The federal Endangered Species Act has been called the world’s gold standard for environmental protection. Passed in 1973, it strengthened earlier federal protections for animals that had been nearly wiped out by humans, including bald eagles, humpback whales and California condors.

But the act has faced opposition from those who believe it unfairly protects animals that sometimes poach livestock and that it unfairly restricts land use.

At a recent hearing to discuss “modernizing the Endangered Species Act,” Sen. John Barrasso, R-Wyo., head of the Senate Environment and Public Works Committee, said the ESA “is not working today.”

On the House side, Natural Resources Committee Chairman Rob Bishop, R-Utah, said the act “has never been used for the rehabilitation of species. … It’s been used to control the land. We’ve missed the entire purpose of the Endangered Species Act. It has been hijacked.”

A former U.S. Fish and Wildlife Service director at the Senate hearing responded to calls that the law needed a dramatic change by reminding committee members of how the law is viewed in other parts of the world. “The Endangered Species Act is the world’s gold standard” for conservation and protection of animals, said Daniel Ashe, now president and chief executive of the Association of Zoos and Aquariums.

“It’s not perfect. It can be better,” Ashe said, “Your goal is to make it … stronger and better.”

The world’s flora and fauna are experiencing a global extinction crisis caused by human activity, according to many experts. But humans have also learned how to protect species and help them make a recovery. Here are eight species that would probably have disappeared already were it not for the Endangered Species Act.

Black-footed ferret

If raccoons could get plastic surgery, they’d transform themselves into black-footed ferrets. This variety of ferrets is the only one that’s native to the Americas, but Americans are shoving them off their habitat with development, and they inadvertently introduced a plague to their primary food source, prairie dogs. Along with development that causes prairie dogs to scatter, the sylvatic plague, which caused the bubonic plague in humans, wiped out entire prairie dog populations and spread to ferrets.

Black-footed ferrets exist on about 2 percent of their historic range. Listed as endangered in 1967, they were twice considered extinct in the 20th century before a population of about 20 was found. the Fish and Wildlife Service partnered with the Smithsonian Conservation Biology Institute in Front Royal, Va., and zoos to breed the animals in captivity and reintroduce them into the wild. The federal agency is also experimenting with using drones to scatter plague vaccine, encased in peanut butter tablets. There have been hopeful signs of a small rebound, but as development continues to slice up their habitat, their fate is still dicey.

Humpback whale

This is the famous singing whale. No one really knows why, but males, who do all the crooning, sing for up to 20 minutes at a time, sometimes all day, pausing only for a breath – an underwater opera. The singing nearly stopped in the 1960s, when more than a century of commercial hunting took its toll. Humpback whales were listed as endangered in 15 habitats worldwide, and their numbers fell to 1,600.

They were listed as endangered in 1967 under the Endangered Species Conservation Act that preceded the current act. Nearly five decades later, they’ve rebounded to about 20,000 individuals across the world, enough to be delisted in all but a few of their habitats.

Bald eagle

The bald eagle was designated as the national bird in 1782, just before the end of the Revolutionary War. Although the Americans and British stopped fighting, hostilities didn’t end for eagles.

Name a threat, and bald eagles have faced it: illegal shooting, poisoning, habitat destruction and contamination of its food with DDT. By the 1970s, when they were listed as endangered, bald eagles were on the verge of extinction. Wildlife officials teamed up with state and federal lawmakers to save them.

The first step was to ban DDT, ushering in a bald eagle recovery through conservation, including the protection of nesting areas. Bald eagle populations recently climbed to an estimated 70,000 birds from a low of about 400 breeding pairs in 1963, a recovery that the Fish and Wildlife Service called remarkable. They were removed from the endangered list in 2007.

American alligator

Basically a dinosaur, the alligator has been on Earth for 200 million years, hunting other animals and devouring them.

Humans nearly put an end to their existence through hunts and habitat destruction.

Like the black-footed ferret, alligators were listed as endangered in 1967. Authorities banned hunting to protect it, the American alligator quickly rebounded, and in 1987 the service declared that the animal had fully recovered. American alligators exist from North Carolina to Florida on the Atlantic coast and from Florida to Texas on the Gulf coast.

They abound in the Florida Everglades, where invasive Burmese pythons have begun to challenge them for the top spot on the food chain.

Grizzly bear

Half a million grizzlies once roamed the Lower 48 states, from the northwestern corner of Washington to southern Wyoming. About 1,800 remain.

Grizzlies were listed as threatened in the lower 48 states in 1975 after being reduced to 2 percent of their historic range. In their southernmost territory, inside Yellowstone National Park, they were fiercely protected, and hunting outside the park was ended. The bears’ numbers have grown from about 130 to 700 in the Greater Yellowstone Ecosystem, and now Fish and Wildlife is considering removing them from the list, sparking an ongoing fight with ranchers on one side, conservationists on the other and the federal government in the middle.

Florida manatee

Manatees have lived in Florida for 45 million years. They are big, gray, lumbering and docile marine mammals, plus they eat sea grasses, so people call them sea cows.

As Florida developed into a vacation and retirement paradise that swelled its population to 20 million, making it the fourth-most-populous state, sea cows found themselves in the path of boats. Sharp propellers butcher the animals, and they are harassed by snorkelers. Florida first acted to protect manatees as far back as 1893, and the federal government first protected them as an endangered species in 1967.

Aerial surveys in 1991 proved that their numbers had dropped from the tens of thousands to fewer than 1,300. In 2013, there were a record 800 manatee deaths caused mostly by humans. Faced with the extinction of the Florida manatee, the state and federal government stepped up protections. They created manatee protection zones marked clearly for boaters,

Manatee numbers are rebounding, with about 6,200 in the most recent annual count. Last year in January, Fish and Wildlife proposed to downgrade their status on the endangered list.

California condor

They are among the largest flying birds in the world, speeding at up to 55 mph in search for carrion. Thousands of years ago, they weren’t just California condors. They existed as far away as Florida.

But like every other animal on this list, it couldn’t overcome human expansion into America. “As people settled the West, they often shot, poisoned, captured and disturbed the condors, collected their eggs, and reduced their food supply of antelope, elk, and other large wild animals. Eventually, condors could no longer survive in most places,” according to the California Department of Fish and Wildlife.

A few remaining condors were backed into the Southern California mountains by the turn of the last century. It’s been illegal to kill a condor in California for 100 years. But the same chemical that nearly doomed eagles, DDT, was an invisible killer of condors. Compounding the problem, carrion killed by lead shot poisoned birds that fed on the carcasses. They’ve been protected by federal law since 1967 and state law since 1971.

By then, they were already too far gone. In 1982, only 23 were left worldwide. Five years later, they were rushed into a captive breeding program. In 1992, federal officials started releasing a few into the wild, and now there are 410 birds.

One of the first things the Interior Department did under its new secretary, Ryan Zinke, was rescind an Obama administration regulation that outlawed hunting with lead shot.

Gray wolf

As humans colonized the east and expanded to the west, ranchers came to despise wolf packs that killed cattle on instinct learned over thousands of years of hunting prey. They were hunted, shot, trapped and poisoned throughout the lower 48 states until they were on the brink of extinction.

Gray wolves were listed as endangered in 1978. Although some progress was made toward their recovery, they continue to be listed as endangered in 39 states and parts of another five because shooting and trapping still happen. A distinct north Rocky Mountain population was delisted due to recovery six years ago.

Foes of the endangered listing are working to remove wolves from the list, as conservationists complain that there isn’t a sufficient number to escape extinction. But it’s clear that protection has led to a rebound.