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Western monarch butterflies have been vanishing. This week, a sanctuary saw thousands return.

A western monarch butterfly flies about a garden created to support pollinating insects and birds at the Gardens at Lake Merritt in Oakland, Calif., on March 18, 2021. (Melina Mara/The Washington Post)

Before the sun was up Thursday, a team from the Pacific Grove Museum of Natural History arrived at its monarch sanctuary, with binoculars and data sheets in hand. Last year, no butterfly clusters were observed at this site in Northern California, so Natalie Johnston, a community science coordinator at the museum, said she wasn’t sure what to expect.

Two weeks ago, the team observed a single butterfly fluttering in the outdoor space filled with eucalyptus trees. Less than week later, there were more than 1,300. On Thursday, Johnston said they arrived when the butterflies would still be resting — easier to count — the silhouettes of their wings visible as light entered the space.

They found more than 2,500 monarchs.

Johnston said the counts at the sanctuary — one of the primary locations for the butterflies that stop along the California coast during their winter migration — happen in the early morning because butterflies are more active after the temperature warms up to around 55 degrees.

“As the sun begins to warm up, the butterflies can move. Suddenly an entire cluster will break apart and go into the skies at the same time,” she said. “That sight hasn’t been seen since 2019, and it was awe-inspiring and it gave us hope.”

The early signs of an uptick in the number of western monarchs this year don’t necessarily signal a long-term rebound for the insects, according to experts. The western monarchs have seen a major decline, as the butterflies vanish in part because of habitat loss and the changing climate. Still, groups that track them welcomed the recent sightings.

In 2020, there was a historic low for a population of western monarchs that has struggled in recent years, according to the Xerces Society for Invertebrate Conservation. Just 1,914 migratory western monarchs were counted in total last year, according to the organization. The group conducts annual counts of the butterflies across hundreds of coastal sites — including one that will begin next month.

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In 2019, fewer than 30,000 of the butterflies were counted, which Emma Pelton, senior conservation biologist with Xerces, said was already more than a 99 percent decline from where the population was in the 1980s.

“This is hopeful,” Pelton said about the recently observed uptick. “It doesn’t mean that we’re not really in a precarious situation with the population as a whole.”

Matt Forister, an insect ecologist and professor at the University of Nevada at Reno, said the recent counts are “good news, but personally, I don’t make too much of it in terms of informing a long-term trajectory.”

Insect populations, he said, are “famously stochastic, up and down, unpredictable things.”

“We just never make much out of one year’s worth of data,” he said. “If we get a few years like this, that’ll be awesome. But if you look at the long-term trajectory of the monarchs and most other butterflies in the West, you’ve got this slow downward spiral with lots of ups and downs.”

The monarch butterfly populations have been affected by human activity. And as the migrating butterflies traverse the West every year, they are affected by that environment, Forister said. The migrating western monarchs spend winter along the California coast, as well as some overwintering sites in Baja California, before spreading across Western states in the spring.

“So they’re facing all of the challenges that all butterflies in the West are, which is to say development, pesticides, climate change,” Forister said. “When we think about insect declines in general, it’s those things: It’s loss of habitat, it’s contamination of remaining habitat and then it’s climate change, which is altering resources across the whole landscape.”

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Pelton said that while the stress placed on the butterfly populations from the changing climate may not be reversible overnight, there are ways to address the habitat issues, such as by planting native plants — including native milkweed critical to the insect’s reproductive and breeding life stages — or by reducing pesticide use.

“Signs of hope are also signs that we need to keep doing what we’re doing,” Pelton said. “We need to scale it up.”

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