Western monarch populations reach highest number in decades

Western monarch populations reach highest number in decades

  • The western monarch butterfly population reached its highest number since the year 2000, with more than 335,000 butterflies counted during the annual Thanksgiving Western Monarch Count in California and Arizona.
  • Western monarchs winter in California and migrate thousands of miles every year, in a migratory cycle that takes three or four generations. They are counted annually in their by volunteers at these sites.
  • The population rebound is a positive development, but the species is still considered endangered and far from its population numbers in the 1980s when millions of butterflies could be seen in the trees.
  • Conservation efforts include protecting overwintering sites, planting native plants, reducing pesticide use and supporting conservation initiatives; the public can also participate in community science projects and make simple changes in their gardens and communities.

The population of western monarch butterflies reached its highest numbers since the year 2000, with more than 335,000 butterflies counted at their California and Arizona overwintering sites during the 26th annual Thanksgiving Western Monarch Count.

“We can all celebrate this tally,” Emma Pelton, a conservation biologist at the Xerces Society and western monarch lead said in a press release. “A second year in a row of relatively good numbers gives us hope that there is still time to act to save the western migration.”

More than 250 individuals took part in the 2022 Western Monarch Count, in which they examined sites along the California coast and a few others in California’s interior and Arizona in November and December. The volunteers counted groups of monarchs as they came together to spend the winter in groves of trees, often made up of nonnative eucalyptus species.

Monarch butterflies cluster together in treetops for protection over winter. The overwintering sites need to provide particular combinations of protection from weather, dappled sunshine, and nectar. This cluster is in Pacific Grove, California. Xerces. Photo by Isis Howard / Xerces Society.

The largest gathering of butterflies, 34,180 individuals, was at a private site in California’s Santa Barbara County. Santa Barbara, Ventura and San Luis Obispo counties have sites hosting more than 20,000 butterflies. The San Francisco Bay area also saw a considerable increase over the past few years, with around 9,000 individuals found.

A few publicly accessible sites in California where the public can see monarch clustering include Pismo Butterfly Grove in San Luis Obispo County, the Pacific Grove sanctuary in Monterey County and Natural Bridges in Santa Cruz County.

Although this population rebound from fewer than than 2,000 counted in 2020 to more than 335,000 in 2022 is positive news, scientists estimate that we are still down over 90% from historic numbers in the 1980s and into the early 90s, when millions of monarchs filled the California overwintering sites.

“We know we still have a long way to go to reach population recovery,” Pelton said, “and the storms that hit right afterwards mean we’ll start the spring with far, far less than this total.”

Chart showing the size of the overwintering population of monarch butterflies in California from 1997 to 2022 (green bars). The chart also shows the number of sites visited each year (blue line). Image from Xerces Society.

Scientists don’t know exactly why we have seen an uptick in numbers over the past two years. But, Pelton said, butterfly numbers, like many other insect populations are “bouncy” fluctuating from year to year in response to temperature, rainfall, and food availability.

The iconic monarch butterfly has been listed as Endangered by the International Union for Conservation of Nature (IUCN), a global authority on the conservation status of species. An Endangered listing means the species is likely to go extinct without significant intervention.

Monarchs meet the criteria to be listed under the U.S. Endangered Species Act, which would ensure protection for the butterflies and their habitats, but the U.S. Fish and Wildlife Service announced in December 2020 that the species would not be listed, saying that other species are a higher priority.

“Unfortunately, there continues to be very little meaningful protection for the species or its habitat. Overwintering sites in particular continue to be destroyed and damaged each year,” Isis Howard, a conservation biologist with the Xerces Society and coordinator of the count, said.

A monarch butterfly (Danaus plexippus).
A monarch butterfly (Danaus plexippus). Image by John Banks via Pexels (Public domain).

Right after the Thanksgiving count ended, the California coast was hit by extremely heavy rainfall events known as atmospheric rivers, which resulted in flooding, falling tree branches and trees being uprooted. Some overwintering sites were unaffected, but volunteers at others reported an increase in the number of monarch butterflies lying on the ground and blown away from their clusters due to the bad weather, making them more susceptible to cold and wet conditions as well as predation.

“Small populations are particularly vulnerable to being snuffed out by extreme weather, so we are lucky these storms occurred in a relatively good year,” said Pelton. “We don’t want to count on luck alone to ensure the survival of the western monarch migration.”

There are two populations of migratory monarch butterflies in North America, both renowned for their impressive overland journeys spanning up to 6,400 kilometers (4,000 miles). Eastern monarchs spend the winter in Mexico, while the western monarchs winter in California. In the spring, all monarchs migrate north, some as far as Canada. This migratory cycle covers thousands of miles and takes three or four generations. Monarch population estimates are taken at these overwintering grounds.

A map showing the migration patterns of the monarch butterfly in North America.  Monarchs overwinter along the California coast or in Central Mexico (both areas show in red). From those areas they spread northward during spring and summer to breed on milkweed, before the last generation of the summer journey to the overwintering areas. Southern Florida is colored purple to indicate that this is an area in which monarchs do not migrate, but live all year. Image courtesy of Xerces Society.

Conservationists say we must make a stronger effort to secure existing overwintering sites and to make them more resilient to the impacts of climate change. This can be achieved by replacing dead or dying trees, designing the sites to prevent or reduce flooding and growing native plants as nectar sources.

“The plain fact is that if we lose overwintering sites in California, we could lose migratory western monarchs,” Howard said. “Development, eucalyptus removal and tree trimming all need to be managed thoughtfully if we are to leave space for these animals to survive.”

After leaving their overwintering sites, western monarch butterflies depend on locating suitable habitats and securing breeding grounds across multiple western states. Gardeners, park managers, schools and others can play a role in the recovery of the western monarch population by making simple but effective changes, such as:

  1. Growing native milkweed (Asclepias) plants.
  2. Growing a variety of nectar plants, preferably native to your region.
  3. Reducing or avoiding the use of pesticides.
  4. Encouraging policymakers to support initiatives like the Recovering America’s Wildlife Act and the Monarch Action, Recovery, and Conservation of Habitat Act.
  5. Participating in community science projects that monitor monarchs, such as the Western Monarch Milkweed Mapper, Western Monarch Mystery Challenge and the nationwide Integrated Monarch Monitoring Program.

“So many people and organizations have come together to try and protect this butterfly and its habitats. From planting native milkweed and reducing pesticide use to supporting the protection of overwintering sites and contributing to community science, we all have a role to play in making sure this iconic insect makes a full recovery,” said Anna Walker, a member of the IUCN Species Survival Commission Butterfly and Moth Specialist Group who led the IUCN monarch butterfly assessment.

“It is difficult to watch monarch butterflies and their extraordinary migration teeter on the edge of collapse,” she added, “but there are signs of hope.”

Monarch caterpillars feed on milkweed plants. Photo by Bernard Spragg. NZ via Flickr. Public Domain.

Banner image of Monarchs Clustering on Monterey Pine in California. Photo courtesy of Photo courtesy of Carly Voight / Xerces Society.

Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter @lizkimbrough

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