When it comes to wildlife conservation, miracles can happen. Take the American bald eagle, whose story you may be familiar with—or the Arabian oryx.

The eagle’s populations once dwindled to fewer than 500 nesting pairs, thanks in part to the widespread use of a pesticide called DDT, which thinned their eggs, as well as habitat loss and hunting. The U.S. banned DDT in 1972, and the bird was protected under the U.S. Endangered Species Act of 1973. Thirty four years later, bald eagles had recovered sufficiently to be removed from listing.

Then there’s the Arabian oryx, a medium-sized antelope with long, straight horns. It was hit even harder than the bald eagle, hunted (mostly for trophies) until it disappeared from its native range in the Middle East in the 1960s. The International Union for Conservation of Nature, which uses hard data to assess the conservation status of wildlife around the world, classified it then as “extinct in the wild.”

But the Arabian oryx held on in private reserves and zoos around the world. Reintroduction efforts began in 1980, and its numbers quickly grew. By 2011, there were more than 1,000 of them living in the wild, and the IUCN reclassified it as “vulnerable,” a significant improvement in its conservation status. It’s the first animal ever to revert to a “vulnerable” listing after being classified as extinct in the wild.

These two classification systems are both used for the protection of animals, but they are different—here’s how we decide which animals are endangered and threatened.

Following the science

The IUCN periodically assesses every animal for which there is enough data to make an informed decision, explains Jon Paul Rodríguez, chair of the IUCN Species Survival Commission. This commission consists of more than 8,000 scientists in 162 countries.

The organization then decides where to place the animal on a spectrum, from worst to best: extinct, extinct in the wild, critically endangered, endangered, vulnerable, near threatened, and least concern.

Animals that are critically endangered, endangered, or vulnerable are all considered “threatened.” To make their decisions, the group considers data such as population size, number of breeding adults, geographic range, and the like. Rodriguez says that the group trains scientists intensively how to make these determinations. “It’s a fairly involved scientific exercise,” he says.

View Images The Arabian oryx (Oryx leucoryx) is the only species to have gone from being listed as extinct in the wild to being listed as vulnerable by the International Union for Conservation of Nature. Photograph by Joel Sartore, National Geographic Photo Ark

The IUCN is all about producing good science—it doesn’t, by itself, have legal force. “We produce data,” he says, “and then encourage society at large to consider those assessments in their decision-making.”

The Endangered Species Act, however, does have legal force in the U.S.

Signed into law in 1973, following public outcry over the decline of animals like the bald eagle, the act lists wildlife as “endangered” or “threatened.” Each of these designations provides a specific set of protections. When an animal is listed, habitat critical for its survival is demarcated, and there are some restrictions on what can be done in these areas. Listed animals also generally cannot be killed or harassed, unless somebody gets a special permit, and the penalty for doing so can involve fines and/or jail time.

The IUCN’s data is often used to make Endangered Species Act listing decisions, but scientific information from other forums—such as NatureServe Explorer, a collection of population information on tens of thousands of American species—is used as well.

Nearly 1,700 species are listed under the act, and it has prevented the extinction of nearly 300 of those, according to a study published today in the journal PeerJ. While it remains one of the world’s most influential environmental laws, it has been controversial, with some arguing that too many species are listed without being recovered, while others posit the opposite. (Read more: How effective is the Endangered Species Act?)

Making the list

There are several ways to get protection under the act, says Noah Greenwald, the endangered species director for the Center for Biological Diversity, an environmental group based in Tucson, Arizona.

The first way is the that the U.S. Fish and Wildlife Service, the agency tasked with enforcing the act, can designate a species as in need of protection. Then, the agency can either create a proposed rule for its protection or put it on a candidate list.

If the service proposes a rule, the public has a period in which they can comment on it, either to support the proposal or raise concerns. If everything goes smoothly, the species will receive official listing as endangered or threatened within a year of the proposal.

If it’s put on a candidate list, it receives no protection. These species are supposed to be assessed every year by law for listing and subsequent protection—but that often doesn’t happen, Greenwald says, because the agency has a large backlog.

Meet Some of the World's Most Endangered Animals Across the globe, species are vanishing at rates 1,000 times greater than ever. Meet five of the world's most endangered species, from tigers to turtles, through their portraits taken for National Geographic Photo Ark by Joel Sartore.

The second way a species can gain protection: A group like the Center for Biological Diversity, or even a private citizen, can propose a species for consideration. When this happens, the agency is supposed to issue a “90-day finding” (although it usually takes more than 90 days) as to whether the petition has enough evidence for the species to warrant further consideration.

If the petition passes this first test, then the agency has a year to decide whether the species fits in one of these three categories: warranted, not warranted, or warranted but precluded.

“Warranted” means the species deserves protection under the act, and the service issues a proposed rule, Greenwald explains, usually setting up listing within a year. “Not warranted” means the petition is denied, and it goes no further. “Warranted but precluded” means the agency decides there’s enough evidence is to justify listing the species as threatened or endangered but cannot immediately do so “because its limited resources must be devoted to other, higher priority actions,” according to the service.

In this case, the species is often put on the candidate list, to be re-evaluated every year. (Read more: Inside the effort to kill protections for endangered animals.)

But the process often doesn’t work that way, and species can sometimes languish on the backburner for years—or decades. Take for example the trispot darter—a small, handsome finned fish found only in a few places in the southeastern U.S. It was first considered to be listed as threatened or endangered in 1975, but it was not immediately reviewed by the service.

The fish remained a candidate until 1994, when the service determined that the fish could warrant protection, but scientific data was lacking. However, administrative changes two years later removed it from consideration altogether. In 2010, environmental groups including the Center for Biological Diversity petitioned the service to list it. Finally, in 2019, it was officially listed as “threatened.”

Greenwald notes that different administrations have very different track records when it comes to the Endangered Species Act. Under the Clinton administration, more than 500 species were listed, compared to 62 under George W. Bush (during much of that time, the Interior’s top lawyer was David Bernhardt, now acting Interior Secretary). Only a handful have been listed under Trump, he adds.