Monarchs in California form a distinct group from the larger, eastern population. Separated by the spine of the Rocky Mountains, the two clusters have different migration routes. While eastern monarchs pass through the central U.S. to overwinter in Mexico, western monarchs ride out the cold in coastal California between Mendocino County and Baja California. The Xerces Society coordinates groups of volunteers to perform annual counts throughout the western range each Thanksgiving.
During the 2017 count, volunteers reported a total of 192,668 butterflies across 263 sites — itself a gradual decline of more than 90% from historical populations in the millions. The following year, however, something particularly shocking happened. Results from 2018 tallied only 28,429 butterflies, a decline of 86% in a single year and of 99.4% since the 1980s. By their estimates, only one butterfly remains alive today for every 160 in the past, placing the species below a critical threshold of 30,000 needed to guard against extinction.
This year, the number of monarchs visiting overwintering sites totaled only 29,418, another disappointingly low count. The situation has become so dire that the U.S. Department of Fish and Wildlife Services has been petitioned to list the species as endangered. In Pacific Grove, the Monarch Sanctuary counts likewise dropped an order of magnitude, from 7,350 in 2017 to 815 in 2018. In the most recent release, the Sanctuary reported only 642 butterflies.
Source: Monterey Herald, Jan 23, 2020