Climate Change Is Decimating Monarch Populations, Research Shows

Western monarchs have lost 99.9 percent of their numbers since the 1980s

Monarch butterfly on Blue Mistflower
The butterflies have experienced major losses in populations on both the East and West coasts. TexasEagle via Flickr under CC BY-NC 2.0

In many ways, monarch butterflies are the poster child of the insect world. Amateur and professional entomologists alike celebrate the insects’ iconic black-and-orange wings.

However, the beautiful butterfly is under severe threat. Its numbers are dwindling precipitously and scientists are not sure why. Populations of eastern monarchs have declined more than 80 percent in the past two decades while western monarchs have fallen 99.9 percent since the 1980s.

A new study has linked climate change a major driver of monarch population loss. Researchers examining data from 18,000 monarch counts in the United States, Canada and Mexico learned the species is extremely sensitive to weather conditions in its spring and summer breeding grounds.

“Yes, climate change is happening,” study author Elise Zipkin, an ecologist at Michigan State University, tells Adam Vaughn of New Scientist. “It looks like it’s affecting monarchs. Now we have this information, we can have a smart plan for what we might do for conservation of monarchs and other wildlife.”

Published July 19 in Nature Ecology & Evolution, this study reviewed information culled from 25 years of population counts. Zipkin and her colleagues discovered that rainfall and temperature deviations from long-term averages over the past 15 years were seven times more impactful than other causes, including herbicides, pesticides and habitat loss.

Scientists plowed through numbers from two periods: 1994 to 2003 and 2004 to 2018. For the most recent timeframe, weather fluctuations in the spring and summer had the most impact on populations, severely damaging breeding cycles and growth stages.

However, from 1994 to 2003, monarch populations also dropped steeply, but that decline seems to be driven by herbicide use, as well as changing weather patterns.

“Unless more data become available somewhere, it’s impossible to say with certainty what caused the decline during that earlier time period,” Zipkin tells Kate Baggaley of Popular Science.

The monarch butterfly is seen as an indicator species, so what happens to it may foretell similar problems for other insects.

Butterflies, honeybees, bumblebees and other pollinators are crucial for the reproduction of many plants, including 35 percent of the world’s food crops, according to the Natural Resources Conservation Service, an agency within the U.S. Department of Agriculture.

In an effort to turn the tide and help monarchs make a comeback, volunteers are planting milkweed at eight sites across California, reports Erin McCormack of The Guardian. The poisonous plant is essential for the butterfly’s life cycle and an important food source for larvae, providing color later for their final-stage wings. Caterpillars devour milkweed leaves before entering the pupa stage and then emerging as butterflies.

California contributed $1.3 million to restore nearly 600 acres of habitat with 30,000 native milkweed plants. The effort is being led by River Partners, a nonprofit group that provides assistance with land restoration and reforestation projects.

To get a better understanding on what is happening, researchers are turning to the public for help. Several universities have banded together for the Western Monarch Mystery Challenge. Using the iNaturalist smartphone app, gardeners and backyard scientists are encouraged to snap photos of monarchs and include details of the sightings. Scientists say this effort will provide a better understanding of factors impacting the species.

“There are big gaps in our knowledge about monarch biology and behavior,” says researcher Cheryl Schultz of Washington State University in a report about the challenge.

Conservationists hope they have time to save the butterfly. Hillary Sardiñas, pollinator coordinator for the California Department of Fish and Wildlife, says it is critical to end this “death by a thousand cuts.”

“Monarchs are incredibly iconic,” she tells McCormack. “It would be horrible to lose these incredible butterflies that have captured people’s imagination for hundreds of years.”

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