Monarchs currently spend winters in oyamel fir forests in mountainous areas of central Mexico. Winters there are cool, but as long as they aren't too wet, the insects survive to begin the migration north in the spring. But the knowledge that the earth is facing climate changes prompted Oberhauser and colleague A. Townsend Peterson of the University of Kansas to determine whether future winter conditions in the Mexican refuges would continue to support butterfly populations.
Using the technique of ecological niche modeling, the researchers determined the fine-scale climatic traits common to sites suitable for monarchs. Those traits allowed them to predict current locations of monarch populations with a high degree of accuracy. They then compared the monarch-friendly traits to fine-scale climatic traits predicted for sites in Mexico during the next 50 years.
All current climate models predict more storms in places where monarchs spend the winter, said Oberhauser. When the already cool Mexican forests become wetter during the overwintering period, the increased moisture will result in more frequent storms, pushing the climate outside the range of conditions necessary for monarchs to live.
"The conditions that monarchs need to survive the winter are not predicted to exist anywhere near the present overwintering sites," said Oberhauser. "The temperatures won't change much, but the combination of coolness and increased rain will hurt."
The plight of the monarchs is worsened by human activity in the area, she said. Logging, clearing of forests for agriculture, harvesting of wood for home fires, livestock grazing and intentional forest fires have all shrunk habitat suitable for monarchs in the winter. Furthermore, winter storms in the Mexican fir forests in 2002 caused massive monarch mortality, an indication that storms are an important determinant of monarch survival.
"This study demonstrates that it is important to consider a changing climate when making conservation decisions," said Oberhauser. "Organisms need to have habitat that supports them both now and in the future. It is also an example of the far-reaching impacts of a changed climate. Organisms have adapted to survive in very specific conditions, and the combination of habitat loss and a changing climate could have large consequences."
The study represents the first use of ecological niche modeling to predict the seasonal distributions of a migratory species under climate change. Oberhauser and Peterson will now use the technique to study how monarchs will fare in their breeding areas within the United States. If the summer breeding areas should move north with global warming, monarchs may need to migrate farther, Oberhauser said.
Although monarch butterflies may not form the cornerstone of any ecosystem, their loss will and should be felt, she said.
"If we lose monarchs, we lose a species with a unique and interesting biology, and an organism that helps many people connect to the natural world," said Oberhauser. "They're an example of how we can value components of the natural world without putting a dollar figure on them."
The work was supported by the National Science Foundation.