Every fall, in a monumental migration, millions of monarch butterflies leave their summer breeding grounds and travel thousands of miles to reach overwintering sites in Mexico. ©Eric Rock

In recent years, reports on monarch butterfly populations have been confusing: just two years ago, we were hearing that the monarchs were teetering on the edge of extinction. More recently, however, conservation news headlines have alerted us to increases in monarch butterfly populations—as much as 144 percent in one year. So, how, exactly, are the monarchs doing?

Part of the confusion is caused by the fact that there are two populations of monarchs, the eastern and the western. The eastern population, found east of the Rocky Mountains, makes up the vast majority of the North American monarch population and migrates to a specific area in Mexico for the winter, known as the Monarch Butterfly Biosphere Reserve. The smaller western population migrates to a much wider area along the central and southern California coast, collecting in scattered groups at lower densities than their eastern counterparts.

Lately, the eastern population has seen encouraging growth. The western population, however, is not doing as well. Numbers are plummeting, down more than 86 percent in 2018 from 2017 and more than 99 percent down from the western population high in the 1980s.

This map depicts the two migratory monarch butterfly populations in the U.S. and their migration patterns. ©Monarch Joint Venture

Why is this so, and what are the prospects for monarch butterflies in the future?

How do we measure monarchs?

Not only are there two different populations of monarchs experiencing disparate growth rates, they are each counted differently, too. World Wildlife Fund in Mexico and the Monarch Butterfly Biosphere Reserve measure the eastern monarch population in hectares, or the area the overwintering insects cover. Due to the sheer number of butterflies and the density in which they gather—anywhere from 10 to 50 million butterflies per hectare—there’s no reliable way to count individual insects. Thus, measuring the population in hectares gives a reliable evaluation of its year-to-year increase or decrease.

In contrast, the western population does not concentrate in such tight numbers. Because of this and the fact that there are far fewer butterflies in the western population overall, it’s possible to estimate the number of individual butterflies during the Western Monarch Thanksgiving Count, a citizen-science effort coordinated by Xerces Society for Invertebrate Conservation, an international nonprofit organization that protects the natural world through the conservation of invertebrates and their habitats.

Not only are there two distinct monarch butterfly populations—each experiencing different growth rates—they are each counted differently, too. ©Fernando Romo

Why are monarchs of the West waning?

Monarch butterflies west of the Rocky Mountains spend their winters in coastal tree groves in California. In the spring, they fan out to Arizona, Idaho, Nevada, Oregon, Utah and Washington, laying eggs on milkweed plants and feeding on flowers.

According to the Xerces Society for Invertebrate Conservation’s 2018 Western Monarch Thanksgiving Count, the number of West Coast monarchs spending the winter in California had plunged to only 20,456 butterflies—a drop of 86 percent since the previous year. Scientists predict that if this population continues to decline at that rate, we will completely lose migratory monarchs in the western United States over the next several decades; there is a 63 percent chance of extinction in 20 years and an 84 percent chance in 50 years.

Declines in U.S. monarch populations are attributed to farmers’ increasing use of pesticides that kill milkweed plants and other native vegetation; human development that has wiped out the butterflies’ habitats, destroying roosting forests in California and Mexico; and climate change. Increasing carbon dioxide levels in the atmosphere may be making milkweed—the only food monarch caterpillars will eat—too toxic for them to tolerate.

Monarch wings are getting larger, possibly because climate change is forcing the butterflies to travel longer distances as breeding grounds shift farther north. ©Eric Rock

Why are monarch wings growing?

Climate change isn’t just affecting monarchs via milkweed. It’s also shaping the butterflies more directly. Micah Freedman, a Ph.D. student in ecology and evolution at the University of California, Davis, visited museum collections around the country in 2017. He measured thousands of monarchs dating back to the 1870s. Monarch butterflies come in a range of sizes, with wingspans from 3.5 to 4.8 inches, so it was only when he began to analyze his measurements on the computer that he found a small but consistent 4.9 percent increase in wing size over the past century and a half.

In his study, published in the science journal Animal Migration in December 2018, Freedman states that he believes that one reason wing size could be on the rise is climate change. In fact, an earlier study funded by the U.S. Geological Survey and the U.S. Fish and Wildlife Service showed that the top three factors that led to lower monarch numbers were loss of habitats and higher temperatures both in the spring and the late summer. Warmer weather could be pushing spring and summer breeding grounds farther north, which means a longer return trip to Mexico in the fall. Since monarch size corresponds to the distance they migrate, Freedman says, monarchs with larger, longer wings have a major advantage over their smaller fellows.

The Thanksgiving tally

There are things that you can do to keep monarch butterflies flying into the future. Create new monarch habitats by planting milkweeds that are native to your area. Don’t spray pesticides. Work to understand why our climate is getting warmer, what the solutions are and then take action to implement them.

Monarch butterflies are beautiful—and ephemeral. Each adult butterfly lives only about four to five weeks. ©Eric Rock

You can also become a citizen scientist by tracking the monarch migration, milkweed growth, caterpillars and other important aspects of monarch survival. Coming up soon is the Western Monarch Thanksgiving Count. Participate, if you can.

Lastly, the 3,000-mile monarch butterfly migration is one of the most epic on Earth. This Thanksgiving, give thanks for that bounty.

Here’s to finding your true places and natural habitats,