When the Endangered Species Act was signed into law 50 years ago, nothing about it was controversial. The legislation had passed unopposed in the Senate and by a near-unanimous majority (355-4) in the House. No one in Congress, the news media, or the scientific community seems to have had any qualms about enacting what is now one of the most potent and rigorous environmental laws on the federal books.

“Essentially no skepticism was expressed about either the law’s conservation goals or its regulatory strategies,” University of California, Berkeley professor Holly Doremus, a leading environmental law scholar, wrote in the Journal of Law & Policy. “Legislators appear to have regarded it as an opportunity to deliver ringing rhetoric that would please the environmental movement without facing any immediate political costs.”

Accepted at face value was the assurance that with the new statute in place, endangered plant and animal species would be protected and their populations restored to healthy levels. That was the reason for the Endangered Species Act, explicitly spelled out in the text. Section 2(b) says the law’s purpose was “to provide a program for the conservation of such endangered species and threatened species.” The law also defined those key words, “conserve” and “conservation.” They mean “to bring any endangered or threatened species to the point at which the measures provided pursuant to this chapter are no longer necessary.”

Measured against that yardstick, the Endangered Species Act has been a failure.

To begin with, there are more endangered and threatened species than ever before. When the law went into effect in 1973, fewer than 130 species were on the government’s list of domestic plants and animals at risk of going extinct. Today, according to the US Fish and Wildlife Service, there are well over 1,600. And just as the ESA has not prevented the endangerment of many additional species, it has not achieved its mission of bringing endangered flora and fauna populations back to health. All told, only 57 domestic species (3 percent of those listed) have recovered, while 11 (1 percent) have gone extinct. After a half-century under the stringent safeguards established by the law, 96 percent of the endangered-species list is still, by the federal government’s reckoning, endangered.

Attempting to put lipstick on a pig, defenders of the status quo cheer the fact that 99 percent of listed species haven’t gone extinct. But as Jonathan Adler of the Center for Environmental Law at Case Western Reserve University observes, that is like celebrating because 99 percent of patients admitted to the emergency room years ago are still on life support. The whole point of treatment is for patients to get well, not to linger in ill health.

Similarly, the whole point of the Endangered Species Act was to conserve at-risk plants and animals, not just to list them. What went wrong?

The essence of the problem lies in how the law treats property owners. The ESA makes it illegal to harm an endangered species, including by destroying or changing the habitat where that species lives. Since two-thirds of species on the endangered list are found on private property, owners can face crippling restrictions on the use of their own land. Rather than encourage property owners to protect the habitat on which such species rely — for example, by reimbursing them for the profit they stand to lose if a planned development is halted or a stand of timber goes unharvested — the law threatens them with painful losses if they don’t. Often their response has been to preempt that threat by eliminating the evidence before the government learns about it.

“When it comes to private land, ESA encourages not preservation of endangered species, but their quick shooting,” science writer Ronald Bailey wrote in 2003. “If an endangered species is found on a farmer’s property, she has every incentive to, as they say, ‘shoot, shovel, and shut up.’”

Sam Hamilton, a career Fish and Wildlife Service official who became the agency’s director under President Barack Obama, grasped the issue. “The incentives are wrong here,” he once said. “If I have a rare metal on my property, its value goes up. But if a rare bird occupies the land, its value disappears. We’ve got to turn it around to make the landowner want to have the bird on his property.”

Sadly, Hamilton died just months after being named to head the Fish and Wildlife Service and was never able to effect that turnaround. As the Endangered Species Act embarks on its second half-century, it is clearer than ever that the law needs an overhaul. Will it take another 50 years to make that happen?