Change in monarch winter abundance over the past decade: A Red List perspective

Abstract

Assessing invertebrate species for the IUCN Red List under Criterion A frequently involves fitting an appropriate statistical model to available abundance data and calculating a 10‐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 recommended. 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. Our results suggested that monarch population change rates have not been constant over the study period. 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. We recommend that researchers evaluating monarch conservation status consider using abundance models with variable change rates, as models with constant change rates may not accurately predict the trajectory of monarch abundances into the future.

Dramatic Recent Declines in the Size of Monarch Butterfly (Danaus plexippus) Roosts During Fall Migration

“…Debate over the conservation status of monarch butterflies has largely focused on how to weigh two very different sets of population trends, with clear declines at overwintering sites in Mexico (3,4,8) but possible relative stability in the summer breeding range (despite clear summer declines in regions where use of the herbicide glyphosate is particularly high, for example) (12). This disparity in population trends for winter versus summer monarchs has led to confusion and controversy over the conservation status of monarchs, perhaps best exemplified by the recent listing, and then delisting, of monarchs as "endangered" by IUCN (45). Our findings suggest that steep declines among fall-migration monarchs might bring these seemingly irreconcilable trends into alignment (see also 7,16,46).…”

Changes in landscape and climate in Mexico and Texas reveal small effects on migratory habitat of monarch butterflies (Danaus plexippus)

“…The eastern population declined from the 1990s to 2014, with no statistically detectable trend from 2014 to 2018 21 and recent analyses indicate the population has been relatively stationary, but low, for the last 10 years 22 . Population declines in the mid-1990s and early 2000s were associated with the rapid adoption of herbicidally tolerant GMO corn and soybeans across the midwestern US from ~ 1994 to ~ 2006 and glyphosate application, which caused large declines in milkweed and monarch butterfly egg production 10 , 23 , 24 .…”

What monarch butterfly waystation names and characteristics can reveal about program participants

“…These declines have led to monarch butterflies being petitioned for listing under the US Endangered Species Act, with a decision that listing was warranted but precluded (USFW, 2020). Furthermore, in 2022 the International Union for Conservation of Nature listed the eastern migratory subspecies of monarch butterflies as endangered (Walker et al, 2022) but later down-listed the monarch to vulnerable (IUCN, 2023) due to disagreement with the population models used for assessment (IUCN, 2023;Meehan & Crossley, 2023).…”