The familiar orange monarch butterfly is famous for its southward fall migration. It flies from the United States and southern Canada to its wintering grounds in Mexico and coastal California, only to return northward in the spring. This journey takes three to four butterfly generations to complete. At one time, a billion or more monarchs took part in the 2,500-mile journey.
The number of butterflies making the trip varies greatly from year to year, but over the past decade, there has been a steep and steady decline. It is estimated that the migrating population is currently about 35 million butterflies. Once, wintering butterflies occupied as much as 45 acres of forest in Mexico; this winter, it is less than 2 acres, which is about the size of a football field.
A two-year stretch of bad weather is responsible for much of the recent decline. A Great Plains heat wave in 2012 disrupted the northern migration that year and unusual cold spring weather in Texas delayed the breeding season last year. Smaller overall numbers make the population far more vulnerable to such weather events.
Of greater concern is the loss of habitat for the butterflies. Monarchs only lay their eggs on milkweed, and that plant has been disappearing in the Great Plains because of herbicides being used around genetically modified corn and soybean crops.
The colorful and amazing spectacle of the monarch buttery migration is one of the most visible victims of changes to the natural environment.
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Web Links
Migration of Monarch Butterflies Shrinks Again Under Inhospitable Conditions
http://www.nytimes.com/2014/01/30/us/monarch-butterflies-falter-under-extreme-weather.html?_r=0
Photo, taken on November 5, 2012, courtesy of William Warby via Flickr.
Earth Wise is a production of WAMC Northeast Public Radio. Support for Earth Wise comes from the Cary Institute of Ecosystem Studies in Millbrook, NY.