Monarch butterflies perform annual migrations across North America which have been called “one of the most spectacular natural phenomena in the world."
Starting in September and October, eastern/northeastern populations migrate from southern Canada and the United States to overwintering sites in central Mexico where they arrive around November. They start the return trip in March, arriving around July. No individual butterfly completes the entire round trip; female monarchs lay eggs for the next generation during the northward migration, and at least five generations are involved in the annual cycle.
Monarchs also perform small distance migrations in Australia and New Zealand. There are also some populations, for instance in Florida and the Caribbean, that do not migrate.
It may be the most familiar North American butterfly, and is considered an iconic pollinator species. The eastern North American monarch population is notable for its annual southward late-summer/autumn migration from the United States and southern Canada to Mexico. During the fall migration, monarchs cover thousands of miles, with a corresponding multi-generational return north. The western North American population of monarchs west of the Rocky Mountains often migrates to sites in California but has been found in overwintering Mexican sites as well.
On 8/14/14, the Center for Biological Diversity filed a legal petition requesting Endangered Species Act protection for the monarch and its habitat. The monarch butterfly is listed as a Species of Special Concern in Ontario.
167 million acres of monarch habitat have been lost since 1996. The reduction in milkweed habitat in agricultural regions of North America has been cited as a major cause of population declines. Prior to the introduction of genetically-altered corn and soybeans, milkweed was common in the crop fields. Conservationists cite the use of pesticides and herbicides as a cause of population decline. Milkweed habitat is also destroyed by the expansion of urban and suburban areas. Conservationists also call attention to the decreased habitat that allows the growth of nectaring plants.
Climate change threatens to disrupt the monarch butterfly’s annual migration pattern by affecting weather conditions in both wintering grounds and summer breeding grounds. Colder, wetter winters could be lethal to these creatures and hotter, drier summers could shift suitable habitats north. WWF’s 2013 report from Mexico showed that the number of monarch butterflies wintering there was at its lowest in 20 years. The number is measured by the amount of forest they occupy, and in 2013 the number of butterfly acres decreased from approximately seven to three. Abnormal patterns of drought and rainfall in the U.S. and Canada breeding sites may have caused adult butterfly deaths and less plant food for caterpillars. Fewer butterflies up north mean fewer then migrate south to Mexico for the winter. Monarchs need mountain forests in Mexico for their winter habitat, however nearby human communities also rely on them and create pressure on forests through agriculture and tourism activities.
On 6/20/14, President Barack Obama issued a Presidential Memorandum entitled "Creating a Federal Strategy to Promote the Health of Honey Bees and Other Pollinators." The Memorandum established a Pollinator Health Task Force, to be co-chaired by the Secretary of Agriculture and the Administrator of the Environmental Protection Agency, and stated:
The number of migrating Monarch butterflies sank to the lowest recorded population level in 2013-14, and there is an imminent risk of failed migration.
The range of the monarch is worldwide and while monarch butterflies are not endangered as a species, the migration of the eastern north american population may be an endangered phenomenon.