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Illegal logging at wintering grounds in Mexico contributes to the monarch butterfly’s decline.

FRANS LANTING/MINT IMAGES/SCIENCE SOURCE

U.S. agency sidesteps listing monarch butterflies as endangered

The U.S. Fish and Wildlife Service (FWS) announced today it will not yet protect one of North America’s best known butterflies under the Endangered Species Act. The agency concluded that the iconic black and orange monarch (Danaus plexippus) has suffered population declines steep enough to possibly qualify for federal protection, but FWS will take no action at this time because 161 species already being considered for the list are a higher priority for support from the agency’s limited budget.

The decision, which came after 6 years of consideration, “is the right one at this time,” says Orley Taylor, an insect ecologist at the University of Kansas, Lawrence, and founder of a monarch conservation organization called Monarch Watch. “It acknowledges the need for continued vigilance, emphasizing the need to continue support for programs that create and sustain habitats for monarchs.”

Monarchs are now found throughout the world, but the two major populations are found in North America. One group, once made up of many millions of butterflies, flits up and down the east coast of North America as part of a 4800-kilometer migration to and from wintering grounds in Mexico. That population represents more than 90% of all North American monarchs. A second population in western North America heads only as far as the southern California coast in the fall.

Over the past 2 decades, surveys of these two wintering populations have documented steep declines, with numbers dropping to levels that threaten extinction. The eastern monarchs, which converge onto a small region in the Mexican highlands, congregate on trees in such dense clusters that they can’t be counted. Instead, researchers estimate numbers by measuring the area of trees the butterflies cover. During the 2019–20 season, that area dropped to 2.83 hectares, half of the previous winter and down from 18 hectares in the 1990s. The U.S. Geological Survey has predicted 6 hectares are required for monarchs to stay afloat in the long run.

The western monarchs have suffered even steeper losses. Their overwintering numbers dropped to about 29,000 in 2019—about 1% of its historic population—and as low as 2000 just a few weeks ago.

There are many reasons for the decline. It takes several generations for monarchs to complete each one-way trip and, along the way, habitat loss—particularly the decline in milkweed that sustains its caterpillars and plants that supply nectar to migrating adults—and pesticide use have taken a toll. Drought, fires, and other problems related to climate change have also stressed the insects. The monarchs that winter in Mexico have also lost habitat to illegal logging and been battered by winter storms.

In 2014, several conservation organizations and a private individual petitioned the U.S. government to consider adding monarchs to the endangered species list. But FWS didn’t find the species to be a very high priority. So, the agency took little action until last year, when a federal court ordered it to complete an assessment by today.

The agency’s decision, which will be formally published in the Federal Register on 17 December, was based on data evaluated by state and federal scientists. Even though the smaller western population is in more dire straits, these experts focused on the much larger eastern population, in part because the agency only considers listing whole species of invertebrates, and not just subpopulations, as it does for vertebrates. 

“We prioritize our workload based on the needs of the candidate and listed species,” FWS ecologist Lori Nordstrom explained at a virtual press conference. But it will revisit the issue, she said; the monarch remains under consideration as a candidate for federal protection and by 2024, FWS expects to be ready to propose whether to classify the species as threatened or endangered. In the meantime, FWS “will review [the monarch’s] status yearly to see if the listing priority changes,” she said.

One reason for the “precluded” status is that monarchs are already getting attention from FWS, as well as other government agencies, conservation organizations, and private individuals. Many groups are promoting the planting of milkweed, for example, and establishing pockets of wild meadows in agricultural areas. Those efforts “have made and continue to make a big difference,” says Charles Wooley, FWS regional director for the Great Lakes region.

But the difference may not be big enough. In 2015, then-President Barack Obama’s administration announced plans to increase the eastern population to 225 million butterflies—equivalent of 6 hectares in Mexico—by this year. That did not happen.

Not everyone supported federal protection for monarchs. “I don’t think the monarch should be listed,” says Anurag Agrawal, an insect ecologist at Cornell University. The species is very abundant in Spain, Hawaii, Australia, New Zealand, and other places where it’s been introduced, so it won’t go extinct. “We aren’t there yet,” Taylor says, but he thinks climate change may lead to a need for listing in the next decade or so.

There’s also an element of uncertainty about what the monarch numbers collected by surveyors really mean. “The year-to-year fluctuation in monarch numbers makes it difficult to put an exact number on the degree to which monarch populations have declined,” says Karen Oberhauser, a conservation biologist at the University of Wisconsin, Madison, who has studied monarchs since 1985. Data from as far back as the 1950s show “it is very clear that monarch butterflies are a very high fluctuation species in terms of their population dynamics,” Agrawal agrees. Populations that crash can recover. Females lay hundreds of eggs, only two of which need to survive for the population to survive. And because four generations occur per year, even if most of the butterflies in Mexico die one year, “there is opportunity for the population to recover.”

It is possible, however, that monarch migrations might cease, Agrawal says. In the West, “monarch butterflies have been disappearing from large parts of their range, and we don’t know why,” says Elizabeth Crone, an ecologist at Tufts University who has been tracking this group of monarchs. One possibility is that the insects are no longer moving away from wintering grounds, but instead are settling in there as some monarchs have done in Florida. “So, we might not lose the species, but we could stop seeing them in places like Nevada, Oregon, Washington, and even the Central Valley of California,” she says.

If that happens, conservationists say wintering territories in California will need to be protected, particularly in light of the dwindling numbers there. “For the western population, protection was needed yesterday,” says conservation biologist Tierra Curry at the Center for Biological Diversity, one of the groups that sued FWS to force it to evaluate monarchs. “Getting put on the candidate waiting list is better than getting denied,” she says, adding that she hopes President-elect Joe Biden’s incoming administration will take more protective steps.

Oberhauser, however, worries that without the resources that come with federal protection, the monarch situation will not improve. “We need to do more,” she argues. And Curry concurs that quicker action is better: “The longer listing is delayed, the more difficult and expensive recovery planning becomes.”