Colorado River Clams Provide Benchmark
When naturalist Aldo Leopold explored the Colorado River delta in 1922, he found a "milk-and-honey wilderness." But 27 years later, he wrote that "I am told the green lagoons now raise cantaloupes." Conservationists have long contended, largely in impressionistic terms, that 70 years of American dam building and water diversion have destroyed the biological richness of the delta, a key nursery of marine life at the end of the Southwest's great watercourse. Now researchers have confirmed those suspicions, using an important ecological player as a quantitative marker.
Washed up. Despite the periodic accumulation of shells (above), the Colorado River now supports 95% fewer clams than in decades past.
CREDIT: KARL FLESSA
"Basically, we've used clam shells to quantify what things were like before the dams and found they were vastly different," says Karl Flessa, a geoscientist at the University of Arizona in Tucson who led the four-university team's work, which is reported in the December issue of Geology. The work, says Sally Walker, an invertebrate paleontologist at the University of Georgia, Athens, "shows paleontology can be extremely useful for solving environmental questions by establishing an ecosystem's long-term past before humans altered it. That's powerful."
Flessa and his colleagues in Virginia and Mexico studied clams in the delta because, unlike other animals that decay and are lost to the geological record, clams leave behind hard shells to tell of past abundance. Clams furthermore stand as a "proxy" for "the whole marine ecosystem and its health," says Flessa, who notes that numerous fish, mammals, and migratory shorebirds depend directly on them for food. Flessa and his colleagues hoped that an analysis of the vast islands of gleaming white shells, using paleontological, geochemical, and geochronological methods, would allow them to estimate the delta's biological productivity both before and after the river's water was diverted.
To do so, the researchers carried out a series of simple mathematical calculations. First, they used satellite images, trenches excavated in shell-rich beaches, and field measurements of ridge density to estimate that the remains of some 2 trillion clams lay entombed in great shell ridges and islands in the delta. Then they dated 125 shells by analyzing changes in their amino acids and calibrating the results with radiocarbon dating. Virtually all those 2 trillion shells accumulated over the 1000 years from A.D. 950 to 1950, they found. Finally, they used stable isotope profiles recorded in shells to calculate the population turnover rate, which allowed them to calculate that 6 billion mollusk bivalves flourished at any given time in the area. From that number, they calculated an average density of 50 clams per square meter over the last millennium. In contrast, earlier this year seven sample areas yielded estimates of just three individuals per square meter.
Michal Kowalewski, a geobiologist at Virginia Polytechnic Institute and State University in Blacksburg and one of the project's leaders, believes that the productivity of the delta system has fallen at least 95% since the 1930s, when Hoover Dam was built. "That's a big drop, but in fact our calculations are so conservative it's probably much worse than that--maybe 10 times worse," Kowalewski says. He blames reduced fresh water and nutrient flows to the delta. About 90% of the river, or about 13.5 million acre-feet of water a year, is now diverted to support the fields and booming cities of the Sun Belt.
The new work could have both local and global implications. In the Southwest, the clam counts could help environmentalists secure greater water flows to the delta to restore its species. "You need numbers to negotiate with and litigate with, and [Flessa's work] gives us numbers," says environmentalist David Hogan of the Tucson-based Southwest Center for Biological Diversity, which has been active on the issue.
The clam research also offers a dramatic example of how the methods of paleontology can be used to address environmental problems elsewhere, suggests Walker. Techniques like Flessa's and Kowalewski's can provide quantitative historical baselines even when long-term ecological monitoring cannot, she says. "Applying methods like these can give you a numerical sense of the scale of what has happened and then a metric, or benchmark, for attempting remediation," says Walker.
To Flessa, the numbers provide mute testimony on "what has been lost" during 70 years of aggressive water management in the region. He says that the federal dam builders in the Southwest too often ignored the costs of irrigating fields and slaking the thirst of sprawling desert cities. "Now," he says, "we're providing some quantitative assessments of those impacts. That they're huge will help, I hope, to sharpen future policy."
Mark Muro writes from Tucson, Arizona.
Issue of 15 Dec 2000,
Copyright © 2000 by The American Association for the Advancement of Science.