Monarch butterfly populations decline

A monarch butterfly lands on swamp milkweed in Michigan. The U.S. Fish and Wildlife Service recently named the butterfly as a candidate for listing under the Endangered Species Act. 


The U.S. Fish and Wildlife Service has found after a thorough assessment that adding the monarch butterfly to the list of threatened and endangered species is warranted, but it won’t be protected as endangered at this point because its precluded by other, higher-priority listings. 

Instead, the monarch butterfly becomes a candidate for listing under the Endangered Species Act, and its status will be reviewed each year until it is no longer a candidate.

The Endangered Species Act provides for a warranted-but-precluded finding when the service does not have enough resources to complete the listing process because the agency must first focus on higher-priority listing rules.

“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. 

“However, before we can propose listing, we must focus resources on our higher-priority listing actions,” Aurelia Skipwith, U.S. Fish and Wildlife Service Director, said. “While this work goes on, we are committed to our ongoing efforts with partners to conserve the monarch and its habitat at the local, regional and national levels. Our conservation goal is to improve monarch populations, and we encourage everyone to join the effort.” 

During the past 20 years, scientists have noted declines in North American monarchs overwintering in Mexico and California, where these butterflies cluster. 

Numbers in the larger eastern population are measured by the size of the area they occupy. 

At a density of roughly 8.5 million monarchs per acre, it is estimated that the eastern population fell from about 384 million in 1996 to a low of 14 million in 2013. The population in 2019 was about 60 million. 

The western population, located in California, saw a more precipitous decline, from about 1.2 million in 1997 to fewer than 30,000 in 2019. 

In 2014, the service received a petition to list the species and published a 90-day finding in December 2014. 

In 2016, the agency began an in-depth status assessment, looking at the global population as well as focusing on monarchs in North America, where 90 percent of the world’s population occurs. 

“The Association of Fish and Wildlife Agencies has been working side-by-side with the U.S. Fish and Wildlife Service on voluntary monarch butterfly conservation since 2014,” Sara Parker Pauley, director of the Missouri Department of Conservation and president of the Association of Fish and Wildlife Agencies said.  

“We’ve made great strides, including raising awareness and restoring key habitat, but we still have a long way to go for a sustainable, long-term impact. 

“Moving forward, we’ll continue to be a close partner in our collaborative efforts to restore this iconic species.”

In Colorado and Chaffee County, some of threats monarch butterflies face are fewer milkweed stocks available, collisions with vehicles during flight and natural predators, said April Estep, wildlife biologist with Colorado Parks and Wildlife. 

Adult monarch butterflies, easily recognized by their orange and black markings, feed on nectar from flowers. Monarchs lay their eggs exclusively on milkweed, the sole source of food for monarch caterpillars.

To help the butterflies as they migrate across the country, Estep said leave milkweed in place. 

“Monarchs lay their eggs on milkweed leaves and the caterpillars eat milkweed; it is their only food source,” Estep said. 

Because monarchs travel long distances, it is critical to maintain reliable sources of nectar plants for them to feed on and ample milkweed on which to lay their eggs, according to the fish and wildlife service.

Estep also said people can help the butterflies by providing water for them, and not disturbing any chrysalis. 

A chrysalis is the form a caterpillar takes before it emerges from its cocoon as a fully formed moth or butterfly.

There are two long-distance migratory monarch populations that occur in North America; the largest is east of the Continental Divide and overwinters in the mountains of central Mexico. 

Monarchs west of the Continental Divide overwinter primarily along coastal California. 

Estep said monarchs east of the Continental Divide begin their journey towards Mexico in September and October.

Monarchs also inhabit about 90 other countries, islands or island groups around the world, but these monarchs are believed to have originated from the North American population.

While the butterfly won’t be protected under the Endangered Species Act at the moment, other efforts to protect monarchs are underway. 

Conservation efforts ongoing across the continent, include partnerships with states, tribes, Canada and Mexico, local communities and conservation organizations, to address threats to the monarch and to bolster milkweed abundance and other habitat needs, according to U.S. Fish and Wildlife. 

Because monarchs are solely dependent on milkweed during the caterpillar stage, efforts to boost the number of milkweed stems across the country are the basis for many monarch conservation plans.

“The Rights-of-Way as Habitat Working Group is focused on conserving monarchs and other pollinators through habitat conservation on energy and transportation lands throughout the United States,” Iris Caldwell, program manager of the University of Illinois-Chicago’s Energy Resources Center said. “The Service’s decision underscores the important work we have ahead of us, particularly over the next couple of years. 

“We will continue to encourage energy and transportation partners to join the agreement as we all come together to save the monarch.”

More information and tips on how to help conserve monarch butterflies is available at