While analyzing the mountains of data harvested from three decades of butterfly counting in the Midwest, a Michigan State University Ph.D. candidate had seen enough studies to be braced for bad news.

But, as reported in this week’s Proceedings of the National Academy of Sciences, finding that none of the 136 species of butterflies’ populations increased over that time took Wendy Leuenberger aback.

“We expected to find that at least some species had done well over the past 32 years,” she said.

Leuenberger and colleagues report the results after they combined more than 4.3 million observations in the U.S. Midwest from 1992-2023 to characterize changes in butterfly biodiversity. The results: Fifty-nine of the 136 species declined in abundance over the study. That means for every 10 species and 100 individuals present at a county at the beginning of the study period, there are now only nine species and 60 individuals.