FEATURE - Mexican butterfly death raises questions on numbers
Date: 25-Feb-02
Country: MEXICO
Author: Fiona Ortiz
But while the ground is a cemetery, the air is vibrant with butterflies, millions of them painting the trees orange, the combined force of their wings making a windy sound.
The huge numbers of dead and of survivors have awakened a big question among scientists: Just how many butterflies come each year to the pine-covered mountains of Michoacan, Mexico?
Scientists previously believed that 25 million to 170 million butterflies reached these mountains at the end of their mysterious migration, which begins as far as 3,000 miles (5,000 km) away in the northern U.S. and southern Canada.
But one scientific count of the dead after a January cold snap was 270 million, or double the high end of previous estimates of the migration to Mexico.
"One thing this mortality study showed is that we underestimate the number of butterflies by at least a factor of five," said Lincoln Brower, renowned monarch expert from Sweet Briar College in Virginia, who calculated the dead.
He now thinks 50 million butterflies inhabit each 2.5 acres (1 hectare), instead of 10 million earlier thought, for a total population of perhaps more than 500 million.
While high death figures this year would not wipe out the species, it could point to the incredible migration's future and the effect logging has on the butterfly's Mexico habitat.
COUNTING THE DEAD
This year biologists expected a record number of monarchs in Michoacan. The population varies from year to year, depending on the weather and the health of the milkweed plant in the United States, where monarchs lay their eggs during the spring.
It is not unusual for the cold to kill large numbers of butterflies in Mexico. Scientists believe that deforestation in and around the reserve exacerbates the problem by removing the forest cover that protects butterflies from the cold. Since 1968 about 44 percent of the woods in which they take sanctuary have been depleted, much of it by illegal logging.
Brower said he and some visiting scientists came up with the 270 million death toll the week after the cold snap, based on counts taken in different square-yard (meter) sample areas.
Brower estimates some 75 percent at each of two big colonies in the reserve died. But he also said it was very hard to say how many survived and the percentage of dead, because previous estimates may have been completely off.
Every year researchers estimate the total butterfly population in November and December, counting the hectares that butterflies are covering in colonies inside and outside of the government-protected 54,000 hectares (133,400-acre) reserve.
The count is made based on a 10 million butterflies per hectare (2.5 acres) standard. But Brower said he developed that standard in 1977 and that he had always considered it conservative.
"Without cutting down a tree and counting every butterfly on it, which we're loath to do, we could never really get a decent estimate," Brower said.
"It's not like counting buffalo," said Juan Bezaury, the director of the World Wildlife Fund in Mexico, which helped to finance Brower's study.
Tourists visiting the Monarch Butterfly Biosphere Reserve this year are in for the traditional treat, despite the deaths in January. Scientists say that since there were millions of survivors, there is no way the untrained eye can tell the colony was depleted.
In Sierra Chincua, the location of one of the colonies with a huge death toll, visitors can still gawk at air thick with butterflies and trees dripping with layers and layers of the orange and black insects.
MORTALITY WAS HIGHER THAN USUAL
Ernesto Enkerlin, president of Mexico's National Commission for Protected Areas, said his numbers show about 40 percent of the total died but agreed the mortality was high.
"This is the biggest death we've registered in the reserve ... but the most important thing is that according to us there are at least 70 million butterflies alive," Enkerlin said.
The debate
