Last month, government officials gathered in Washington, DC, for the monarch butterfly’s first summit, just as the dawn of the “Monarch Waystations” that are now ubiquitous on the American turf was beginning to blossom. Like everyone else, they were worried about the fate of the iconic insect, after decades of noticeable population decline in the butterfly’s winter colonies.
There are two different (but genetically identical) populations of monarchs in the United States, and both are migratory. Monarchs west of the Rocky Mountains spend winters in southern California, while those east of the mountain range fly thousands of miles from northern Ontario to central Mexico, where they wait for months. cold in oyamel spruce stands. Since the mid-1990s, scientists have found that the number of butterflies arriving in Mexico has dropped by about 70 percent. They blame bad weather, deforestation and car collisions for the fall.
In 2020 alone, 26 percent fewer eastern monarchs arrived in Mexico than the previous year, after being hit by storms and droughts. Those who survived the trip found their already tiny wintering areas reduced by illegal logging. In 2019, researchers concluded that the Western monarch “was on the verge of near extinction” after a 97 percent reduction in that subpopulation since the 1980s.
Therefore, it may be surprising — and perhaps controversial — for a recent study to be published in the journal Biology of global change suggests that some populations of monarch butterflies are actually found in the go up. “There is no apocalypse of the monarch butterfly,” says Andrew Davis, a professor of ecology at the University of Georgia (UGA) and co-author of the study. “Not in the United States, anyway.”
His group’s work is unusual because it focuses on insect breeding sites, not on their migratory scales. In other words, the team analyzed counts made in the summer in the U.S., not in the winter in Mexico or Southern California. Davis and his fellow researchers relied on more than 135,000 observations of monarchs made on both sides of the Rockies between 1993 and 2018 during the North American Butterfly Association (NABA) annual count. These events require citizen scientists to record all butterflies they see within a 15-mile radius for two days in early July.
Although the research team noted that there has been a slight decline in some regions of the US, especially in the Midwest and New England, areas such as the Southeast and Northwest Pacific are seeing more monarchs. Taken together, the data suggest an annual global increase of 1.36 percent over the summer range of the species, meaning that over the 25-year period, the summer population of monarchs in the U.S. has increased about 35 percent.
Davis says his team’s findings show that breeding butterflies in the summer is offsetting the losses insects experience during the winter. “They are able to recover and repopulate their entire breeding area each year, no matter how many there are in the winter colonies,” he says. “They’re just math. A single female can lay 500 eggs. If the conditions are right, the population explodes.”