Dwindling Monarch Butterfly Populations Wintering in Mexico

Last Updated: March 18, 2013 by
Categories: Monarch Butterflies, Winter, UNESCO

Every year millions, up to a billion, of Monarch butterflies migrate from the U.S. and Canada, where they live during the summer, to Mexico to winter in the oyamel fir and pine trees in mountainous forests west of Mexico City. They stay in Mexico from October to March and hibernate in these moderate climates.

In all but one of past seven years the population of the butterflies wintering in Michoacán and western Mexico State has decreased and this year by 59 percent. The populations are 1/15 what they were in 1997.

Monarch Butterfly Biosphere Reserve was established through a series of presidential decrees in the 1980’s and then given UNESCO (United Nations Educational, Scientific and Cultural Organization) World Heritage site status in 2008. It provides 200 square miles of refuge and protection for the butterflies, of which only a very small portion is open to the public.

Illegal logging in the Monarch Butterfly Biosphere Reserve has long been suspected as being a major factor in the decrease in Monarch numbers. In recent years Mexico has cracked down on the logging. An aerial survey performed in 2012 found almost no illegal logging. Yet a Mexican environmentalist, Homero Aridjis stated, “low intensity logging, not detected in satellite image analysis, continues unabated in and near critical overwintering habitats.”

Also water availability at the reserve affects Monarch numbers. The butterflies do not drink any water during their trip south. And mountain streams in the reserve have suffered by drought and use by local inhabitants.

The World Wildlife Fund and its partner organizations said in a statement, "The decrease of monarch butterflies...probably is due to the negative effects of reduction in milkweed and extreme variation in the United States and Canada."

Omar Vidal, director of the World Wildlife Fund in Mexico, said: "The conservation of the monarch butterfly is a shared responsibility between Mexico, the United States and Canada. By protecting the reserves and having practically eliminated large-scale illegal logging, Mexico has done its part. It is now necessary for the United States and Canada to do their part and protect the butterflies' habitat in their territories...”

Yet, Lincoln Brower, entomologist at Virginia’s Sweet Briar College, stated, “To blame the low numbers of monarchs solely on what is happening north of Mexico is misleading. Herbiciding of soybean and corn fields that kills milkweed is a serious problem, but the historical decline over the past 19 years had multiple causes.”

The Monarch butterflies cluster so closely on the trees that their numbers are measured by how much space they cover. Branches of the trees where they cluster can sag and change colors.

Mexpro.com has posted articles on the butterfly migrations for several years, but thought this year we should give it special attention since the populations are causing such concern among scientists. If you would like to learn more please visit http://worldwildlife.org/species/monarch-butterfly