Washington: A new research has suggested that humans and nature itself are making it tough for rivers to continue in their central role to support fish species.
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.
"Floods and droughts shorten the food chain, but they do it in different ways," said John Sabo, a biologist at Arizona State University.
Alan Tessier of the National Science Foundation (NSF) said: "The length of food chains is a crucial determinate of the functioning of ecosystems," said
"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."
Sabo and co-authors suggested 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.
The researchers studied rivers and streams in the US ranging in size from the Mississippi and Colorado Rivers, down to small tributaries.
The study employed naturally occurring stable isotopes of the element nitrogen to measure how top-predators were faring in the food chain.
"Floods simplify the food web by taking out some of the intermediate players so the big fish begin to eat lower on the chain.
"With droughts, it`s completely different: droughts eliminate the top predator altogether because many fish can``t tolerate the low oxygen and high temperatures that result when a stream starts drying out," said Sabo.
Sabo outlined a classic scenario that humans face during drought years.
As drought takes hold, the need for water for irrigation and agriculture increases and leads to a draw-down of natural river flows.
The effects downstream can be devastating.
"We would not have guessed that the infrequent drought that results would have a big effect on a stream, but our results show that it does.
"Some streams affected by drying five to ten years ago are still missing large-bodied fishes, compared with same-sized streams that never dried.
"Food webs can recover sooner after a flood, in roughly a year, but it takes far longer to recover in the case of drying or drought," said Sabo.
The study has hinted that competing users of a river``s water-for agricultural production and recreational uses like fishing-need to work out amenable uses of rivers and streams that not only look to the immediate future, but also project long-term effects.
"The question becomes: can you have fish and tomatoes on the same table?" asked Sabo.
"They compete for the same resources, and society depends on both: agriculture for grain, fruits, vegetables, and fish for protein, particularly in the developing world.
"Humans may need to make hard decisions about how to allocate water so that we grow the right food, but still leave enough in rivers to sustain fish populations," he added.
The findings appeared in the journal Science.