Inside the Cross-Border Partnership to Replant Trees for Monarchs

The number of eastern monarch butterflies that spend winters in central Mexico is down more than 80 percent compared to the 1990s, according to the conservation nonprofit Xerces Society. The Butterfly Pavilion — a nonprofit invertebrate zoo in Colorado — and the Mexican government are working to revive the population by restoring the habitat where the monarchs wait out the winter season.

In a forested, mountainous region northwest of Mexico City called the Monarch Butterfly Biosphere Reserve, the effort will plant 100,000 oyamel fir trees, which the butterflies rely on to stay dry and maintain their body temperature during the winter months. 

“It's such a powerful partnership,” Shiran Hershcovich, lepidopterist manager at Butterfly Pavilion, told TriplePundit. “It's a species that crosses countries and ignores our artificial borders.”  

The great monarch migration

North American monarchs are known for their extensive migrations, with those born east of the Rocky Mountains traveling up to 3,000 miles from Canada and the United States to central Mexico, National Geographic reports. 

“They have these ancestral migration routes that they have been following for countless generations, so they follow wind patterns, and they orient themselves with the angle of the sun and even the magnetic fields of the Earth,” Hershcovich said. “They're using all of these very complex conditions to guide themselves and reach, every year, the richly scented oyamel forests.”

But monarchs face a host of threats along that journey, including shrinking overwintering sites as parts of the oyamel forests are lost to things like legal and illegal logging and the effects of climate change. Last year, the winter butterfly count was just under 1 hectare (2.2 acres), 59 percent lower than the year before, according to an annual population survey. Due to the large number of monarchs concentrated in one space, they cannot be individually counted and are tracked by the space they take up instead, Hershcovich said.

Last year’s count is the second lowest so far. The record low was in 2013, when colonies covered a mere 0.67 hectares (1.65 acres), The Guardian Reports.

“Scientists estimate that the minimum threshold they need to cover is six hectares,” Hershcovich said. “But for the last few years, the monarch colonies have been under that threshold, so it's kind of a perilous time.”

Restoring the monarchs' wintering grounds

The partnership between Butterfly Pavilion and the Mexican government aims to restore five hectares (over 12 acres) of butterflies, bringing the total population up to the threshold needed to stabilize the migration. In addition to the oyamel trees, the organizations are planting native nectar plants and milkweed around the region, Hershcovich said. The nectar plants are an important food source, and milkweed is the only plant monarchs lay their eggs on.

Planting won’t begin until the region’s rainy season, which occurs in the summer. For now, the team is focused on preparation, she said. They’re growing saplings from oyamel seeds collected from the existing forest in a nursery in Michoacán state.

“They're a long lived species, and they take such a long time to grow that the best time to plant them is 100 years ago,” Hershcovich said. 

The Association of Zoos and Aquariums — a nonprofit that certifies zoos and aquariums based on things like animal care, welfare and conservation — is funding about 40 percent of the restoration via its Conservation Grant Fund, she said. Funds were also secured through other avenues.

“We're still in the fund development process,”Hershcovich said. “While we have our first year of the program fully funded, there is so much more work to be done.”

This isn’t the first attempt at restoring the oyamel forests. Reforestation as an ongoing battle with multiple factors at play, including continued loss of habitat due to wildfires, tree mortality, and rising temperatures that make some of the existing oyamel forests inhospitable for the monarchs, Hershcovich said.

“We're hoping to plant in some more climate resilient areas, which is why this restoration effort is a little bit different,” she said. “We're using some of the latest research to inform our decisions and the replanting. Higher elevation areas are going to be a little bit more resilient to climate stress because they're a little bit cooler and a little bit wetter. So it's going to be, hopefully, a more successful restoration effort.”

With the long and arduous migratory route, restoring the monarch population depends on more than just planting oyamel trees in Mexico’s Biosphere Reserve. While reforesting their winter home will go a long way toward remedying the butterfly’s waning numbers, action needs to be taken throughout North America.

“We can't just protect one little corner and call it a day,” Hershcovich said. “There needs to be work across the entire migratory range.” 

Just about everyone along the route can participate in the work by caring for their local environment and planting native nectar plants and milkweed, she said. Hershcovich also encourages people to rethink their pesticide and herbicide usage.

“At the end of the day, monarchs are bugs, too,” she said. “And things that kill insects kill all insects.”

Protection is vital, too

Monarchs enjoy special protections in Mexico — where they are recognized as environmentally and culturally significant — and Canada. Mexico’s Biosphere Reserve is a federally protected area, and Canada recognizes the monarch as an endangered species. But unsuccessful efforts to classify it under the Endangered Species Act in the U.S. date back to 2014. Monarchs could finally be designated as threatened later this year, NPR reports.

The monarch population can also vary greatly from year to year. Counts in Colorado can see 200 to 300 percent increases during rainier years when more milkweed and nectar plants grow, Hershcovich said. She’s hopeful that increased restoration efforts will help the population rebound.