Every year, millions of monarch butterflies migrate to forests in central Mexico, but the wondrous insects, which have come to symbolize migration, are increasingly being threatened by climate change.

According to the U.S. Fish and Wildlife Service, between 1990 and 2015, a billion butterflies vanished. Scientists fear that as the temperature of the earth’s climate system continues to rise, the monarch butterfly, considered one of the “oldest, most resilient” species, could soon be wiped out.

The butterflies are famous for flying 6,000 miles every year to avoid severe weather. According to the Washington Post, they spend their summers in the northern United States and Canada, breed in the southern U.S. during the fall and spring seasons and experience winters in central Mexico.

In the Latin American country, the species inhabit a stretch of remote forests, each of them smaller than half a football field. Should a storm or heat stroke hit the area, the publication warns, the population of monarch butterflies in the region could be completely killed off.

In 2002, for instance, a winter storm killed about 75 percent of monarchs. A decade later, a heat wave in the Midwest killed tens of thousands of butterflies.

“At every stage in their migration, they are threatened by climate change,” Eduardo Rendón, the monarch butterfly coordinator for the World Wildlife Fund in Mexico, told the outlet.

In Mexico, the abundance of orange-winged butterflies fluttering among blue skies and on crispy leaves is considered “one of the great biological wonders of the world,” according to Lincoln Brower, a late expert on the species.

Across central Mexico, sport teams, schools and businesses are named after monarch butterflies. For locals, the species carry the spirits of their ancestors. For immigrants, the insects symbolize their journeys. For those who inhabited the land before Spanish colonization, they also held great significance, with pre-Hispanic pottery in the area often emblazoned with images of butterflies.

While scientists are still unsure why the monarchs concentrate in the region, they are certain that the butterflies are at risk with the rise of climate change and illegal logging in the forests they occupy.

“If you’re talking 20, 30, 40 years out, we’re not going to be talking about monarchs any more,” Chip Taylor, the founder of Monarch Watch and a professor at the University of Kansas, told the Washington Post.

While some efforts have been made to save the beloved species, including the World Wildlife Fund planting drought-resistant oyamel trees, which would provide shelter to the monarchs when temperatures rise, experts believe it won’t be enough.

“The migration will disappear unless we solve climate change,” Taylor said.