Monarch butterfly population more than halved by atmospheric river storms
After the highest initial count in two decades around Thanksgiving, atmospheric river-fueled storms battered overwintering roosts and led to the loss of the iconic western monarch butterflies.
Western monarch butterfly proponents’ spirits soared after the highest initial annual western monarch counts over Thanksgiving in more than 20 years. Then, three weeks of storms supercharged by atmospheric rivers slammed the overwintering butterflies. The final results after a second January count showed that weather took a toll on the endangered species. Numbers went down almost 60%.
"Although the overall number of western monarchs counted for the New Year’s count remains relatively high compared to recent years — with more than 116,000 butterflies reported — the 58% seasonal decrease exceeds the typical range of 35–49% observed over the previous six winters," said Isis Howard, Xerces Society biologist.
"You have severe storms, and then that means branches come down, trees topple over. We saw flooding at quite a few sites. And then we also have heard reports of monarchs being blown out of their clusters," said Xerces Society biologist Emma Pelton.
"And if it's too wet and too cold, they can't regain that energy to get back up into their clusters. So higher rates of predation and mortality," she continued.
The Western Monarch Count regional coordinator in Monterrey County said the groves in her area showed a 25% drop since Thanksgiving. She said other areas saw a 40% percent drop after the storms. Clusters were entirely gone in some areas.
Storm damage also prevented some observers from going out to the sites.
"On some sites, they're still there in good numbers, and we're hampered because of all the road closures and power outages and flooding. A lot of people just couldn't get out safely," said Pelton.
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Over 250 volunteers counted roosts and flutters from 272 overwintering sites across California and Arizona to estimate the population. The Xerces Society for Invertebrate Conservation runs the survey and counted 335,479 butterflies around Thanksgiving. That number dropped to 116,758 after New Year's.
Over the winter, the butterflies huddled for warmth in groves of trees and faced day after day of tropical storm-force winds and record rainfall. Many of the trees were non-native eucalyptus which is prone to being blown over in high winds and rooted in saturated soil. Observers reported flooding, downed tree limbs and even entire trees uprooted to Xerces.
‘Huge mortality’ and ‘storms take it up a notch’
Xerces scientists just started the count in 2017 and are unsure what is historically normal seasonal decline. Throughout years of drought and storms, the population drops have varied from 30 to 50%.
"We do think that there's a pretty big problem happening where we're seeing huge mortality. And these storms take that up a notch," said Pelton. "So we're losing a lot of butterflies. We don't know that it's 100% mortality."
She suggested that butterflies could choose another area over winter. Also, warming temperatures prompted some clusters to start mating and continue the north and eastward migration. One site posted that observers saw mating behavior which signaled an early end to winter.
Protect overwintering sites to withstand storms and droughts
"The plain fact is that if we lose overwintering sites in California, we could lose migratory western monarchs," said Howard. "Development, eucalyptus removal, and tree trimming all need to be managed thoughtfully if we are to leave space for these animals to survive."
The western monarch population crashed in 2020 to only 2,000 butterflies. The 2021 population rebounded to 250,000. It's a stark contrast to the 1980s when the wintering bugs numbered in the low millions in the West.
"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 of the tragedy after a promising initial count. "We don’t want to count on luck alone to ensure the survival of the western monarch migration."
"This is great compared to three years ago. This is not great compared to three decades ago," Pelton said. "We've seen such wild swings in the population over the last five, ten years that I really don't think two good years in a row is an indication of a trend."
The International Union for Conservation of Nature listed migratory monarchs as "endangered," but the butterflies have not been listed in the Endangered Species Act.
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After spending breeding season to the north and east, the monarchs return to mainly coastal California when the weather gets cold from November to February. Coastal forests offer food of winter flower nectar.
Dr. Arthur Shapiro, Professor of Evolution and Ecology at the University of California, Davis, says there are about 10 core wintering sites in California. He says he doesn’t know what happened to the western monarchs over the past several years.
Shapiro doesn’t believe drought, which reduced favored monarch food like milkweed nectar, is to blame for the precipitous drop in numbers.
"What I can say is that Monarch populations had been in long, slow decline before the 5-year drought and rebounded significantly during the drought," Shapiro told FOX Weather in 2021.
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Shapiro suspects neonicotinoid insecticides may have had a role in the decline. Pesticide use is lower in drought years because many farmers leave their land fallow.
"If pesticides are a cause, I won't say the cause of the declines, this would provide a mechanism to account for improved numbers in dry years," he added.
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Neonicotinoid pesticides accumulate in the nectar and pollen of treated crops and turf. The E.U. banned several neonicotinoid insecticides, used on crops and turf in 2013 to protect pollinators like bees.
A recent U.S. Geological Survey study also suggested that the pesticides played a role in the 57% decrease in the occurrence of the western bumblebee.