Conservationists have long contended that dams and water diversion have destroyed the biological richness of the Colorado River delta, a key nursery of marine life that flows into the Gulf of California. Now researchers have confirmed those suspicions by counting ancient clam shells. They estimate that the river has lost about 95% of its productivity since the Hoover Dam was built in the 1930s.
Unlike soft-bodied animals that decay without a trace, clams leave behind hard shells that record their past abundance. Clams also hint at an ecosystem's health, because many fish, mammals, and migratory shorebirds depend on them for food. A team of researchers led by geoscientist Karl Flessa of the University of Arizona in Tucson hoped that analyzing ancient shells would allow them to estimate the delta's biological productivity both before and after the river's water was diverted.
First the researchers estimated how many clams lie entombed in the delta's massive shell ridges and islands. After analyzing satellite images, excavating trenches in shell-rich beaches, and making other field measurements, they came up with a grand total of some 2 trillion clams. Then they dated 125 shells and inferred that almost all of those 2 trillion shells accumulated between A.D. 950 and 1950. Finally, they calculated the population's turnover rate to estimate an average density of 50 clams per square meter over the last millennium--a number that contrasts with fieldwork earlier this year that yielded just three clams per square meter, they report in the December issue of Geology.
Paleontology studies such as this can "establish an ecosystem's long-term past before humans altered it," says invertebrate paleontologist Sally Walker of the University of Georgia, Athens. And that provides "a metric, or benchmark, for attempting remediation," Walker says.