Gray Whale

Forty years ago this week, the Endangered Species Act was signed into law by President Richard Nixon, making the United States the first nation on Earth to declare a basic right of existence for species other than our own.

The goal of the ESA was simple: keep species from going extinct, no matter the cost. This was a law with a conscience, and by and large it has worked. Today about 40 percent of the plants and animals listed as endangered in the U.S. are stable or improving; many would be gone without it.

Some species have recovered to such extent that they have been delisted - taken off the Endangered Species List. Many are pictured here.

Left: The Gray Whale (Eschrichtius robustus), which has rebounded in numbers after hunting of the sea giants was banned, was the first sea mammal to be taken off the Endangered Species List. It is still guarded by the Marine Mammal Protection Act.

By CBSNews.com senior producer David Morgan

Credit: Merrill Gosho/NOAA

Palau Fantail

A Palau Fantail Flycatcher (Rhipidura lepida), photographed in Koror Palau by Devon Pike, was delisted, as was the Palau Ground Dove (Gallicolumba canifrons).

Forty-seven species have been delisted from the Endangered Species List owing to recovery or to improved data collection about their populations. Nearly two dozen more species have been downlisted from "endangered" to "threatened," as threats to the species and their habitats are reduced.

Credit: Devon Pike/Wikipedia

Virginia Northern Flying Squirrel

The Virginia Northern Flying Squirrel (Glaucomys sabrinus fuscus).

Credit: FWS

Tennessee Purple Coneflower

Tennessee Purple Coneflower (Echinacea tennesseensis), photographed by Dr. Thomas G. Barnes of the University of Kentucky.

Credit: Dr. Thomas G. Barnes, courtesy of USFWS

American Peregrine Falcon

Two sub-species of the Peregrine Falcon, including the American Peregrine (Falco peregrinus anatum), have recovered. Despite delisting, the peregrine continues to be protected under the Migratory Bird Treaty Act.

Delisted species have no legal protective status under the ESA, but under an amendment to the law scientists continue to monitor populations for a minimum of five years after delisting.

Credit: USFWS

Magazine Mountain Shagreen

Magazine Mountain Shagreen (Mesodon magazinensis), a tiny snail found in the Ozarks of Arkansas, was the first invertebrate removed from the federal Endangered Species List, thanks to restoration efforts at its habitat.

Credit: Ron Caldwell, Lincoln Memorial University/Courtesy of FWS

Lake Erie Water Snake

Biologist Angela Boyer holds a Lake Erie Water Snake (Nerodia sipedon insularum) during an annual survey. Listed as endangered in 1999, the species was delisted in 2011.

The Concho Water Snake (Nerodia paucimaculata), found in Texas, has also been delisted.

Credit: Megan Seymour/USFWS

Red Kangaroo

American endangered species protections have also been extended to species beyond U.S. territories, such as the red kangaroo (left, wandering onto the Vines Golf Course in Perth, Australia), in order to ban the import of the species or products derived from them.

Three kangaroo species were listed as threatened in 1974 due to commercial exploitation, but populations have recovered after four Australian states enacted stricter wildlife management programs.

Credit: Stuart Hannagan/ALLSPORT

Hoover's Woolly Star

A native plant of California, Hoover's Woolly-star (Eriastrum hooveri) was delisted owing to the protections afforded by its growth on public lands, and a greater abundance than previously recorded by biologists.

Credit: Bureau of Land Management

Brown Pelican

Pesticides wiped out the Brown Pelican (Pelecanus occidentalis) in Louisiana in the 1960s, but the bird was reintroduced and was brought back from the brink of extinction.

Credit: Thomas Maurer/FWS

Eggert's Sunflower

Eggert's Sunflower (Helianthus eggertii), a plant native to Tennessee, Alabama and Kentucky, was removed from the federal list of endangered and threatened plants in 2005, primarily after successful recovery efforts at Arnold Air Force Base in Tennessee, home to the largest known occurrence of the Eggert's Sunflower.

Credit: FWS

Aleutian Canada Goose

In the mid-1970s, the Aleutian Canada Goose (Branta canadensis leucopareia), a subspecies of Canada geese found on Alaska's Aleutian Islands and in areas of the Pacific Northwest, numbered only in the hundreds. Today, after efforts by the Fish and Wildlife Service and through partnerships with Canada and private landowners, the numbers have rebounded, and the threat of extinction has passed.

Credit: Glen Smart/FWS

Tagging

Alaska Refuge Supervisor Dave Spencer bands an Aleutian Canada Gosling on Buldir Island in the Aleutians in this 1975 photo.

Credit: Jim Bartonek/FWS

Robbins' Cinquefoil

Once close to extinction, the Robbins' cinquefoil (Potentilla robbinsiana), found in the White Mountains of New Hampshire, has recovered, and was the first plant species to be removed from the Endangered Species List.

Credit: USFWS

Bald Eagle

In 1963 nesting pairs of the Bald Eagle (Haliaeetus leucocephalus) numbered as low as 400 in the lower 48 states. The species' dramatic recovery (there are as many as 10,000 nesting pairs today) led to its delisting in 2007.

Credit: Karen Laubenstein/U.S. Fish & Wildlife Service

Maguire Daisy

The Maguire Daisy (Erigeron maguirei) lives only on cliffsides in Southern Utah.

Credit: Michelle Dela Cruz, National Park Service

Columbian White-Tailed Deer

Habitat conservation efforts, including the founding of the Julia Butler Hansen Refuge in Washington State, have led to the recovery of the Columbian White-tailed Deer (Odocoileus virginianus leucurus), found in Oregon and Washington.

Credit: USFWS

American Alligator

Hunting and habitat loss led to the decline in Southern states of the American Alligator (Alligator mississippiensis). Listing on the ESA led to its recovery, but it shares some protections when it shares habitat with the endangered American Crocodile, which is closely resembles.

Credit: Rodney Cammauf/National Park Service

Gray Wolf

The gray wolf (Canis lupus), which has been delisted in some parts of the United States; the Fish & Wildlife Service recently held a public comment period on delisting the species throughout the lower 48 States, while also expanding recovery efforts for the Mexican Wolf (Canis lupus baileyi).

By CBSNews.com senior producer David Morgan

Credit: Gary Kramer/FWS