The number of monarch butterflies has fallen sharply in Mexico's wintering grounds
The number of monarch butterflies in their wintering grounds in Mexico fell dramatically this year to the second-lowest level on record, according to an annual survey.
The census, considered a benchmark for the species' health, showed the butterflies covered only about 2.2 hectares of forest in central Mexico, a 59 percent decline from the previous year. Only the winter of 2013-2014 had fewer butterflies.
Scientists say the decline appears to be caused by hot, dry conditions in the United States and Canada, which reduce the quality of available milkweed, the only plants monarch caterpillars can eat, as well as the availability of nectar from many species of flowers, which they feed like butterflies.
“It tells us that we need to step up conservation and restoration measures,” said Jorge Rickards, general director of the World Wildlife Fund in Mexico, which conducted the study with the National Commission for Protected Natural Areas and other partners.
Migratory monarchs are listed as vulnerable or at risk of extinction by the International Union for the Conservation of Nature's Red List of Threatened Species, the world's leading scientific authority on species status. They were initially classified in the more threatened category of endangered animals, but their status was adjusted in September.
The United States government has placed monarchs on what is essentially a waiting list for protection under the Endangered Species Act; the species is eligible for protection, officials say, but others take priority.
It's normal for insect population numbers to fluctuate dramatically up and down, but declines become dangerous when they are chronically eroded, as in monarchs, says Karen Oberhauser, professor emeritus of entomology at the University of Wisconsin-Madison, who has studied monarchs for decades. .
“If we start with low numbers, a catastrophic event could send the population into a tailspin from which it will be difficult to recover,” said Dr. Oberhauser.
A decline of this magnitude has never occurred after a figure as low as last winter, she added. Dr. Oberhauser also noted that deforestation in the butterflies' winter habitat in Mexico was low last year, so it does not appear to be a factor in the decline.
Monarch numbers during the summer breeding season were only slightly lower than last year, with some areas, such as the Northeast, showing higher totals. But drought conditions in the south-central United States and northern Mexico likely led to lower nectar availability and less successful fall migration, said Dr. Oberhauser.
The study, made public Wednesday, measures eastern monarchs, which live east of the Rocky Mountains. Western monarchs, on the other hand, winter primarily in California. Their annual count is conducted differently and this year the number of butterflies numbered 233,394. That was less than last year, but much higher than the dangerous low of less than 2,000 in 2020.
Yet western monarchs made up just five percent of their population in the 1980s, when numbers were often in the low millions, according to Xerces Society, a nonprofit organization dedicated to insect conservation that participated in both studies.