In late 2021, I stood in a forest about two hours from Mexico City and watched as a river of butterflies passed overhead. They were monarchs — the iconic, Halloween-colored butterflies — and they were coming here from the US to rest for the cold winter months.

Nearly all of the monarchs east of the Rocky Mountains spend winter in this one forest in Mexico. They often fly more than 2,000 miles to get here, pollinating plants as they go. And by December, there are tens of millions of them in the branches of native fir trees, which droop under their weight. Scientists believe the trees provide just the right temperature and humidity for the butterflies to survive winter before they return north.

While monarchs here seem hyper-abundant — I remember having to watch my step around puddles to avoid crushing them — their species is in decline. Scientists assess US monarch populations each winter by measuring the area they occupy in this fur grove in Mexico, and by counting butterflies in coastal California, where monarchs west of the Rockies spend winter.

The results from these assessments, shown in the graphs below, speak for themselves.

In Mexico, for example, the number of acres occupied by monarchs has fallen from an average of about 21 for the first 10 years of monitoring (1993 to 2002) to about 4.4 this past winter. Scientists have linked these staggering losses — part of a broader collapse of butterfly populations across the US — to the destruction of grasslands, insecticides, and drought caused in part by climate change.

“There’s a very clear decline in Mexico, which is where most of our monarchs go,” said Karen Oberhauser, one of the nation’s leading monarch experts and professor emeritus at University of Wisconsin Madison.

That decline underpins a proposal by the Biden administration announced late last year to grant monarch butterflies protection under the Endangered Species Act. The ESA is the strongest wildlife law in the country and one of the strongest in the world.

The US Fish and Wildlife Service, a government agency that oversees the ESA, proposed to list monarchs as threatened, a formal designation meaning they’re likely to soon become at risk of extinction. If finalized, that proposal would make it a federal crime to kill or harm monarchs, with many important exceptions. It would also require that federal agencies make sure their actions, such as permitting a gas pipeline or wind farm, don’t endanger the insects.

This proposal is a big deal — monarchs........

© Vox