The monarch butterfly migration is among the largest ever documented for insects in the world - they migrate up to 3,000 miles to reach their wintering grounds! Monarch butterflies have a seasonal migration that is completed by several generations rather than by single individuals, meaning that monarch butterflies reproduce, die and are replaced by new individuals to complete their yearly migration.
There are two distinct monarch butterfly populations in North America: the western and the eastern populations. The western population lives west of the Rocky Mountains and overwinters along the California coast. The eastern population - which we'll focus on here - is the largest, lives east of the Rocky Mountains and travels to wintering grounds in central Mexico.
Eastern monarch butterflies leave their grounds in the northern U.S. and southern Canada in the fall and migrate up to 3,000 miles to their wintering grounds in Mexico. By early spring they migrate into the southern U.S. states, like Texas, where they breed and their offspring continue flying north. During summer, the monarchs continue to breed, generating offspring that will complete the migration cycle by flying to the northern U.S. and southern Canada. Monarch butterflies migrate because they cannot withstand the year-round conditions in these northern sites, especially cold and freezing temperatures.
Photo courtesy of Mark Musselman, NCTC Image Library, U.S. Fish and Wildlife Service.
Monarch butterfly populations are declining. Scientists believe weather and climate are among the contributing factors, along with herbicide use and deforestation. Climate affects monarch butterflies directly by impacting adult well-being and juvenile development, and indirectly by affecting the abundance and timing of milkweed availability-their host plant.
Climate conditions in Mexican overwintering sites are shifting towards lower and freezing temperatures and extreme precipitation appears to be increasing, which can kill up to 75 percent of the overwintering population.
Temperature is used as the main cue to migrate north during spring and south during fall. Climate change can alter the temperature signals that monarchs use for migration, creating a mismatch between their spring arrival in the United States and the availability of the milkweed plants they depend on.
Monarchs require optimal temperature and precipitation conditions during spring and summer to reproduce and for offspring to survive. Changes in weather, such as sudden cold spells, can disrupt the migration direction (north or south) and extreme weather including drought or heat waves can reduce monarch populations, which cannot withstand extremely high temperatures. Extreme precipitation, wildfires and other extreme events are also harmful for the species.
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