Monarch Butterfly Population Decline Warns of "Terrible" Future Issues
North American monarch butterflies journey thousands of miles during an annual mass migration, but the trip didn’t quite go as planned in 2018. Every year, the eastern population of monarchs travels to high-elevation forests in Mexico, and the western population travels to wooded groves along the California coast. And every year in California, volunteers work to count how many of the pumpkin-colored insects nestle in for the winter. But this year the state’s monarch population reached a record low, an 86 percent decline from 2017.
The Xerces Society for Invertebrate Conservation, the nonprofit that oversees the Western Monarch Thanksgiving Count, says this statistic is pooled from the preliminary count results from 97 sites. In 2017, these sites housed 77 percent of the overwintering population of western monarchs — a total of 148,000 monarchs. In 2018, just 20,456 monarchs were spotted in the same locations.
It’s a decline that Cornell University professor Anurag Agrawal, Ph.D. tells Inverse is “terrible and troubling.” Agrawal, who has studied the dependent relationship monarchs have with milkweed, explains that year-to-year variations — like the difference seen here from 2017 to 2018 — are often driven by climate. California has just gone through some of the worse drought and wildfire seasons in its history, and if climate conditions in the state change for the better in coming years, he expects population numbers to make some gains.
But there’s an overarching problem that goes beyond climate. “Over the long-term, persistent declines are likely caused by other major factors,” Agrawal explains, “and the California population has been declining for about 50 years.”
It seems to be getting worse, faster. There has been a 97 percent decline in the total population of the western monarch butterfly since the 1980s. Overall, both eastern and western groups are not faring well. A 2017 study published in Biological Conservation estimated that there is a 72 percent chance monarch butterflies will be extinct within 20 years. Both populations are believed to be affected by shared threats: a reduced abundance of milkweed plants caused by an increase in genetically modified herbicide-resistant crops, a loss of nectar resources from flower plants, and degraded overwintering forest habitats due to deforestation. Climate change and invasive plants (including invasive milkweeds) are believed to be secondary threats.
“Scientists have been concerned about monarch butterflies for many years, and this particular species is especially vulnerable to dramatic plunges because it migrates through a number of threatened habitats,” Sandra Schachat, a Stanford University Ph.D. student who studies insect morphology and ecological conservation, tells Inverse. “Monarchs’ reliance on various habitats makes this species vulnerable to a ‘perfect storm’ of sorts, in which various factors can come together in a short period of time to cause a dramatic decline.”
The presence of milkweed is essential to monarchs because they deposit their eggs onto the flowering plant, and their caterpillars feed exclusively on its leaves. The eradication of milkweed from agricultural and urban areas, some scientists argue, is linked to their population declines. However, well-meaning milkweed planting by humans looking to help has been a problem as well — the only species of milkweed widely available in the United States is not native to monarch regions, and many of the butterflies who have skipped their migrations to stay with garden milkweed have been infected with parasites.
Joseph Belsky, a Unversity of Arkansas graduate student who’s assessed the major drivers behind monarch declines, says that the planting of non-native milkweed species (commonly called tropical milkweed or Mexican butterfly weed) along the California coast has caused issues. “Monarchs coming into contact with these non-native milkweeds become disoriented,” Belsky tells Inverse, “and engage in breeding during the winter months when they conversely should be overwintering on eucalyptus trees.”
Agrawal explains that the monarch butterflies aren’t alone in their situation — many butterfly populations are in danger, especially migratory species. And he emphasizes that while “any one butterfly may not be a ‘keystone’ for the sustainability of an ecosystem, they are sentinels.” In his opinion, we should see their decline as a wakeup call, alerting us to the continent’s overall poor environmental health. Schachat emphasizes that, because butterflies pollinate plants, a world in which they go extinct means repercussions for the plants they pollinate and the myriad creatures that eat those plants.
“The loss of any plants that rely on butterflies will throw the entire plant community out of balance,” Schachat says, “which will have repercussions throughout the ecosystem.”
As for what can be done, Agrawal advises people who care about the problem to become involved in habitat conservation and restoration — anything from donating to conservation organizations to working to keep lands wild and free from development. Additionally, any steps people can take to minimize their impact on climate, Schachat advises, can help — that can be burning less fossil fuel, eating less meat, and being a responsible tourist when you enter a habitat.
Meanwhile, the Xerces Society will continue to count: All of its site data will be collected and analyzed by the end of January, and group members are keeping their fingers crossed that other sites are hosting monarchs.