Fewer than 2,000 monarch butterflies were counted overwintering this year along the California coast—a staggering drop from the 30,000 last winter and a 99.9 percent decline from the several million butterflies that once spent the winter between San Francisco and San Diego in the 1980s, a report from the Xerces Society said this week.
The plunge in numbers signifies a near collapse of the western population of the species, which spends the winter months in California. The monarchs migrate northward in the western United States during the spring, summer and fall months, producing several generations before an autumn generation migrates back to the West Coast for winter. The Xerces Society, an insect conservation nonprofit, has used community volunteers to count the monarchs at overwintering sites since the 1990s. The number of monarchs counted each year has trended downward, even though the number of sites being monitored has increased.