Breaking The Chain

Could be rough times ahead if you are the big fish atop the food chain in your river. A sophisticated new analysis finds that droughts and floods reshuffle river food webs in dramatically different ways – offering a potential preview of how climate change, dams and other forces may affect top predators such as large fish.

The new work, published in Science, weighs in on a longstanding ecological puzzle: What factors determine the length – or number of links — in a food chain? Theoretical studies have suggested that bigger, more stable ecosystems with abundant supplies of food and nutrients should have longer chains involving more players. Real-world studies, however, have often failed to back that idea. In part, that’s because they could not simultaneously address all of the major factors that influence food chains.

The new study attempts that juggling act. Making clever use of nitrogen isotopes, a team of four researchers analyzed how food webs in 36 North American rivers were affected by changing flows, the size of the watershed, and energy supplies. In essence, the nitrogen isotopes served as a marker for food chain length, since nitrogen accumulates in the tissues of consumers, increasing by 3.4 parts per million with each link in the chain. So by measuring nitrogen isotope concentrations in top predators, the researchers could get a rough idea of how many links were in the food chain beneath them – and how the length of that chain changed under different environmental conditions.

Overall, the team found that the bigger river ecosystems with more stable flows did support longer food chains, but that energy supplies seemed to make little difference. But variations in flow – such as droughts or floods caused by everything from dams to climatic changes – can have a big impact on food webs, particularly on large fish at the top of the chain.

Floods and droughts both shortened the food chain, “but they accomplish this in different ways,” says John Sabo of Arizona State University in Tempe, the study’s lead author. “High flows take out the middle men in the food web, making fish (the top predator) feed lower in the food chain.” Droughts “just eliminate the top predator altogether because many fish just can’t tolerate the low oxygen and high temperatures that follow when a stream starts drying out… The end result in either case is a simpler food web.” And simpler, the researchers note, is not necessarily better.David Malakoff | October 15, 2010

Source: Sabo, J., Finlay, J., Kennedy, T., & Post, D. (2010). The Role of Discharge Variation in Scaling of Drainage Area and Food Chain Length in Rivers. Science DOI: 10.1126/science.1196005

Image © Orlando Florin Rosu |