The number of monarch butterflies overwintering in the mountains west of Mexico City has shown a positive rebound this year, with the area they covered doubling in 2024 despite challenges posed by climate change and habitat loss, experts reported Thursday.
Rather than counting individual butterflies, the annual survey tracks the area they cover as they cluster on tree branches in the pine and fir forests of the mountain region. These monarchs travel from east of the Rocky Mountains in the U.S. and Canada to spend the winter in Mexico.
According to Mexico’s Commission for National Protected Areas (CONANP), this year the butterflies occupied 4.4 acres (1.79 hectares), a significant increase compared to only 2.2 acres (0.9 hectares) in 2023. Last year saw a 59% decline in the area covered, marking the second-lowest level since records began.
After spending the winter in Mexico, these distinctive black and orange butterflies begin their journey north, breeding multiple generations along the way. Their offspring that reach southern Canada later embark on the return trip to Mexico at the end of summer.
Gloria Tavera Alonso, director general of conservation at Mexico’s environmental agency, attributed the improved numbers of monarch butterflies to favorable climate conditions and increased humidity. Last year’s decline had been partly caused by drought along the butterflies’ migratory path.
Changes in weather from year to year are expected to cause fluctuations in monarch butterfly populations. Jorge Rickards, Mexico director general for the World Wildlife Fund, emphasized the need to remain vigilant and continue expanding conservation efforts to protect the species.
Gloria Tavera Alonso credited ongoing initiatives aimed at increasing the availability of plants that monarchs rely on for nourishment and reproduction along their migratory route.
However, monarch populations have been struggling north of the border.
The Xerces Society for Invertebrate Conservation has been monitoring western monarch populations, distinct from those wintering in central Mexico, along the California coast, northern Baja California, and inland regions in California and Arizona for 28 years. The highest count recorded was 1.2 million in 1997. In February, the organization reported just 9,119 monarchs in 2024, marking a 96% decline from 233,394 in 2023, which was the second-lowest number since the survey started in 1997.
A recent nationwide study in the United States has revealed a steady decline in butterfly populations across the Lower 48 states, with an average drop of 1.3% per year since the early 2000s. The study, published in Science, found that 114 butterfly species have experienced significant declines, while only nine species have seen increases.
Experts attribute the risks faced by monarch butterflies across North America primarily to the loss of milkweed, the plant where monarchs lay their eggs. Milkweed has been disappearing due to factors such as drought, wildfires, herbicides, and urban development.
In response to these concerns, the U.S. Fish and Wildlife Service proposed in December that monarch butterflies be classified as a threatened species under the Endangered Species Act.