Western Monarch Numbers Reach Worrisome New Low

A recent California butterfly count suggests that when it comes to saving the iconic insects, it might be now or never

Santa Cruz’s Natural Bridges State Beach boasts many wonders, among them a stunning ocean view, a so-called wildflower wetland, and tide pools galore. But for many-an-eager monarch tourist — this writer included — the Monarch Trail is the main attraction.

photo of monarch butterfly
Every fall, western monarchs head to hundreds of overwintering sites along the California coast to avoid colder winter climates. Last year, however, overwintering monarch numbers hit an all-time low. Photo by Michelle Maani.

During a mid-November visit, I beelined it from the parking lot to the trailhead for the short boardwalk path that leads to the heart of this well-known monarch overwintering site. I could feel my own excitement, and that of those around me, as I approached the main viewing platform in a cluster of eucalyptus trees. As my eyes searched the branches above, I was gleeful to spot my first cluster of butterflies, and then a second cluster nearby. But as I continued to crane my eyes upward, I began to feel worried. And when I realized these two clusters were the only monarchs in sight, I couldn’t help but be disappointed.

Afterwards, I wondered if I’d gotten the timing wrong. Had I arrived too early — or too late — to enjoy peak monarch? Or maybe I hadn’t looked hard enough. Were there more monarchs tucked high in other trees?

But I had arrived within the perfect window — late October through the end of November is considered the best time to spot the butterflies at Natural Bridges. And I probably hadn’t missed many hidden monarchs. As I learned later, fewer butterflies than usual arrived at the park this winter.

A Thanksgiving Weekend Count — which is organized by The Xerces Society and engages citizen scientists in counting and extrapolating monarch numbers across the state — found just 1,120 butterflies at Natural Bridges last November, down from 9,000 the same weekend in 2017. Numbers have fluttered between 500 and 9,600 at the site over the past 15-plus years. But they used to be much higher. In 1999 there were 15,000 butterflies. In 1998 there were 60,000. And in 1997, there were 120,000.

Western monarchs spend their lives west of the Rocky Mountains, migrating from breeding grounds in California, Nevada, Idaho, and Oregon to the Pacific Coast every fall to escape colder weather. They begin to arrive in October, and typically disperse from overwintering sites by mid-February. (The Western monarchs are not to be confused with the Eastern monarchs, which travel up to 3,000 miles from breeding grounds east of the Rockies to overwinter in Mexico every year. Eastern monarchs are faring slightly better than their western counterparts, but scientists estimate their numbers have declined by 90 percent since 1996.)

The Xerces Society — which has been compiling data on overwintering monarch numbers for more than 20 years — says the same dramatic downward trend seen at Natural Bridges has been observed at overwintering sites across California. In the 1980s, it’s estimated that anywhere between 4.5 million and 10 million butterflies made their way to the Pacific Coast every fall. But the 2018 Thanksgiving Count found only 28,429 butterflies across 213 California overwintering sights, the lowest number on record. That represents a 99.4 percent decline from the estimated population in the 1980s (and an 86 percent drop from the same Thanksgiving count just the previous year). Put differently, for every 160 butterflies overwintering in California eucalyptus groves in 1980, there was just one in 2018.

“To picture what this means for monarchs, imagine that the population of Los Angeles had shrunk to that of the town of Monterey,” says Emma Pelton, a monarch conservation expert with the Xerces Society.

There are several factors contributing to the steep declines. Habitat loss in summer breeding locations as well as in overwintering regions is one. Herbicide use is another. Specifically, herbicide use killing off milkweed — upon which the monarchs lay their eggs, and the only plant that young caterpillars feast on — poses a major threat to the insects. And of course, as with most things these days, climate change can’t be ignored: The increasing frequency and intensity of extreme weather events is an additional stressor on the already stressed pollinators.

All together, these factors threaten to wipe out the western population of the monarchs. In fact, experts say the 2018 overwintering numbers may have dipped below a critical threshold. A 2017 study published in Biological Conservation, for example, used a theoretical extinction threshold of 30,000 overwintering monarchs based on estimates of how many butterflies would be required for mating and thermoregulation.(Monarchs cluster together on tress to stay warm.)

But conservation groups are still pushing for the insects. The Xerces Society, for one, has released an action plan to help bring western monarchs back from the brink, which includes protecting overwintering sites from development and excessive tree trimming; planting milkweed and other native flowering plants along key migratory routes; protecting and restoring summer breeding habitat; and cutting herbicide and pesticide use, particularly use of neonicotinoids, which are absorbed by plants and then ingested by insects feeding on leaves, nectar, and pollen. And earlier this year, the Western Association of Fish and Wildlife Agencies (WAFWA) — a collaborative of government agencies across the Western US and Canada that aims advance conservation across western North America — adopted a conservation plan for western monarchs. Like that of the Xerces Society, the WAFWA plan outlines approaches to protect and restore monarch habitat. It also sets goals for public education, scientific research, monitoring, and coordination among state, federal, nonprofit, academic, and private actors.

“Can we promise that monarchs will recover and fill California’s skies again?” said Xerces Society Executive Director Scott Hoffman Black. “Sadly, no. But we are not going to be the generation that witnessed this loss and stood by and did nothing.”

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