Rapid decline in Western monarch butterflies leads to phenological and geographic Allee effects

Allee effects are processes that become disrupted at low population sizes, causing further declines to eventual extinction. In 2018, the Western monarch butterfly population fell below the threshold that scientists predicted would trigger Allee effects. To identify components of Allee effects, we compare monarch performance before and after the 2018 crash. The population has expanded more slowly during spring and summer breeding and filled a smaller breeding range. Other components of the monarch life cycle have not changed after the crash. However, winter survival is much lower and wing sizes of overwintering monarchs are larger now than in the 20th century, suggesting higher mortality and stronger selection than in the past. Western monarchs now face new challenges to recover from low population size, in addition to the original causes of declines.