Last month, the government Officials held the first Monarch Butterfly Summit in Washington, D.C., when the milkweeds in the “Monarch Butterfly Station” ubiquitous on U.S. lawns began to bloom. Like everyone, they fear the fate of the iconic insect after decades of significant declines in the butterfly’s winter habitat.
The United States has two distinct (but genetically identical) populations of monarchs, and both are migratory. Monarchs west of the Rocky Mountains spend the winter in Southern California, while those east of the mountains fly thousands of miles from as far north as Ontario to central Mexico, where they wait in the pine forests for the colder months. Since the mid-1990s, scientists have found that the number of butterflies entering Mexico has dropped by about 70 percent. They blamed bad weather, deforestation and car crashes for the decline.
In 2020 alone, eastern monarchs traveling to Mexico fell 26 percent from the previous year due to storms and drought. Those who survived the trip found their already small wintering grounds dwindled by illegal logging. In 2019, researchers concluded that Western monarchs were “hovering at their quasi-extinction threshold” after the subpopulation declined by 97 percent since the 1980s.
So a study recently published in the journal may be surprising, as well as controversial Global Change Biology suggests that some monarch butterfly populations are actually rise“There is no monarch butterfly apocalypse,” said Andrew Davis, professor of ecology at the University of Georgia (UGA) and co-author of the study. “Not in America anyway.”
His team’s work is unusual in that it focuses on the insects’ breeding grounds, rather than their migratory stopovers. In other words, the team looked at counts for the entire U.S. summer, not the Mexican or Southern California winters. Davis and his colleagues relied on more than 135,000 monarch butterfly observations on both sides of the Rocky Mountains between 1993 and 2018 during the North American Butterfly Association’s (NABA) annual tally. The events required citizen scientists to record all the butterflies they saw within a 15-mile radius over two days in early July.
While the team noted a slight decline in some parts of the United States, particularly the Midwest and New England, regions such as the Southeast and Pacific Northwest saw an increase in the number of monarchs. Altogether, the data indicate an overall annual growth rate of 1.36% within the species’ summer range, which means that the summer population of American monarchs has increased by about 35% over a 25-year period.
Davis said his team’s findings suggest that butterfly reproduction in summer is compensating for insect losses in winter. “No matter how many colonies are in winter, they bounce back and repopulate the entire breeding range every year,” he said. “It’s just math. A female can lay 500 eggs. If the conditions are right, the population will explode.”