Western monarch butterfly numbers rebound in Santa Cruz, but below historic levels

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SANTA CRUZ — Western monarch butterfly numbers have bounced back from dismally low-figures recorded in 2020 — at least temporarily — according to the Xerces Society for Invertebrate Conservation, which Tuesday released its 2021 Thanksgiving Count data.

Community scientists documented more than 247,000 monarch butterflies across the West. That’s more than a 100-fold increase from the previous year’s total of less than 2,000 monarchs, said Xerces Society Western Monarch Lead Emma Pelton, and the highest count recorded since 2016.

In Santa Cruz County, just 711 monarchs were recorded during the 2020 Thanksgiving count. Last year, more than 5,400 butterflies were documented clinging to Eucalyptus trees and fluttering through overwintering sites.

A monarch butterfly takes flight at Natural Bridges a few weeks ago. (Shmuel Thaler/Santa Cruz Sentinel) 

“We’re ecstatic with the results and hope this trend continues,” Pelton said during a webinar Tuesday. “There are so many environmental factors at play across their range that there’s no single cause or definitive answer for this year’s uptick, but hopefully it means we still have time to protect the migration.”

Monarch butterfly populations in Santa Cruz, and across the state, have dropped dramatically over recent years. The trend is linked to to habitat degradation and pesticide application, as well as the impact of climate change. The butterflies overwinter from approximately October through March across the California coast from Mendocino County south to San Diego.

Xerces Society has sponsored the Thanksgiving count since 1997, which is carried out by volunteers. Since then, the iconic species population has plummeted by 95%, according to Xerces Society data, which shows numbers were in the millions.

RELATED: Migration, interrupted: Monarchs find new rhythms in San Francisco Bay Area

“So this year is not recovery, and it will take multiple more years to understand if this is the beginning of a trend or just a blip,” Pelton said.

The 2021 count was marked by record turnout: community scientists observed 283 overwintering sites. Numbers of the butterflies trended south, reported Isis Howard, an Xereces Society Conservation biologist.

Monarch counts were more concentrated on the Central Coast, but even more so in Southern California.

Volunteers in the San Francisco Bay Area and Central Coast typically see the bulk of monarch numbers during the annual count. But this year, from Mendocino to San Mateo County, scant butterflies were documented. Santa Cruz County marked the first sighting of butterfly clusters of more than 1,000 strong, Howard said. Pacific Grove saw 14,000 monarchs overwintering at sites, and further south volunteers in San Luis Obispo County documented more than 90,000 monarchs.

Scientists are yet sure why the butterflies flocked to southern regions.

Weather could be at play. According to Pelton, dry spring and summer conditions can coax first-generation Monarch butterflies out of their cocoons. Those first generation butterflies that breed in California and at Santa Cruz landmarks such as Lighthouse Field and Natural Bridges, are crucial for the species’ population numbers to sustain.

It’s also unclear what factors — weather or elsewise — led to the boom in 2021 population numbers. Pelton cautioned that one good year doesn’t foretell conditions to come for the beloved butterfly, which is scheduled to be listed under the Endangered Species Act in 2024.

“These numbers are going to continue to fluctuate until we really deal with the underlying reasons we’ve seen these huge declines over decades,” Pelton said.

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