The U.S. government has spent billions of dollars trying to save more than 1,500 animal and plant species listed as endangered or threatened.

A number of House Republicans say that just 2 percent of protected species have been taken off the list and have called for an overhaul of the 1973 Endangered Species Act. But environmentalists credit the act with saving species from extinction and say that hundreds more are on the path to recovery. The Endangered Species Act enjoys fervent support among many environmentalists, whose Democratic allies on Capitol Hill have thwarted past proposals for change.

Here’s a look at five species and how they have fared since being added to the list:

1. Grizzly Bears

(Marc Cooke/Associated Press)

Grizzlies were listed as threatened in the Lower 48 states in 1975, after being nearly wiped out over their historical range. But the bruins have been coming back, particularly in and around Yellowstone National Park, where they number more than 700. They’re doing so well that the U.S. Fish and Wildlife Service is considering removing federal protections for the Yellowstone grizzlies. But some scientists warn against it, saying that climate change has devastated whitebark pine trees, a key food source for the bears. Another 1,000 grizzlies live outside the Yellowstone area, while 30,000 of the bears in Alaska have never been listed as threatened.

2. Gray Wolf

(Dawn Villella/Associated Press)

More than 6,000 gray wolves roam the Lower 48 states after they were wiped out in the Northern Rockies and only a small population was left in the Great Lakes by the mid-1990s. The federal government spent more than $100 million on wolf recovery, and the Fish and Wildlife Service has proposed removing the predator from the endangered list across the United States, except for a small population of wolves in the Southwest. Despite the rebound, environmentalists point to the drop in wolf numbers in the Northern Rockies after Congress lifted federal protections there in 2011. Since then, wolf population numbers have declined 7 percent because of new hunting and trapping seasons in Montana, Idaho and Wyoming.

3. Spotted Owl

(Abby Hehmeyer/AFP/Getty Images)

The northern spotted owl was listed as threatened in 1990 because of loss of old-growth forest habitat to logging. Lawsuits led to the establishment of millions of acres of reserves on national forests to protect not just the owl’s habitat but also that of threatened salmon and a host of other species. Despite the logging limits, the owl has continued to decline by about 3 percent a year. Scientists have now identified the top threat to its survival as the invasion of the barred owl, a more aggressive and adaptable cousin that migrated across Canada from the East Coast and is driving spotted owls out of their territories. Last year, the Fish and Wildlife Service began an experiment to remove up to 3,600 barred owls from Oregon, Washington and Northern California to see if that will provide enough save havens for spotted owls to reverse their decline.

4. Bald Eagle

(Gene J. Puskar/Associated Press)

The official symbol of the United States nearly became extinct through hunting and the widespread use of the pesticide DDT. In 1963, there were just 417 of the birds documented in the nation. More than $574 million was spent on the eagle’s recovery through 2007, the year its population reached about 10,000 mating pairs in the Lower 48 states and it was taken off the list. It is still illegal to kill a bald eagle under a 1940 federal law. The Fish and Wildlife Service says the bald eagle is now known or believed to be in all Lower 48 states, along with Alaska, where it was never considered threatened.

5. Caribbean Monk Seal

(Dennis Fujimoto/Associated Press)

Some animals protected under the Endangered Species Act go extinct anyway. The Caribbean monk seal, which once swam in the waters off Florida, Puerto Rico and the Virgin Islands, was taken off the endangered list in 2008 due to extinction. The only subtropical seal native to the Caribbean Sea and the Gulf of Mexico once numbered more than 250,000, but overhunting left the population unstable. The last confirmed sighting was in 1952.