We’re halfway through our theme week on World Heritage Sites. Today, we’re talking about the Monarch Butterfly Biosphere Reserve in Mexico, which was named a World Heritage Site in 2008. Located 62 miles northwest of Mexico City, this is where most of the monarch butterflies from Eastern North America migrate for the winter. These butterflies occupy just a fraction of the 350 square miles of the reserve from October to March each year.
There are 14 major butterfly colonies on the reserve. It’s estimated that up to a billion butterflies spend the winter in the rugged forested mountains. They congregate in clusters onto pine and fir trees, making the trees appear orange and the branches sag beneath the weight. In the spring, these butterflies migrate back north, some going as far as Eastern Canada.
Migrating butterflies can travel up 3,000 total miles to reach their final destination. They can go over 100 miles in a day with the right conditions. They coast on air currents to move quickly while conserving energy. Over the time it takes them to make this journey, 4 generations of monarch butterflies are born and die. A typical lifespan is just 2 to 6 weeks. The monarchs that migrate are the last generation each year, and live up to 9 months. Most of that time is spent migrating south, staying there for the winter, migrating north, and finally breeding in the spring.
None of the monarchs that migrate south have ever been to their destination before. But they know exactly where they’re headed. There have been many theories put forth to try and explain monarch migration. Scientists believe they use cues like magnetism and sunlight to navigate, but the rest is still pretty much a mystery.
Not all monarch populations migrate. Some populations in Florida, the Caribbean, Central America, and Northern South America stay put all year round. Scientists aren’t sure if they stick around because of the mild weather, a genetic difference, or a combination of factors.
Sadly, the number of monarchs overwintering in Mexico has shown a long-term downward trend. Monarch populations have dropped 85% over the last 2 decades, due to things like climate change, habitat loss, and urban development. There are some things we can do to help the monarchs though, including planting milkweed (which is the only food source for monarch caterpillars), planting native nectar-rich flowers, shrubs, and plants, and avoiding the use of pesticides. Learn more here.
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