Semmens, Brice X.Semmens, Darius J.Thogmartin, Wayne E.Wiederholt, RuscenaLopez-Hoffman, LauraDiffendorfer, James E.Pleasants, John M.Oberhauser, KarenTaylor, Orley R.2016-04-062016-04-062016-03-21Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus) Brice X. Semmens, Darius J. Semmens, Wayne E. Thogmartin, Ruscena Wiederholt, Laura López-Hoffman, Jay E. Diffendorfer, John M. Pleasants, Karen S. Oberhauser, Orley R. Taylor Sci Rep. 2016; 6: 23265. Published online 2016 March 21. doi: 10.1038/srep23265https://hdl.handle.net/1808/20639The Eastern, migratory population of monarch butterflies (Danaus plexippus), an iconic North American insect, has declined by ~80% over the last decade. The monarch’s multi-generational migration between overwintering grounds in central Mexico and the summer breeding grounds in the northern U.S. and southern Canada is celebrated in all three countries and creates shared management responsibilities across North America. Here we present a novel Bayesian multivariate auto-regressive state-space model to assess quasi-extinction risk and aid in the establishment of a target population size for monarch conservation planning. We find that, given a range of plausible quasi-extinction thresholds, the population has a substantial probability of quasi-extinction, from 11–57% over 20 years, although uncertainty in these estimates is large. Exceptionally high population stochasticity, declining numbers, and a small current population size act in concert to drive this risk. An approximately 5-fold increase of the monarch population size (relative to the winter of 2014–15) is necessary to halve the current risk of quasi-extinction across all thresholds considered. Conserving the monarch migration thus requires active management to reverse population declines, and the establishment of an ambitious target population size goal to buffer against future environmentally driven variability.This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/Article10.1038/srep23265openAccess