NEW YORK (NYTIMES) - The monarch butterfly is threatened with extinction but will not come under federal protection because other species are a higher priority, US federal officials announced Tuesday (Dec 15).

Monarchs have long captured human hearts, fluttering through yards, parks and fields on wings that look like miniature works of art. But their numbers have been decimated by climate-change-fuelled weather events and pervasive habitat loss in the United States.

"We conducted an intensive, thorough review using a rigorous, transparent science-based process and found that the monarch meets listing criteria under the Endangered Species Act," Aurelia Skipwith, director of the US Fish and Wildlife Service, said in a statement. "However, before we can propose listing, we must focus resources on our higher-priority listing actions."

As part of the decision, monarchs' status will be reviewed each year by the agency, and conservation efforts will continue.

The number of eastern monarchs - which undertake an astonishing, multi-generational migration from as far north as Canada to overwinter in central Mexico - has declined by 75per cent since the 1990s, scientists estimate. Across the Rocky Mountains, western monarchs have seen an even more alarming drop.

Some of this collapse is tied to a need for milkweed, the only plant that monarch caterpillars can eat.

Milkweed has declined across monarch breeding grounds throughout the United States since farmers started using crops that are genetically modified to tolerate Roundup, a brand of weedkiller. Milkweed often grew among crops but cannot survive spraying.

In recent years, as the monarchs' plight has grown more dire, a movement has emerged to sustain the species by planting milkweed for caterpillars along with other native plants that nourish the adults.

Everyday citizens, advocacy groups and government agencies have planted 500 million milkweed stems, officials said, providing a lifeline for monarchs.

But given the increasing toll from climate change, which is fuelling winter storms that wipe out millions at a time in Mexico, droughts that kill them in the United States and temperature changes that may cause them to migrate too early or too late, efforts to protect monarchs have not been enough.

"While all of these people that care about monarchs are doing a lot of positive things, there are a lot of negative things happening at the same time," said Karen Oberhauser, a conservation biologist at the University of Wisconsin who has studied monarchs since 1985.

"We're running as fast as we can to stay in the same place." Federal protection would have helped, Oberhauser said. But officials said Tuesday they do not have the money or resources to protect all the species that need it.

"We have to work within the funding resources that we have," said Lori Nordstrom, assistant regional director for ecological services for the Fish and Wildlife Service's Midwest region.

Officials announced Monday that the northern spotted owl merits an increase in protective status from threatened to endangered, but the change was also scuttled to focus on higher priority species. Currently, 723 animal species are listed as endangered or threatened in the United States.

In addition to being threatened by climate change and habitat loss, monarchs are killed by pesticides sprayed on crops, and in cities, towns and backyards for mosquito control.

Increasing habitat is critical to helping species survive climate change, scientists say, especially given that its effects are already being felt. A drought in the Corn Belt, for example, wouldn't be as catastrophic for monarchs if there were robust numbers in the Northeast. But habitat is only part of the solution.

"One, we restore a lot of habitat," said Chip Taylor, founder and director of Monarch Watch and an emeritus professor at the University of Kansas. "And two, we try to convince our fellow citizens and particularly our politicians that we have to do something about greenhouse gases."

On Tuesday, the Trump administration made another announcement about the Endangered Species Act, limiting the very definition of "habitat" under the law. Environmentalists say the move could prevent the protection of land that species need to adapt to climate change.

While the Biden administration can undo it, the process will require resources that are sorely needed to take on a biodiversity crisis that has put a million species at risk of extinction around the world.

"The Biden administration is going to unfortunately be spending quite a bit of time simply bringing us back to where the status quo was four years ago," said Ya-Wei Li, director for biodiversity at the Environmental Policy Innovation Centre.

In the meantime, officials and monarch advocates are calling on Americans to plant milkweed and other nectar-rich native plants in their backyards or wherever they can. Rallying around monarchs, they point out, will create habitat that nourishes species up and down the food chain.

In Osage, Iowa, where Wayne Fredericks farms soybeans and corn, he has several acres of land in a federal program that pays him to maintain habitat that supports pollinators. He sees hundreds of monarchs during the summer, and also pheasant and quail.

"It's a delight," he said, and a win-win for his bottom line and biodiversity.

Scaling up these and other efforts is critical to saving monarchs, said Wendy Caldwell, the director of Monarch Joint Venture, a partnership of government agencies, conservation groups, businesses and academic programs to protect the monarch migration across the United States. "We need all hands on deck."