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The International Union for Conservation of Nature (IUCN) officially classified monarch butterflies as endangered. The group’s red list, which includes over 147,000 species, is used by NGOs, private companies, and global policymakers to inform conservation efforts for threatened and endangered species.
The U.S. Fish and Wildlife Service considered the monarch’s status in 2020 and determined that classifying the butterflies as endangered was warranted but that there were higher priorities. The species will be assessed again in 2024 to determine whether it will receive protections under the Endangered Species Act. These protections include the designation of critical habitat—land on which habitat-damaging activity and development by government agencies and private citizens would be prohibited, and the banning of “taking,” trapping, or killing the monarchs.
“If nothing else, the process of reviewing monarchs and getting people to talk about their decline and conservation has generated a lot of momentum,” said Wendy Caldwell, executive director of the nonprofit Monarch Joint Venture. “We’re making a lot of progress there and can continue to do so with or without that [classification].”
Both Eastern and Western monarch populations showed a slight increase during the most recent counts. Western monarchs stay west of the Rocky Mountains and spend winters in Southern California and the Southwest, while Eastern monarchs, whose migration is better-documented, spend winters from Central Mexico to lower Canada.
Although the population uptick sparked hopes of recovery among conservationists, the insect still faces a long road to reaching the population level it held three decades ago when an estimated 700 million made the annual migratory journey. “This is an improvement, but larger numbers are needed to significantly improve the population,” Karen Klinger, a GIS specialist examining monarch habitats at the Field Museum in Chicago, told Stacker.
Both populations make a multigenerational journey to breed throughout the summer and return home for the winter. Though intensive, monarch migration is also incredibly delicate, relying on favorable climate conditions and an abundance of the only plant monarch caterpillars eat—milkweed. Shifting landscapes and climate change have contributed to the monarch’s long-term decline.
Stacker looked at the decline of Eastern and Western monarch butterfly populations and contextualized the challenges facing the species, citing data from the World Wildlife Fund and the Xerces Society.
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Emma Rubin // Stacker
After record-low counts over the past four years, Western monarch winter populations showed an uptick in 2021
When the Xerces Society first began winter counts of Western monarchs in 1997, it surveyed just over 100 sites along the California coast. Today, its Thanksgiving counts cover over 283 sites in the Southwest and California. Even with the expanded survey range, a Stacker analysis showed that monarch counts declined by 80%. When normalized by the average count seen at every site, the population is nearly half of what it was. Historic droughts, wildfires, and encroaching developments have shrunk overwintering habitats, and pesticide use has impacted breeding areas.
Emma Rubin // Stacker
Monarchs occupied 35% more land in Mexico last winter than in the previous winter
Last winter, monarchs occupied 2.84 hectares of forest habitat in Mexico—a 35% increase from the previous year, but still a significant dip from the counts realized in the 1990s. Insect populations naturally fluctuate with climate conditions. Favorable temperatures and precipitation can be a boon to monarch populations, meaning year-to-year counts can offer a limited scope compared to long-term trends. When looking at population averages, the past five years represented a 68% decline from the 1993-97 time period.
In 2016, the U.S., Canada, and Mexico set a goal to reestablish Eastern monarchs in six hectares of forest in Mexico by 2020, a range that would protect the species from extinction thresholds even with the natural dips in its population. Though the deadline has passed, conservationists are still working toward that goal. A 2017 U.S. Geological Survey report estimated that over 1.3 billion stems of milkweed were needed throughout the Eastern monarch’s migration corridor. All but one of the 16 scenarios the USGS developed to reach that goal rely on land from the agriculture, residential, conservation, energy, and transportation sectors. Agriculture is the only sector that has the coverage area to restore the needed milkweed by itself.
