Mexico's Monarch Butterfly Biosphere Reserve and the migration it shelters

Background

The Monarch Butterfly Biosphere Reserve is a UNESCO World Heritage property in Mexico, inscribed under natural criterion (vii). The World Heritage Centre describes a 56,259 ha biosphere that lies within rugged forested mountains about 100 km northwest of Mexico City. Every autumn, millions, perhaps a billion, butterflies from wide areas of North America return to the site and cluster on small areas of the forest reserve, colouring its trees orange and literally bending their branches under their collective weight. UNESCO records that up to a billion monarch butterflies return annually, from breeding areas as far away as Canada, to land in close-packed clusters within 14 overwintering colonies in the oyamel fir forests of central Mexico, and that the property protects 8 of these colonies and an estimated 70% of the total overwintering population of the monarch butterfly's eastern population. In the spring, these butterflies begin an 8 month migration that takes them all the way to Eastern Canada and back, during which time four successive generations are born and die. How they find their way back to their overwintering site remains a mystery. UNESCO's Man and the Biosphere pages add that the biosphere reserve covers an area of 56,259 ha of temperate pine, oyamel and oak forests. Because the overwintering colonies occupy only small areas of forest, a very large share of the species' migratory population is packed into a few patches of oyamel forest at one time.