"You can expect some battles - time, place, and players to be determined," says one congressional source.

Enacted in 1973, the law was designed mainly to save animals such as eagles, wolves, otters, and salmon - those noble, brave, and cute "charismatic megafauna" that all Americans recognize and can perhaps identify with.

But over the years, the list of animals and plants threatened with extinction has grown to more than 1,000, including such obscure species as the Shivwits milk-vetch herb in Utah and the Tumbling Creek cavesnail in Missouri. At last count, 1,244 plant and animal species had been listed as endangered or threatened, and another 236 "candidate species" awaited listing.

The law is a kind of a modern-day Noah's Ark, designed to save the last few individuals from being wiped out and then provide a way for them to revive. Once an organism makes the list, federal officials must design and implement a recovery plan - an expensive process that can impact private property to devastating economic effect. It's also a drawn-out process that has seen several dozen species go extinct while awaiting rescue.

Aside from questions about balancing economic and biological values, the law increasingly highlights the debate over what constitutes an endangered species and whether or not some species are worth saving.

Supporters of the act are on the defensive these days. A National Academy of Sciences panel recently declared that federal officials may have erred last summer in cutting off water to farmers in the Klamath Basin of Oregon and California in order to protect three endangered fish species. A federal judge has declared there's no difference between endangered wild Pacific Coast salmon and hatchery fish. (A point many biologists argue strongly against.) And opponents charge that government scientists deliberately planted evidence in order to support the contention that the Canadian lynx needs more habitat to prevent its extinction.

"The ESA has become a wrecking ball in this country, devastating personal finances and regional economies," says US Rep. James Hansen (R) of Utah, chairman of the House resources committee. "It's time we reform this law, grounding it in sound science, not political ideology."

Along the Oregon-California border last summer, some 1,400 farms and ranches had their federal water cut off when it was determined that two species of lake fish and coho salmon in the Klamath River - all listed under the Endangered Species Act (ESA) - had first dibs on the water. The basin's rural economy lost about $134 million as a result, while protesting farmers broke open irrigation headgates and faced off against federal law enforcement officers.

But the issue there is far more complicated than fish versus farmers. The area is also home to several large wildlife refuges, including the wintering area for upwards of 1,000 bald eagles. At the same time, the Klamath Indian Tribes have water rights dating back to 19th-century treaties with Washington. Many experts say the water has simply been over-allocated during the past century, leading to a steep decline in fish and waterfowl.

In the case involving endangered lynx in Rocky Mountain wilderness areas, the scientists and their defenders say they were merely trying to test the system used to distinguish similar species. But that has not stopped longtime critics of the ESA from charging that "junk science" was used to promote a proenvironment agenda by those who sometimes call themselves "combat biologists."

At the heart of the issue is the ESA's requirement to use the "best available" science in determining listings and recovery programs. This can present a moving target for biologists and other experts, leading to findings that are not entirely satisfactory. In the Klamath Basin case, for example, the panel of university experts gathered by the National Academy of Sciences also determined that significantly lowering lake water levels to benefit farmers as the US Bureau of Reclamation (and the Bush administration) wants to do would pose an "unknown risk" to protected fish. The panel's final report won't be released until next year.

Meanwhile, the legal and political maneuvering continues. Conservative groups and developers are preparing lawsuits challenging other endangered species listings. Environmentalists are suing federal agencies for failure to sufficiently protect "rare and sensitive" plant and animal species in Pacific Northwest old-growth forests. Bush administration officials are reevaluating the designation of "critical habitat" in California and other parts of the West. Such designations can strictly limit development, but can also be crucial to species survival.

On Capitol Hill, several dozen bills deal with endangered species. Most address what critics say is the need for "sound science" in determining ESA listings, as well as a greater say for property owners, developers, and other economic interests. A bill recently introduced by Sen. Gordon Smith (R) of Oregon, for example, would give greater weight to commercial or scientific empirical and field tested data.