Posted: Oct 19, 2010 4:49 pm
A river ran through it
Nature and humans leaving mark on rivers and streams, affecting aquatic food webs
Public release date: 15-Oct-2010
Rivers and streams supply the lifeblood to ecosystems across the globe, providing water for drinking and irrigation for humans as well as a wide array of life forms from single-celled organisms up to the fish humans eat.
But humans and nature itself are making it tough on rivers to continue in their central role to support fish species, according to new research by a team of scientists including John Sabo, a biologist at Arizona State University.
Globally, rivers and streams are being drained due to human use and climate change. These and other human impacts alter the natural variability of river flows.
Some affected rivers have dried and no longer run, while others have seen increases in the variability of flows due to storm floods.
The result is that humans and nature are conspiring to shorten food chains, particularly by eliminating top predators like many large-bodied fish.
"Floods and droughts shorten the food chain, but they do it in different ways," said Sabo.
Sabo is the lead author of a paper reporting results of a study of 36 rivers in this week's issue of the journal Science.
"The length of food chains is a crucial determinate of the functioning of ecosystems," says Alan Tessier, program director in the National Science Foundation (NSF)'s Division of Environmental Biology, which funded the research.
"Ecologists have long sought to explain why food chain length varies among different ecosystems. This study provides a quantitative answer to that question for stream ecosystems, and provides critical evidence for the importance of flow variation."
High flows "take out the middle men in the food web, making fish [the top predator] feed lower in the food chain," said Sabo. "Droughts completely knock out the top predator."
"The result is a simpler food web, but the effects we see for low flows are more catastrophic for fish--and are long-lasting."
Sabo and co-authors--Jacques Finlay, University of Minnesota, St. Paul; Theodore Kennedy, U.S. Geological Survey, Southwest Biological Science Center, Flagstaff, Ariz.; and David Post, Yale University, New Haven, Conn.--suggest that the fate of large-bodied fishes should be more carefully factored into the management of water use, especially as growing human populations and climate change affect water availability.
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