Monarchs overwintering in Mexico. (Photo: Estela Romero)

“On March 6 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 2024–25 monarch butterfly population counts. In December 2024, monarchs occupied 1.79 hectares, compared to 0.9 hectares at the same time in 2023. This represents a 99% increase, the biggest jump we’ve seen since the monarch overwintering area increased from 2.48 hectares in December 2017 to 6.05 hectares in December 2018, a 144% jump. While any increase is great news, this number is still below the average for the past decade, 2.81 hectares, which is itself far less than the area when researchers first started measuring colony size in the early 1990s (see graph). The decline in their numbers led to the December 2024 recommendation by U.S. Fish and Wildlife Service that monarchs be designated as threatened and thus receive the protection of the Endangered Species Act.

“At the wintering sites in central Mexico, where most monarchs from east of the Rocky Mountains overwinter, monarch population size is compared from year to year by the number of hectares (one hectare = 2.5 acres) occupied by trees with clusters of monarchs. WWF and CONANP have been 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 it is about 21 million.”

Read the full article by Karen Oberhauser at Monarch Butterfly Fund»