The monarch butterfly (Danaus plexippus) has a unique life history whereby an annual migration cycle is completed by multiple generations each year.  North American monarchs are split by the Rocky Mountains into an eastern and western population that overwinter respectively in an isolated mountain range in Mexico and along the California coastline. Despite intense scientific interest in this species, very little is known about their continent-wide population dynamics, in large part because focused, single-investigator studies cannot examine trends at a large enough spatial scale.  We have used the North American Butterfly Association’s long-running count program to track patterns in monarch dynamics throughout its reproductive range in North America.  This program has used volunteers to conduct counts at hundreds of sites throughout the United States and Canada for the past 35 years.  We also used estimates of overwinter colony size in Mexico to track that portion of their life cycle. Our main goal was to determine if monarch populations show significant trajectories (increases or declines) in any portion of their range and whether overall patterns are more driven by dynamics on the summer vs. wintering grounds.  

Results/Conclusions

Monarch populations in eastern N. America show substantial short-term (2-3 year duration) fluctuations throughout much of their range, especially in the Midwest where they have their center of abundance. The eastern population shows no uniform trend towards increase or decline, although the high variability may mask underlying dynamics.  The western population has shown a steady decline during the past decade that may indicate that monarchs in this region are facing serious threats to population persistence. Comparisons between patterns in Mexican overwinter colony size and summer breeding abundance in the eastern range suggest that monarch populations can recover from extreme losses during the winter phase.   We will discuss potential causes of the population dynamics, regional declines and links between summer and winter population patterns.