For decades environmentalists have been guided in their work by what became known as the “precautionary principle.” This decision-making guide was first put forward in environmental terms by pioneering naturalist and biologist Aldo Leopold in his landmark 1940s essay “Round River.”

His focus was the complexity of the environment.

“If the biota, in the course of aeons, has built something we like but do not understand, then who but a fool would discard seemingly useless parts? To keep every cog and wheel is the first precaution of intelligent tinkering,” Leopold wrote.

This is the major logic behind the Endangered Species Act, the strongest environmental law ever written. For the United States to allow a species to go extinct, it must go through an exhaustive process that is politically perilous.

This imperative has strong support. John Turner, the director of the U.S. Wildlife Service under George H.W. Bush, was a Republican president of the Wyoming Senate and a rancher. He regularly told a story of how his grandfather had kept all of the broken farm equipment he ever owned.

“My granddad and my dad used to say ‘It’s important to save all the parts,’ ” Turner said. “You never know when you’re going to need them.”

Protecting all the parts was a daunting task before. In the face of climate change that could dramatically transform or destroy ecosystems across the globe, it has become impossible.

With biologists predicting as many as 20 to 40 percent of all species could be lost in this century due to human-caused climate change, holding on to “every cog and wheel” may undercut efforts to preserve the ecosystems and their inhabitants that can survive the coming ecological bottleneck.

Making such choices isn’t easy. Nor should it be.

I have reported on scientists and other scholars who say we must be prepared to take risks to preserve the ecological treasures we value. This means balancing the need to reduce greenhouse gases with the need to protect biodiversity and other values.

With the recognition we must adapt to change, a new principle is taking hold among a new generation of environmental thinkers. Writers like Emma Marris, author of “Rambunctious Garden,” are urging society to expand its vision of conservation to previously ignored areas like canal banks, dried up croplands and blighted urban neighborhoods. She and Richard Louv, author of “The Nature Principle,” argue we can turn these areas into ecosystems that can bring nature to people and provide ecological services like filtration and carbon sequestration.

This doesn’t take away from the value in protecting wild lands. It adds to it.

Leopold himself outlined an approach in his 1948 book, “A Sand County Almanac,” that can guide these new conservationists. I call it the Resiliency Principle.

“A thing is right when it tends to preserve the integrity, stability and beauty of the biotic community,” he wrote. “It is wrong when it tends otherwise.”

This resiliency principle was for Leopold the foundation for conservation choices. It could be the primary guide for the future in a warmer world.

Rocky Barker covers the environment for the Idaho Statesman and is the author of two books, including “Saving All the Parts: Reconciling Economics and the Endangered Species Act.”