It's a topic of debate:
Last month, the U.S. Fish and Wildlife Service (FWS) declared that one of North America’s best known butterflies, the monarch, might be in trouble. But the agency put off protecting the insect under the federal Endangered Species Act. In the meantime, researchers are continuing to debate how best to gauge the health of monarch populations.
In recent months dueling preprints and publications have intensified the debate. In one camp: researchers who have documented drastic declines in the number of monarchs in Mexico and other areas where some butterflies spend the winter. They believe the species needs immediate help, particularly by protecting and expanding the milkweed-filled meadows where its larvae feed. In another: scientists who have tallied butterfly numbers in areas they occupy during the warmer months and concluded there is less cause for alarm. As a species, monarchs “don’t really need saving,” says Andrew Davis, an ecologist at the University of Georgia, Athens.
At issue is the fact that, given the butterfly's complex life cycle and migration patterns, there's no one reliable way to measure the insect's overall population. One way to measure is by estimating the amount of Mexican forest their overwintering population occupies; by this measurement, the butterfly's population has been trending downward.
But a different picture emerges from data collected north of Mexico during other parts of the year, Davis and others say. Each season, scores of scientists and thousands of volunteers tally butterflies as they fly past recording stations, and count monarch eggs and caterpillars they find on milkweed, the larvae’s only food source. When Davis and colleagues examined 20 of these data sets, covering time spans from 15 years to more than 100 years, they saw little evidence of drastic declines. Winter and spring populations had shrunk a little but summer and fall surveys showed few losses, they reported in October 2020 in a preprint posted on Preprints.org, which has not yet been peer reviewed.
Davis acknowledges his view that monarchs are not in danger of extinction is unpopular: “No one wants [it], ironically,” he says. “They want to keep saving them.”
And the preprint has drawn mixed reviews. Insect ecologist Anurag Agrawal of Cornell University calls it “important. … Some of these data have never been put together this way.” But Leslie Ries, an ecologist at Georgetown University, rejects its conclusions. “The picture painted … includes many faulty conclusions based on the studies or data sets cited,” she says.
Volunteer surveys of butterfly populations are useful but have limitations, especially as to where and when the surveys were taken. Volunteers might be focusing on places where the butterflies currently are, rather than where they used to be, and monarch generations - there are four in a year - can expand or contract independently of one another. There are other complex factors effecting monarch populations - climate, weather, and milkweed availability - that need to be considered as well.
While the overall data seems to point to a gradual reduction in the monarch population, more research is needed before the feds will take action.
Given such uncertainties and the fact that conservation efforts are already underway, U.S. officials say the monarch is not yet a prime candidate for federal protection, especially because resources are limited and other species are in greater need of help. But they plan to decide on the monarch’s status in 2024, FWS ecologist Lori Nordstrom explained last month at a virtual press conference.
The delay worries Tyler Flockhart, an adjunct ecologist at the University of Maryland Center for Environmental Science’s Appalachian Laboratory who has been modeling monarch population dynamics. “We run the risk of studying this problem to death by not taking actions until we are completely certain.”
My advice is the same as always: plant that milkweed.