A rapid decline in US butterfly populations

Butterfly populations across the United States are in alarming decline, according to a new study, with total abundance falling by 22% in just 20 years. Such widespread and worrisome losses portend broader environmental threats and emphasize the urgent need for conservation action. “Our national-scale findings paint the most complete – and concerning – picture of the status of butterflies across the country in the early 21^st century,” write the authors. The decline of biodiversity has been extensively documented worldwide. Among these losses, the decline of insects is particularly concerning, given their crucial roles in many ecological processes. Butterflies – the most systematically monitored insect group in the U.S. – have been tracked through volunteer and expert-led programs for decades. However, previous studies have been geographically fragmented and methodologically inconsistent, making it difficult to discern broad-scale trends in butterfly populations. To address this, Collin Edwards et al. compiled and harmonized an extensive and diverse dataset of butterfly monitoring across the contiguous U.S., integrating 12.6 million individual butterfly observations from more than 76,000 surveys conducted between 2000 and 2020.

Edwards et al.’s analysis found that butterfly abundance across the contiguous U.S. declined by 22% over the last 20 years, with an annual decline of 1.3%. While declines were observed in all regions, butterfly losses were particularly severe in the southwest. According to the findings, the widespread decline was largely driven by sharp population losses in many individual species, with 33% of species showing significant declines and only 3% experiencing gains. Over 100 species declined by more than 50%, including 22 that suffered losses exceeding 90%. What’s more, species traits such as wingspan and host plant preferences did not reliably predict which species were in decline, highlighting the complexity of the factors driving these losses. Despite these challenges, Edwards et al. note that butterflies have the potential for rapid recovery under favorable conditions, highlighting the importance of proactive conservation strategies to prevent further declines and to support long-term population stability. “Although Edwards et al. used a huge quantity of survey data, there are still many parts of the US for which there is very little data,” writes Brian Inouye in a related Perspective. “Increased monitoring and spatially replicated monitoring, even for short-term studies, would help resolve uncertainties about the status of rare species and help tease apart potential causes of changes in butterfly abundance.”