High on a remote mountaintop, Alfredo Cruz Colin gazed at the giant pines and firs where millions of orange-and-black monarch butterflies spend the winter after flying as far as 2,000 miles across North America. He saw two things: the winged wonders and the trees that could be cut and sold for $300 each.

“We can contemplate the butterflies,” said Cruz, a lawyer. “Or we can send our children to school and feed our families” with cash from the cut trees.

The winter migration of monarch butterflies to Mexico, a stunning sight that draws vast numbers of tourists to mountain forests 100 miles west of Mexico City, has been devastated this year. One of the chief causes is logging that destroys butterfly sanctuaries, according to Mexican and U.S. environmentalists.

The butterfly population this winter is the lowest since researchers began detailed surveys 12 years ago and perhaps the smallest since the 1970s, when international scientists discovered the colonies in central Mexico, according to Lincoln P. Brower, a biology professor at Sweet Briar College in central Virginia and an authority on monarch butterflies. He estimated that the population was at least 75 percent smaller than last year’s.

The reason for the dramatic drop appears to be a combination of particularly cold, stormy weather in North America in recent years, herbicide use in the United States and Canada that is killing milkweed plants where butterflies lay their eggs, and persistent illegal logging in Mexico, according to a report issued last week by a panel of monarch researchers chaired by Brower.

The northeast face of this mountain has “been stripped of forest and burned,” destroying long-established butterfly sanctuaries and leaving only one small butterfly area this year, said Brower, who has visited the site almost every year since the mid-1970s.

Conservationists are also concerned about threats from herbicides. While genetically engineered crops such as soybeans and corn are resistant to the chemicals, the weedkillers are causing massive destruction of butterfly eggs on milkweed leaves, they said.

“Why should we care?” Brower said. “For the same reasons we should care about the Mona Lisa or the beauty of Mozart’s music.”

Scientists agree that the monarch has a great capacity to recover from dramatic die-offs. In the winter of 2001-02, as many as 80 percent of the butterflies in Mexico perished in an unusual winter storm, and the following year their numbers rose again.

But scientists said they were more disturbed by the steady deterioration of the butterflies’ North American habitat.

“All of us firmly believe that the butterfly is capable of rebounding, but there is a limit,” Brower said. “How many bales of hay can you put on a camel’s back before the last straw breaks it?”