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Food for thought: It's the size of the ecosystem that counts

Researchers found that in larger lakes, a longer food chain with more links occurred between the top predators and organisms at the base of the chain  
ENN



July 14, 2000
Web posted at: 1:07 p.m. EDT (1707 GMT)

Researchers from Cornell University and the Institute of Ecosystem Studies have turned up new food for thought: The number of mouths food passes through in an ecosystem is determined by the size of an ecosystem, not by the amount of available food energy in the ecosystem.

Previously it was thought that the length of a food chain is determined by the amount of energy available to top predators.

All organisms, from bacteria to blue whales, are part of complex food chains, which represent the feeding relationship between all living things in an ecosystem. For more than 75 years, ecologists have been trying to figure out why some ecosystems have longer and more complex food chains than others.

The number of feeding links between organisms at the bottom and the top of the food chain is important not only to ecosystems but also to humans who interact with them.

"A central goal of conservation is to protect and preserve the ecosystem functions and processes upon which all life depends," said David Post, a graduate student of ecology at Cornell University and co-author of the study. "Because food-chain length affects these ecosystem functions and processes, we need to understand how food-chain length varies and what causes this variation."

To test the influence of ecosystem size and productivity on food chain length, the researchers collected fish that feed at the top of the food web from 25 lakes in and around New York and in central Wisconsin. The lakes ranged in size from about 7 hectares to 2,600,000 hectares in Lake Erie. They ranged in food energy levels from low to very high productivity.

The researchers found that in larger lakes a longer food chain occurred, with more links between the top predators and the organisms at the bottom of the chain. Highly productive lakes did not have long food chains.

The researchers suggest that larger lakes might have longer food chains for two reasons. First, they have top predators such as lake trout that are not found in smaller lakes. Second, some of the fish, such as black bass found in small lakes and larger lakes, ate higher up on the food chain in larger ecosystems.

In larger ecosystems food chains might become more diverse, allowing greater dietary specialization that could result in longer food chains.

The same principle may apply to food chains in terrestrial ecosystems such as large forests and smaller forest fragments, Post said.

The research may shed light on how endangered species and humans are affected by contaminants in the environment such as pesticide residues.

In ecosystems with long food chains, a process called biomagnification can cause top predators to carry high levels of contaminants because the concentration of some contaminants increases with each predator-prey link in the chain.

"Because food-chain length influences the bioaccumulation of contaminants, understanding the causes of variation in food-chain length will help us understand the variation of contaminant concentrations," Post explained. "That could be important for the conservation of species that are adversely affected by contaminants and for humans who eat contaminated species.

"If we wish to preserve the diversity of natural ecosystem processes, we must preserve the larger ecosystems where the longest food chains are found," Post warned. "As ecosystem size decreases (e.g., through the fragmentation of forest patches), we may see the loss of large, often rare, top predators. Those top predators that can survive in smaller ecosystems will likely feed lower in the food chain, which may alter the feeding relations and community structure through out the food web."

Post's report, "Ecosystem size determines food-chain length in lakes," was published in a recent issue of the journal Nature.

Copyright 2000, Environmental News Network, All Rights Reserved




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