BBC News Online
Monarch butterflies may lose their winter habitat within 50 years because of climate change, say researchers.
The monarch butterfly travels thousands of kilometres
Each autumn, Monarchs migrate thousands of kilometres from North America to the oyamel fir forests of Mexico, where they spend the winter months.
Scientists fear that increased rainfall may render the forests unsuitable for the butterflies - which can only survive under very specific conditions.
The results appear in the Proceedings of the National Academy of Sciences.
Monarch butterflies (Danaus plexippus) have a very complex life-cycle. They breed east of the Rocky Mountains during the summer months, before embarking on a Herculean journey to reach their wintering grounds in central Mexico.
Exactly how they get there is one of nature's mysteries. They navigate south by using the Sun - but their precise destination is a mere pinprick on the map, so the Sun cannot provide the whole answer.
"If I was navigating my way to Mexico City using the Sun I would never get there," said Karen Oberhauser, of the University of Minnesota, US. "You would need a really smart person to work out how they do it. You would need to think like a butterfly."
The Monarchs from North America feed on milkweed during the summer. In the winter they must migrate to a place where the climate is not so harsh. But crucially they need somewhere cool enough to keep their metabolism low, because there is no milkweed for them to eat.
"They have to survive the winter entirely on the reserves they built up over the summer," said Dr Oberhauser.
Habitats that fit the Monarch's specific needs are rare. Their winter home is restricted to the mountainous oyamel fir forests, where they huddle in great colonies around the trunks of the trees.
The cool but relatively dry conditions are just right for the Monarchs, and they cannot cope with much fluctuation.
Scientists have already noticed that Monarch populations take a nosedive if there are any "blips" in their winter climate.
In February 1992, for example, a single storm caused 82% of a colony to die, and 42% died after a snowfall in 1981.
Dr Oberhauser and her colleagues used a technique called ecological niche modelling to predict how the Monarchs' winter home will change over the next 50 years.
They found that, while the temperature in the oyamel forests is unlikely to change much, it will rain more, which the butterflies cannot stand because the wet increases their chances of freezing to death.
The range of locations that fit the Monarch's requirements is likely to shrink even further, because of deforestation to make way for agriculture.
Between 1971 and 1999, 44% of the butterflies' winter habitat was degraded, which means it was either cleared or thinned so that it was no longer suitable for the Monarchs.
Scientists are worried that if this north-south migratory population is wiped out, it will prevent them from solving the mystery of how the Monarchs know to follow the same route every year.
"The next step is to take the research on to a continental scale, to see if there are other places they can spend the winter," said Dr Oberhauser. "We need to think to the future, to preserve sites that might be suitable."