Are migratory monarchs really declining in eastern North America? Examining evidence from two fall census programs

Abstract

Abstract. 1. The status of the eastern North American monarch butterfly population is a highly sensitive issue, given that winter and breeding habitats are being lost at an alarming rate each year, and because of this, most believe the population to be declining, although there has been little empirical data to support this idea. In a recent forum article of this journal, Brower et al. (2011) report a statistically significant decline in winter colony size over a 17‐year period and suggest that this is the first sign of impending collapse.2. I conducted an analysis of numbers of migrating monarchs from two fall monitoring stations in the United States (Cape May, NJ and Peninsula Point, MI), which span 15 and 19 years, respectively, and at both locations there was no significant linear trend in average monarch numbers counted over time, which is in marked contrast to the conclusion drawn by Brower et al. (2011).3. Although I identify several possible reasons for the discrepancy between the fall census counts and size of the overwintering areas, these differing patterns argue for a more balanced perspective regarding the status of this population, and certainly for considering more than one phase of the life cycle. Even though it is difficult to imagine how monarchs will fare in the future with so many threats to their population, the data presented here suggest that the population remains stable for now, probably because of the high fecundity of the species and its ability to rebound from small winter numbers.