Climate threatens butterfly's 2,000-mile migration

The unique life cycle of the monarch butterfly - which migrates more than 2,000 miles to its wintering grounds - could come to an end within 50 years, according to a study published yesterday.

Each year, monarchs migrate south to the high mountain forests of Mexico from their summer breeding sites as far north as the border between the United States and Canada. Scientists have discovered that the oyamel fir trees of Mexico on which the butterflies spend the winter are highly vulnerable to the changes in climate that meteorologists have forecast for the next 50 years.

By then, according to computer predictions made by Karen Oberhauser of the University of Minnesota and Townsend Peterson of Kansas University, few of the oyamel forests will be able to provide the vital protection that the monarchs need to survive a cold winter. "In fact, when current oyamel distribution was included in models to be projected to future climates, none of the present wintering sites was predicted to be suitable in 50 years' time," the researchers say in the journal Proceedings of the National Academy of Sciences.

With its bright orange-and-black markings, the monarch is one of the most beautiful butterflies. It is also one of the most intriguing because of its extraordinary migratory route, which is believed to have evolved since the last Ice Age.

When winter comes to an end, millions of the monarchs fly north out of the Mexican mountains to feed and breed on the spring and summer forest flowers virtually throughout the entire North American continent. In September, four or five generations later, the monarchs newly emerged from their chrysalises as far north as the Great Lakes begin their long journey to the Mexican mountains, sometimes finding the same tree used by their great-great-grandparents the previous winter.

Dr Oberhauser said that the oyamel fir forests of central Mexico provided a unique microclimate for the butterflies that allowed them to spend up to five months in their wintering grounds.

Although climate change is not expected to affect significantly the winter temperature of the oyamel forests, rain and snow is predicted to increase. This would make it more difficult for a butterfly that requires dry weather.

Dr Oberhauser added: "The relationship between winter mortality and weather conditions suggests climate-change may have important impacts on monarch butterflies."

It is not just climate change that could see off the monarch. "Degradation of the forests in which monarchs overwinter is a real and serious threat to their survival." Logging and the conversion of woodland into farmland, as well as forest fires, are all putting pressure on the long-term survival of the monarch. "Our analyses suggest that climate-change effects may pose an additional long-term risk to monarchs," Dr Oberhauser said.

The scientists point out that the monarch may be expected to survival climate change better than some animals that cannot migrate. However, their highly specific requirements for surviving the winter months make them especially vulnerable to any changes.

Monarchs also face threats on their northern breeding grounds, where milkweed, the plant that larvae feed on exclusively, is considered a noxious weed and often destroyed.

A series of studies published last year demonstrated that monarchs were able to navigate vast distances using an internal clock.

They use the clock to calculate the correct direction in which to travel, with the Sun serving as a compass.