Bob Hilscher // Shutterstock
Monoculture and large-scale use of pesticides have impacted pollinator populations
Unlike other plants that spread through seeds or runners, milkweed spreads via underground stems called rhizomes. “We probably, through agriculture, expanded the quantity of milkweed just by disturbing the landscape through cultivation,” Caldwell said. But changes in agricultural practices, including wider use of pesticides, consolidation of smaller farms, and the introduction of herbicide-resistant, genetically modified crops have reduced the spread of milkweed.
“Those agricultural fields used to be really productive for monarchs. They’re not anymore, but we want to bring that back again,” Mark Johnston, a GIS analyst with the Field Museum, told Stacker. “There is a big push, at least in our region, to try to get back to sustainable agriculture,” he said. This means scaling back heavy conventional production in favor of more diverse crops and avoiding herbicides like glyphosate, which decimate milkweed.
The U.S. Geological Survey has also recommended converting margins of monoculture farms to environmentally diverse habitats under the Conservation Reserve Program. In the application-based system, farmers are paid annual rent to halt agricultural production over a 10-15 year period, instead planting species that can support habitat and improve environmental quality. Such alternatives have different goals, with some focused on mitigating soil erosion or improving water quality and others targeting pollinators and endangered species.
Beyond agriculture, “right-of-way” corridors have also offered an opportunity to support pollinators in non-residential zones. Under an agreement with the U.S. Fish and Wildlife Service, the University of Illinois at Chicago has, since 2017, been partnering with transportation and energy companies to provide monarch habitats on those companies’ land in exchange for regulatory easement on other portions of land. Companies participating in the agreement are subject to additional conservation regulations under the Endangered Species Act, and could continue activities that are estimated to have a minimal or temporary impact on monarch habitats.
IrinaK // Shutterstock
Climate change has also disrupted monarch migratory patterns
Eastern monarchs, living anywhere from two to six weeks, make their 3,000-mile journey through a migratory relay race. Traveling 25-30 miles daily, they stop at breeding grounds to lay eggs for the next generation that will continue the trip north. Decreasing day length, temperature fluctuations, and milkweed quality are environmental cues that signal monarchs to delay reproduction and begin their return south. But climate change has made that process more fragile.
“What monarchs need most is well-timed resources,” Caldwell said, and what scientists call “phenological mismatch” has become more prevalent. This could be a late freeze killing swaths of milkweed in the South where the first generation of monarchs will stop to breed, or it could be later blooming times that make it harder for monarchs to find nectar—in essence, it is a demonstrable circumstance that alters the timing of a species’ life cycle patterns.
The nature of monarchs’ multigenerational journey can also amplify climate’s impact. In a year where temperature and precipitation are favorable, more caterpillars will survive to complete the next leg, exponentially increasing the number of eggs they’ll lay further north and the number of butterflies that will eventually return south.
Courtesy of Phyllis Schulte
Home gardeners can help.
Even though the agriculture sector has the biggest opportunity to restore milkweed, the USGS emphasized an “all hands on deck” approach that involves planting across rights-of-way, agricultural, and residential sectors.
Klinger, whose work with the Field Museum specializes in examining monarch habitats in metropolitan areas and along rights-of-way, said that the most successful gardens have many blooming flowers and diverse types of milkweed. Some species, like swamp milkweed, are preferred for egg-laying, while others may offer advantages based on seasonal circumstances. It’s also important that gardeners plant native milkweed because non-native varieties, such as tropical milkweed, can become invasive.
Regardless of space, Klinger emphasized that many habitats can support monarchs. “We have people monitoring patches of all shapes and sizes, from one or two plants to hundreds of plants in their garden,” she said. Through the Field Museum’s citizen science monitoring project in the greater Chicago area, they’ve seen a single potted milkweed support a butterfly’s full life cycle. Monarchs have been seen flying as high as 11,000 feet, so even skyscraper gardens can be beneficial. The above photograph, submitted to the project by a participant, shows a milkweed plant covered in caterpillars—evidence that even the most modest footprints can have a significant effect.
This story was written by Stacker and has been re-published pursuant to a CC BY-NC 4.0 License.