GEOFF BENNETT: This year marks the 50th anniversary of the Endangered Species Act.

It was the most comprehensive legislation for preserving species at risk of extinction and made the U.S. a global leader in environmental law.

But, today, Congress is divided over how far its protections should go.

William Brangham reports on the act's legacy and uncertain future.

WOMAN: I'm going to see if he's got that wound on the bottom still.

WILLIAM BRANGHAM: For the last three months, this juvenile sea turtle named Kempsville (ph) has been receiving intensive care at the National Aquarium in Baltimore.

WOMAN: There we go.

That's it.

WILLIAM BRANGHAM: Kempsville was rescued in Virginia, and is now fighting off multiple infections, and recovering from surgery, where medics removed a fishing hook from its throat.

Kempsville is named after its species, the Kemp's ridley.

Found mainly in the Gulf of Mexico, it's the world's smallest and most endangered sea turtle.

The principal threat, says the aquarium's rehabilitation manager, Caitlin Bovery, is human activity.

CAITLIN BOVERY, Rehabilitation Manager, National Aquarium: The Kemp's ridley is threatened by fisheries bycatch, by habitat degradation from development, by all marine debris, to pollution that is related from run-off and things like that, as well as our increasing climate temperatures.

WILLIAM BRANGHAM: For Bovery's team, each turtle they save is a win in the fight against extinction.

CAITLIN BOVERY: Every Kemp's ridley is critically endangered, so it's incredibly important to make sure that they return to sea.

WILLIAM BRANGHAM: The species has slowly rebounded since the 1980s, when there were only a few hundred nesting females.

Today, there are an estimated 20,000 of the turtles.

CAITLIN BOVERY: It's really one of the most incredible conservation stories and recovery stories that we have from the Endangered Species Act.

WILLIAM BRANGHAM: Under the act, when a species is deemed near or at risk of extinction, it's put on a federal list and legal protections kick in that limit activities and projects that could kill or harm that species, or its habitat.

For the Kemp's ridley, that meant protections like creating a new nesting colony and requiring commercial shrimp boats to use special devices in their nets that allow turtles to escape.

CAITLIN BOVERY: We have seen recovery, real, tangible recovery.

WILLIAM BRANGHAM: Today, these turtles are just one of more than 1,600 plant and animal species in America listed under the act.

RICHARD NIXON, Former President of the United States: The time has come for man to make his peace with nature.

WILLIAM BRANGHAM: Signed by President Nixon in 1973, the Endangered Species Act passed with near-unanimous bipartisan support.

ROBERT FISCHMAN, Indiana University: Congress decided that what we choose to say about ourselves as Americans is what we choose to save.

WILLIAM BRANGHAM: Environmental law professor Robert Fischman says the act is unique because it prioritizes saving species over economic cost.

ROBERT FISCHMAN: The Endangered Species Act makes the strong moral statement that, as a matter of federal law, we should not cause fellow animals on Earth to go extinct, whatever the cost.

WILLIAM BRANGHAM: Over the decades, it's been credited with saving iconic species, like the gray wolf, the bald eagle, and the grizzly bear.

PROTESTERS: We need a leader, not a logger!

WILLIAM BRANGHAM: But it's also led to bitter clashes between conservationists and industry.

In 1990, the northern spotted owl was added, halting logging on millions of acres of forest in the Pacific Northwest.

FRANK BACKUS, Logger: I wonder how many of you are like me that are sitting there, wondering how we have come to the brink of losing our jobs and our homes and our communities.

JAMIE RAPPAPORT CLARK, President and CEO, Defenders of Wildlife: The spotted owl issue was a signal that our forests in the Northwest were in trouble.

WILLIAM BRANGHAM: Jamie Rappaport Clark is the CEO of a conservation nonprofit known as Defenders of Wildlife and a former director of the U.S.

Fish and Wildlife Service.

JAMIE RAPPAPORT CLARK: The species that are increasingly in crisis are just the canaries in the coal mine.

They're sounding the alarm that should trigger our initiative to right the wrong before it's too late.

WILLIAM BRANGHAM: While there's still bipartisan support for the law, many in Congress disagree on how it should be implemented.

REP. BRUCE WESTERMAN (R-AR): The Endangered Species Act is a very important piece of legislation.

It was put in place with good intentions, but it's been 50 years since there have been any real changes to it.

WILLIAM BRANGHAM: Republican Representative Bruce Westerman chairs the House committee on Natural Resources and aims to reform the Endangered Species Act.

He supports reinstating Trumpov era policies that conservationists say weakened its protections.

REP. BRUCE WESTERMAN: So, you cannot be shortsighted and say, at all cost, we must save this species.

I think we should do everything we can to save all the species.

We don't want to see anything go extinct.

But we've also got to use a bit of rationale and reason.

What we need to be doing is actively managing these forests and these natural areas to create the habitat that allow these animals to thrive.

It doesn't need to be a tool just to stop all kinds of development.

WILLIAM BRANGHAM: He argues that the law isn't doing what it set out to achieve.

REP. BRUCE WESTERMAN: People will say it's got a 99 percent effective rate, meaning that 99 percent of the animals that have been listed under the act have not gone extinct.

But only 3 percent of the animals that have been listed have actually been recovered.

And that was the intent of the Endangered Species Act.

It was recovery.

WILLIAM BRANGHAM: Clark says blaming the law for the severity of the current extinction crisis is misguided.

JAMIE RAPPAPORT CLARK: And it's going to take a while to bring them back.

It's not a light switch that you flip on and off.

By the time the Endangered Species Act is brought into play, the species is oftentimes in real dire straits.

WILLIAM BRANGHAM: That's why environmental law professor Robert Fischman argues that the federal government needs to go further than the Endangered Species Act.

ROBERT FISCHMAN: It's cheapest and easiest to conserve species before they decline to the point where they're in danger of extinction.

So, what we really need is to extend the safety net beyond the Endangered Species Act.

WILLIAM BRANGHAM: This year, a bipartisan bill was reintroduced in the Senate that would invest more than $1 billion a year in state and tribal conservation efforts to do just that.

For now, experts say, individuals, the private sector, and nonprofits like the National Aquarium are trying to fill in the gaps.

CAITLIN BOVERY: For the animal rescue program here and the sea turtle stranding response nationwide, there's no federal funding available.

WILLIAM BRANGHAM: So, while sea turtles like Kempsville have advocates, for many other animals, the Endangered Species Act remains the last line of defense.

For the "PBS NewsHour," I'm William Brangham.