Dynamics and trends of overwintering colonies of the monarch butterfly in Mexico.

There are two breeding migratory populations of the monarch butterfly ( Danaus plexippus ) in North America. A comparatively small, Western population migrates from states west of the Rocky Mountains to California, and a large Eastern population migrates from southern Canada and the United States to central Mexico. We monitored the dynamics and trends of monarch overwintering colonies in Mexico from the 2004–2005 to the 2013–2014 seasons. Of 19 colonies, 14 were inside the Monarch Butterfly Biosphere Reserve and five were outside the reserve. The number of colonies with butterflies varied among years, and in only three colonies were butterflies consistently present in all seasons. The total cumulative area of forest used by all monarch colonies in all seasons was 106.53 ha: 83.68% inside the reserve and 16.32% outside the reserve. By the 2013–2014 season, however, the surface occupied by monarchs (0.67 ha) had decreased 44% from the previous season, and is the smallest in two decades, far from the highest record of 18.19 ha in the 1996–1997 season. Extensive loss of breeding habitat by eradication of common milkweed ( Asclepias syriaca , the primary food source for monarchs) from herbicide use and land-use changes in the United States, extreme climate conditions in Canada, the United States and Mexico, and deforestation and forest degradation in overwintering sites in Mexico all contributed to the steady decline in the abundance of monarch butterflies. Unregulated tourism also has become a threat to the dwindling colonies in Mexico. Protection of overwintering sites in Mexico is crucial to conserve this butterfly in North America. Given the rapid decline of monarch overwintering sites documented here, it is critical to initiate an immediate and concerted effort to protect and restore habitat along the migratory routes in the three countries. [ABSTRACT FROM AUTHOR]