Monarch Winter 2021–22 Population Numbers Released

Monarchs overwintering in Mexico. Photo: Estela Romero

Monarchs overwintering in Mexico. Photo: Estela Romero

On May 24, 2022, the World Wildlife Fund-Telmex Telcel Foundation Alliance (WWF) and the National Commission of Protected Natural Areas in Mexico (CONANP), released data from the winter 2021–22 monarch butterfly population counts. At the wintering sites in central Mexico, monarch population size is compared from year to year by the number of hectares (one hectare = 2.5 acres, slightly less than two football fields) occupied by trees containing monarchs. WWF and CONANP have been systematically monitoring this area since 2004, with similar data from 1993–2003 collected by the Monarch Butterfly Biosphere Reserve (MBBR). While the number of monarchs in a hectare varies from year to year and is difficult to estimate, our best estimate is that this value is about 21 million.

In December 2021, monarchs occupied 2.84 hectares in ten different colonies, half in the State of Michoacán and half in the State of Mexico. This compares to 2.10 hectares at the same time in 2020, or a 35 percent increase. The average for the past decade, from the winter beginning in 2012 to the winter beginning in 2021, has been 2.62 hectares, and the population has been declining since we began measuring it (see graph).

Graph showing monarch winter population numbers from 1993 to 2021
Area with monarchs roosting on trees (1 hectare = 2.5 acres). Note year-to-year weather-driven variation but long-term decline in monarch numbers illustrated by decade means. Data from 1993–2003: Monarch Butterfly Biosphere Reserve (MBBR, CONANP). Data from 2003–21: WWF-Telcel Alliance, in coordination with the MBBR.

While 59 million monarchs sounds like a lot, the decline over recent decades and ongoing population threats led to a 2020 conclusion by the U.S. Fish and Wildlife Service that officially listing monarchs as a threatened species under the Endangered Species Act was warranted but precluded by the fact that other species were at even greater risk. In 2016, members of the Monarch Conservation Science Partnership published a paper that set a conservation target of 6 hectares. This population size would give monarchs a reasonable buffer against fluctuations that occur from one year to the next. For example, monarch numbers have declined by over 50 percent from one year to the next several times during the time they have been monitored. If that happened after a year with already low numbers, the population might not be able to recover. Thus, while this increase is hopeful, monarchs are still not “out of the woods.”

Monarch numbers are largely driven by climatic factors and habitat availability. Weather conditions during the spring and summer of 2021 were better than they had been the previous year, resulting in fairly robust numbers last summer. But weather conditions predicted by current climate change models suggest the hot and dry conditions that lead to lower populations may become more frequent. And while many individuals and organizations are working to create more habitat for monarchs, we have not come close to replacing the habitat that was lost early in the twenty-first century. Much of this habitat loss was driven by the advent of genetically modified herbicide-tolerant crops and the subsequent loss of milkweeds from corn and soybean fields throughout the Upper Midwest. In the wintering sites in Mexico, forests are becoming more heavily degraded and they are less able to buffer monarchs from temperature extremes, including both warmer days and colder nights.

It is likely that monarch numbers would be even lower without the efforts of dedicated individuals throughout North America. We need to increase and expand these efforts to mitigate climate change and provide habitat for monarchs and the hundreds of species that share their habitats.

—Karen Oberhauser, Arboretum director

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