On its way to becoming law in 1973, the Endangered Species Act passed the U.S. Senate without a single no vote. The rare bipartisan consensus – that species facing extinction deserved federal protection, habitat and all – held until earlier this month, when federal regulators altered the act’s definition of “harm” to omit habitat destruction.

The law has faced pressure to bend to development and industry interests over the years, including a legal challenge that resulted in a 1996 U.S. Supreme Court decision upholding the regulation defining “harm” to include habitat destruction that impairs breeding, feeding, or shelter. In 2019, the Trump administration rolled back automatic protections for newly listed threatened species and made it easier to weigh economic costs when deciding whether a species warrants protection. Below, Stanford environmental law scholar Deborah Sivas and biologist Rodolfo Dirzo explain what the most recent change means and why it matters.

1. The change isn’t just a new definition, it’s a deletion.

Rather than narrow the previous definition of “harm,” which was affirmed in a 1996 Supreme Court decision, regulators eliminated it outright. They pointed to Justice Antonin Scalia’s dissent in that case, which argued that “harm” should require “affirmative conduct intentionally directed against a particular animal or animals” as their new guiding interpretation, Sivas said.

“In essence, the federal regulators have opened the door for developers and landowners to destroy habitat suitable for listed species without needing to get a ‘take’ permit,” Sivas said. Courts, not agencies, will now have the final say on what “harm” means going forward.

2. For most endangered species, habitat loss is the primary threat.

“Most listed species are at risk of habitat loss, which is frequently the primary basis for the listing decision,” Sivas said. Other pressures, such as disease and hunting, are often secondary. A bulldozer clearing forest without directly killing an animal would be considered legal now, according to Dirzo, who called the change a “grotesque narrowing” that puts many species at risk of local extinction even when no individual animal is touched.

3. The species most exposed are the ones that need the largest or most specific habitat.

Species like the northern spotted owl, which depends on old-growth forest, and wide-ranging carnivores like the gray wolf and Canada lynx, whose survival depends on large, connected habitat, will be harder to legally protect now, according to Sivas.

The ocelot, protected since 1982 largely because of habitat loss in southern Texas, is threatened by border-wall construction, which fragments habitat and cuts off genetic exchange with populations in Mexico. This “nonviolent” factor could push the U.S. population of ocelots toward extinction, Dirzo said, even though no ocelot is directly harmed by the wall itself.

4. The real crisis isn’t extinction. It’s populations disappearing in one place at a time.

If Texas’ ocelots disappear, it won’t matter much that healthy populations still exist in Latin America, Dirzo points out. The local ecosystem services they provide, such as rodent control and ecotourism, will be gone for good. In this sense, Dirzo argues the accumulating loss of local species populations is a more immediately pressing concern than their global extinction. Since habitat loss is the leading driver of population loss, the recent change to the Endangered Species Act strikes at the level where damage is hardest to reverse, according to Dirzo.

5. Courts will have the final word, but it may take a long time.

The new rule is facing lawsuits arguing regulators failed to justify the change or comply with environmental review requirements. Courts may ultimately decide what “harm” means case by case. But litigation takes years, and in the meantime, habitat can legally be destroyed, according to Sivas. “Without habitat, there are no wild species,” Sivas said. “That’s a future none of us should want to live in.”