The western monarch butterfly population in California has experienced a dramatic decline of 99.9 percent between 1980 and 2020, with current numbers returning to near record lows after a short-lived increase from 2021 to 2023, according to a May 2 announcement.
This issue is significant as the monarch butterfly’s migration and survival are considered important indicators of ecosystem health. The drastic reduction raises concerns about environmental changes affecting both wildlife and habitats.
Scientists say that habitat loss, pesticide use, and the disappearance of milkweed—the only plant on which monarchs lay their eggs—are major factors behind this decline. Climate change is also cited as an increasing influence by altering weather patterns and disrupting migration timing. Monarch butterflies require temperatures above 50 degrees Fahrenheit during winter months, leading them to cluster in groves of eucalyptus, Monterey cypress, or Monterey pines along the Central and Southern California coast for protection against storms.
Locations such as Pismo Beach, Pacific Grove, Natural Bridges State Beach in Santa Cruz, Fiscalini Ranch Preserve in Cambria, Coronado Butterfly Preserve managed by the Santa Barbara Land Trust, Goleta’s Ellwood Mesa Open Space, Bodega Dunes Campground in Bodega Bay, and Point Mugu State Park have historically hosted large gatherings of monarchs. Although fewer butterflies are seen today compared to previous decades at these sites, they continue to serve as vital refuges monitored annually by volunteers and scientists.
Population data is gathered through community science efforts like the Western Monarch Thanksgiving Count held from mid-November through early December each year. A secondary count takes place from late December into early January. These counts rely on trained volunteers coordinated by organizations such as the Xerces Society.
Members of the public can participate by volunteering at overwintering sites or reporting sightings via projects like the Western Monarch Milkweed Mapper website. Additional support can be provided by planting native milkweed free from pesticides or joining local habitat restoration initiatives. According to organizers of these efforts, “reducing pesticide use, supporting habitat restoration efforts, and participating in local counts can all make a meaningful difference.”
