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In April 2021, I wrote a piece for Harvesting Nature on what appeared to be the imminent extinction of the western Monarch butterfly population. Only about 2,000 butterflies arrived on their southern California winter range in 2020 where approximately five million once clouded the skies and trees. When a population sees a decline of this magnitude, coming back from the brink is rare, particularly in one breeding season, but it seems there is more to the story on the western monarch butterfly.
The 2021 Thanksgiving monarch count saw an unprecedented number of citizen scientists eager to help collect important population data. Across 283 count sites, the western monarch population estimate was over 247,000 individuals – a 100-fold increase in the 2020 count.
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Given the monarch’s astonishing rebound, population limiting factors come into question. What was it about 2021 – a severe drought year in the western U.S. – that was somehow favorable to the marked population increase? According to Emma Pelton, the Western Monarch Lead with the Xerces Society, “There are so many environmental factors at play across their range that there’s no single cause or definitive answer…but hopefully it means we still have time to protect the migration.”
Weather likely played a factor. 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” reported Hannah Hagemann of the Santa Cruz Sentinel.
While the 2021 count was inspiring, the population increase should be taken with caution. The Western Monarch Butterfly Conservation Plan identifies a five-year-average winter count of 500,000 butterflies to represent a sustainable population. Additionally, the U.S. Fish and Wildlife Service is still on track to propose an Endangered Species Act listing for this iconic transcontinental butterfly in 2024.