Western monarch butterflies get a closer look

A recent study documents the butterfly’s decline, while a new project looks at how to improve its population.

 

On warm fall days on the California coast, it’s not uncommon to see the iconic monarch butterfly flitting through the sky.  In some places, so many butterflies are present that it makes an impressive display. “The air is filled with orange,” says Samantha Marcum, the monarch butterfly coordinator for the Pacific Southwest region of the U.S. Fish and Wildlife.

Marcum, who also works on the Western Monarch and Milkweed Habitat Suitability Model Project, is based near the Lighthouse Field State Beach in Santa Cruz, one of the groves where thousands of monarchs come each winter to escape chilly Western winters. On windy, cooler days, the monarchs can be seen up in the Monterey cypress and eucalyptus trees, clinging to the branches with thread-like legs, stained-glass wings winking in the daylight.

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A cluster of monarchs in a tree.
The Xerces Society/Carly Voight

In 1997, an estimated 70,000 monarchs came to the grove. At the last count in 2015, that number was down to 12,000. Lighthouse Field State Beach is one of 50 grove sites recently studied by the Xerces Society in a report published in July, State of the Monarch Overwintering Sites in California, and funded by the U.S. Fish and Wildlife Service. The authors hoped to illuminate the lives of Western monarchs, an understudied population of the species.

The study and the project Marcum works on looking at habitat and breeding corridors represent a two-pronged effort by conservationists and the U.S. Fish and Wildlife to study and restore the regions where significant parts of the migratory monarch lifecycle take place. By restoring these lifelines, they hope to head off an Endangered Species Act listing. Monarchs were proposed for listing in 2014, and the Fish and Wildlife Service has promised a decision in 2019.

The latest study tapped into two decades worth of data gathered during the Western Monarch Thanksgiving Count, an annual assessment of overwintering monarchs on the California coast. The study found a 74 percent drop in Western monarch numbers over the years, the first time that a definitive number has been placed on decline of Western monarchs. What used to be an arriving cloud of 1.2 million butterflies in 1997 to the coast has dwindled to a wisp of 292,674 in 2015.

Monarchs are perhaps best known for their massive migration from the eastern United States to the oyamel forests of Mexico, where many millions of butterflies cling to the trees like a strange lichen. The Western monarchs are a smaller, genetically similar population, which breeds west of the Rocky Mountains and overwinter primarily in California, as well as some parts of Mexico.

But the Western monarchs are far less studied than their eastern counterparts, which endangered species conservation biologist and lead author Emma Pelton says is due to their smaller numbers and the geography of the West.

“So much of the West is so sparsely populated, like the Great Basin, western Montana, eastern Oregon and Washington and those just aren't areas with a lot of humans out there watching them,” Pelton says.

To understand what is causing the decline of the Western monarch, Marcum says that they need more information about where the butterfly’s actual habitat is. While the recent study illuminates exactly how much the Western monarch populations have declined, the precise location of their feeding and breeding grounds is still unknown, as well as how many generations of butterflies it takes to get from their feeding grounds to the overwintering sites in California. Researchers have found that eastern populations with overwintering sites in Mexico take three or four generations of monarchs to get back to the northern United States and Canada. The Western migration is shorter, but much is still unknown about it.

That’s why the Fish and Wildlife Service, in collaboration with the Xerces Society, is carrying out the Western Monarch and Milkweed Suitability Habitat Project, which will identify key breeding and migratory sites. The project will provide that information to land managers in Western states to either proactively protect the sites, or begin restoration projects on degraded habitat, which most often includes planting native species of milkweed, the singular plant that monarch caterpillars feast on.

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Monarch caterpillars only eat milkweed.
The Xerces Society/Candace Fallon

They’ll also have to contend with a conundrum: the monarch butterfly is widespread and well known, despite its precipitous decline. Since it’s not rare to see one, Pelton says, it can be hard to get the message across that the species is in trouble. It’s also difficult to easily sum up what the problem is since many factors are likely driving their disappearance; less habitat, more pesticides, monoculture practices, and climate change all may contribute, Pelton says. “It’s death by a thousand cuts for monarchs.”

Correction: This story has been updated to reflect Samantha Marcum's correct affiliation with the Western Monarch and Milkweed Suitability Habitat Model Project and the Xerces Society's involvement.

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