Change in Monarch Butterfly Winter Abundance Over the Past Decade: A Red List Perspective

1. Assessing invertebrate species for the IUCN Red List under Criterion A requires fitting an appropriate statistical model to available abundance data and calculating a ten-year change (TYC) estimate based on model-predicted abundances. When the rate of change has not been constant across the available time series, models that accommodate variable change rates are strongly recommended.\n\n2. The monarch butterfly ( Danaus plexippus ) was recently added to the IUCN Red List (Endangered A2ab) based on analysis of a time series of winter abundance in Mexico and the western USA between 1993 and 2020. TYC estimates produced for the assessment came from models that assumed constant change rates. We conducted a Bayesian analysis of the same data using models that accommodated variable change rates and used those models to compute TYC estimates and quasi-extinction probabilities.\n\n3. Our analysis yielded a model-averaged TYC estimate that was not statistically distinguishable from 0%. The Bayesian posterior probability of a TYC value below −50% (Endangered A2ab threshold) was only 3%. Population projections from the best-performing density-dependent state-space model yielded 20-year quasi-extinction probabilities between 1% and 3%, well below the 20% threshold for Endangered status under Criterion E.\n\n4. We suggest that a more thorough analysis of recent overwintering abundances will lead to an improved IUCN assessment for monarch butterflies. We recommend that other researchers evaluating monarch conservation status consider recent changes in population growth rates, as long-term averages from the literature may not accurately predict the trajectory of monarch abundances into the future.\n\n### Competing Interest Statement\n\nThe authors have declared no competing interest.