The American space agency released the relevant satellite pictures last month. The images show the Monarch Butterfly Biosphere Reserve. A Mexican law protects trees growing in the area. The images show that about four hundred and forty­five hectares have been cleared since 2004. Scientists say that, if the clearing continues，monarch butterflies are unlikely to continue using the Reserve as their winter home.

　　For thousands of years, the butterflies have been flying to the same forests. Their trip begins along the border of Canada and the northeastern United States. The insects fly about eighty kilometers a day. They reach central Mexico after about sixty days. No one is sure why the brightly coloured orange and black butterflies chose these forests. However, scientists believe the area might offer the right mix of wetness and cool weather to keep the insects alive through the winter.

　　Lincoln Brower is a professor of biology at Sweet Briar College in Virginia. He has been studying monarch butterflies for fifty­two years. He says they have been migrating along the same path for about ten thousand years. People in Mexico have compared the arrival of the butterflies to dark cloud­like formations (阵式) filling the air. The monarchs arrive in such large numbers that they sometimes cover whole trees.

　　Mexico's President ordered special protection for the Monarch Butterfly Biosphere Reserve eight years ago. The Mexican government has struggled to enforce (强制) the order. Although illegal logging has become one way for poor people to earn money, other Mexicans are fighting to protect the butterflies' home.

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