Monarch butterfly is not endangered

The International Union for Conservation of Nature (IUCN) has decided North America’s monarch butterfly is not “endangered.” Instead, the insect is only “vulnerable” to extinction, the group said last week—adding that it could lower the alarm still further, changing the listing to “near threatened” if an upcoming census suggests the population is stable or growing.

The 27 September decision followed a researcher’s challenge to population models an IUCN team used to justify the endangered designation, conferred just 14 months ago. The team committed a “scientific injustice” by ignoring data showing monarchs are “doing really well,” argued ecologist Andrew Davis of the University of Georgia.

IUCN’s shift marks the latest twist in a scientific debate over the health of the showy black and orange insect. Monarchs are found worldwide, but the North American subspecies, called the migratory monarch (Danaus plexippus plexippus), has become “a poster child of species conservation because of its awesome ecology and migration,” says ecologist Anurag Agrawal of Cornell University, who was not involved in IUCN’s assessment or the challenge.

Migratory monarchs are split into two populations separated by the Rocky Mountains. Each fall, butterflies in the larger eastern population make an epic migration of up to 4000 kilometers from Canada and the northern United States to a forest in central Mexico, where they overwinter. Then, their descendants make a stepwise return to the north, taking up to four generations to complete the journey. The western population winters in Southern California along the Pacific coast and breeds along the Rockies.

Researchers generally agree that the number of monarchs wintering in Mexico declined beginning in the 1990s or earlier but stabilized around 2014 at about 55 million individuals. Many researchers blame the decline on two factors: logging in Mexico’s forests and farming in the U.S. and Canada that increasingly relied on crops modified to resist herbicides. That increased the use of herbicides that kill milkweed, which monarch caterpillars feed on.

A counterscenario, put forward by Davis and others, is that populations were anomalously high decades ago thanks to large-scale clearing of forests for agriculture in the 1800s. The resulting open landscapes allowed milkweed and monarchs to flourish until the land shifted back to forest or intensive farming. Davis also notes that overwintering populations have crashed in the past only to recover the following summer.

In 2022, an IUCN team released an assessment that used annual counts of the wintering population from 1993 to 2020, and two population models, to discern long-term trends in monarch numbers, which can vary year to year. One model rested on a “linear” method that assumed a constant rate of change over time. Modeling found that the population could have shrunk by 22% to 72% over 10 years, meeting IUCN’s criteria for an endangered designation. The assessment also noted that the stability in overwintering monarch numbers seen since 2014 made “current rates of decline less concerning than they were in years past.”

Still, many scientists thought the “endangered” listing was warranted because drought along migration routes or cold winters could tip the population into an extinction spiral. “Monarch populations [are] at a level that most scientists suggest is not sustainable,” says Karen Oberhauser, a conservation biologist at the University of Wisconsin–Madison who was on the assessment team.

But last week an IUCN panel ruled that the linear model “cannot be considered plausible.” Instead, it suggested a more complex model—published in February 2020 in Frontiers in Ecology and Evolution by a team led by Wayne Thogmartin of the U.S. Geological Survey—is better. It shows the eastern monarch population reached an inflection point around 2014, with a steep decline giving way to a slower decline or a slight increase. Such results support a vulnerable designation, the panel said. And if the upcoming winter census shows continuing stability, it said “the most plausible category would become near threatened.”

IUCN’s ruling is not binding on its member nations, including the U.S., but it can influence policy. In 2020, U.S. officials concluded that legally designating the monarch as a threatened or endangered species was “warranted,” but not a priority.

The new designation is drawing mixed reactions. Given the difficulty of forecasting future monarch populations, “the precautionary principle would suggest keeping the listing at endangered, with the potential to uplist” later, says ecologist Leslie Ries of Georgetown University, who was not involved in the assessment or the challenge.

Agrawal, who believes “the original listing of monarchs as endangered was in haste,” is less worried. And Davis says he was “actually hoping that [IUCN] would just list [the monarch] as ‘least concern,’” its lowest threat level. He is considering submitting his own assessment to IUCN.

In the meantime, Anna Walker, an entomologist at the New Mexico BioPark Society who led the IUCN panel that recommended the endangered listing, notes that the vulnerable listing “still indicates a high level of extinction risk.”