On a cool winter morning in the town of Pacific Grove, California, you may think the trees are dangling last summer’s browned leaves. But if you look closely, says Sarina Jepsen, director of endangered species and aquatic conservation for the Xerces Society for Invertebrate Conservation, you’ll realize that those “leaves” are living animals: individual monarch butterflies nestled close together “like paper files in a folder.” The town is an overwintering site for the migratory monarch; only as the day warms will the black and orange insects float into the sun in search of nectar, says Jepsen.
Monarchs are beloved North American creatures: photographed in meadows, welcomed in butterfly gardens, grown in classrooms, celebrated in art. They also are rapidly dwindling in number. Yet until recently, no one knew just how badly the monarch’s western population was faring. Even a 2014 petition to add the butterfly to the Endangered Species List – which the U.S. Fish and Wildlife Service still is considering – focused on the eastern monarch population, found east of the Rocky Mountains. Partly this was because researchers lacked estimates of the historic western monarch population.
Now, a new analysis has quantified the western monarch’s historic population as far back as the early 1980s by combining long-term data sets collected by researchers and citizen…
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