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 from predicted abundances. When the rate of change has not been constant across the entire time series, models that accommodate variable change rates are strongly recommended. 2. The monarch butterfly (Danaus plexippus) was recently added to the IUCN Red List (A2ab Endangered) based on analysis of data on winter abundances in Mexico and the western USA between 1993 and 2020. TYC estimates in the monarch 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. 3. Our results suggested that monarch population change rates have not been constant. The analysis yielded a model averaged TYC estimate of +5.23%, which was not statistically distinguishable from 0% and was considerably different from values of -46% and -72% in the assessment. The Bayesian posterior probability of a TYC value below -30% (A2ab Vulnerable) was 0.15 and that of a TYC value below -50% (A2ab Endangered) was 0.03. 4. 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 using models with variable change rates, as models with constant change rates may not accurately predict the trajectory of monarch abundances into the future.