Eastern Monarch Butterfly Population Declines While Western Population Increases

Eastern Monarch Butterfly Population Declines While Western Population Increases

New reports show a sharp population decline and a loss of habitat in the forests where the Eastern monarch butterfly winter each year. In just one year, the presence of monarch butterflies in their wintering grounds dropped 22%, from 7 acres to nearly 5.5. acres in 2023. This is part of a mostly downward trend over the past 25 years—when monarchs once covered more than 45 acres of forest. The average for the past decade is 2.75 hectares, which is approximately 6.8 acres.

Every year, eastern monarch butterflies travel up to 2,800 miles from Canada and the U.S. to their wintering sites in the forests of Mexico. There, in what is known as the Monarch Butterfly Biosphere Reserve, monarchs cluster in shelter from winds, rain, and low temperatures.

A 2021 analysis shows that the most important factor affecting winter numbers is summer population size. Summer population size is driven by several factors, but the most important is weather in the southern United States in the spring—when monarchs are migrating north from wintering in Mexico. Summer weather, overall herbicide use in crop fields, and late winter population size are also contributing factors. Hotter, drier, colder, or wetter conditions in the southern U.S. are bad when monarchs are moving through. Last spring was very dry – see the drought monitor map from a year ago. One effect of climate change is more extreme weather variability, which may pose additional challenges to monarchs.

The population of western monarch butterflies wintering along the California coast has rebounded for a second year in a row after a precipitous drop in 2020, but the population of orange-and-black insects is still well below what it used to be. Researchers and volunteers tallied more than 330,000 butterflies in California and Arizona in November 2022, the highest number of these insects counted in the last six years. It was a promising rebound after the annual winter count in 2020 recorded fewer than 2,000 butterflies. In 2021, the number recorded was 247,000. It’s not clear why the western population has rebounded but one explanation could be that eastern monarch butterflies, which tend to spend the winter in Mexico, could be mixing with their western counterparts.

The U.S. Fish and Wildlife Service has indicated in its National Listing Workplan that they plan to propose the monarch for listing in fiscal year 2024.

April 14, 2023