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Monarch Butterfly Populations In the West Are Down an Order of Magnitude (qz.com)
An anonymous reader quotes a report from Quartz: Far fewer [monarch butterflies] were heading south this year, and those that have arrived did so a month late, according to Xeres, a non-profit conservation group for invertebrates. One researcher said it was the fewest monarch butterflies in central California in 46 years. Surveyors at 97 sites found only 20,456 monarchs compared to 148,000 at the same sites last year, an 86% decline. It's possible more insects will make the journey late this year, says Xeres, but that now seems unlikely. The minimum population size before the species experiences "migration collapse" is unknown, but a 2017 modeling paper in Biological Conservation (pdf) found that 30,000 butterflies adult butterflies are probably the smallest viable population. Without this critical mass, there aren't enough insects in the western monarch population to continue one of the world's most remarkable lifecycles.