study.
Researchers from Yale, ASU, the University of Minnesota, and the U.S. Geological Survey studied 36 North American streams and rivers. The researchers found that food chains - or the number of mouths that food passes through on the way to top predators - got longer as the size of the body of water increased. The findings are similar to another study conducted by Post a decade ago that found the key factor in food chain length was lake size, not the amount of food resources in a system, as many ecologists had believed.
A longer food chain supports more organisms and larger predators such as big fish but may also increase the concentration of contaminants in larger predators. However, the new study found that the more streams and rivers dried up or flooded, the shorter the food chain. This in turns puts pressure on the ecosystem's ability to support organisms, particularly larger predators. In fact, when water is withdrawn from a stream or a river dries up it may be decades before the food chain recovers.
"Even very large rivers around the world are drying with increasing frequency and global climate models predict many rivers will experience more variable flows, both high and low," said John L. Sabo, professor of ecology, evolution and environmental science at ASU and co-author of the study. "Our results suggest these changes to hydrology will simplify river food webs and increase the likelihood of loosing many top predator fish species from aquatic ecosystems."
As climate variability increases, so will the number of disputes over water use, such as clash a decade ago between farmers and environmentalists over withdrawal of water from the Klamath River Basin, which flows from Oregon into northern California.
"Understanding what determines food chain length will help us make better policy decisions," Post said.
The National Science Foundation supported the research.