The number of Monarch butterflies wintering in Mexico dropped 28 percent this year, according to a report released Thursday, a decline some experts attribute to droughts in parts of the United States and Canada where the butterflies breed and begin their long migration south.
Others say damage to wintering grounds in central Mexico's mountains remains a factor in the decline, citing deforestation of the fir and pine forests they favor.
The numbers of butterflies spending the winter in Mexico have varied wildly in recent years.
Concern rose two years ago, when their numbers dropped by 75 percent in the wintering grounds, the lowest level since comparable record-keeping began in 1993. They partially recovered last year, when the number of butterflies nearly doubled from that record low point.
"Fluctuations in insect populations are normal. In the case of the Monarch, we have shown that these fluctuations are mainly due to climate conditions," said Omar Vidal, head of World Wildlife Fund in Mexico, adding that "during 2011, the abnormal patterns of drought and rainfall in breeding grounds in Canada and the United States ... could have caused high mortality rates and a lack of plants" on which the butterflies feed.
But others were more worried.
Lincoln P. Brower, an expert on monarch butterflies and zoology professor at the University of Florida said this year's number is the third lowest since systematic monitoring began, adding "the current data indicate a continuation of the downward trend."
Brower said the climate argument "ignores the fact that severe degradation of the Oyamel (fir) forest ecosystem has been and still is occurring."
Vidal said a survey indicated that only about an acre (half-hectare) of trees were lost to deforestation last year, down two-thirds from the preceding year.
Illegal tree-cutting destroyed about 3.7 acres (1.5 hectares) in 2010, itself a decrease of 97 percent from 2009. At its peak in 2005, logging devastated as many as 1,140 acres (461 hectares) annually in the reserve.
The survey carried out by the WWF, private donors and Mexico's National Commission on Protected Areas measures the millions of butterflies that arrive each year based on the number of acres of forest they cover. Since the butterflies "clump" together by the thousands on tree branches, apparently to conserve heat, counting individuals would be near impossible.
This year, the Monarchs covered 7.14 acres (2.89 hectares) of forest, compared to 9.9 acres (4 hectares) last year and 4.7 acres (1.9 hectares) two years ago.
The highest documented migration occurred in 1996, when nearly 45 acres (18.19 hectares) of butterflies were spotted in the Monarch reserve, a series of mountaintops in a protected area west of Mexico City.
Vidal said about 18 percent of this year's butterflies had wintered in areas outside the reserve's boundaries, and said activists would ask state governments in the area to extend protection to those nearby areas. The reserve itself has protected status, but is largely owned by communal farmers.
Homero Aridjis, who as Mexico's ambassador to UNESCO lobbied to get the Monarch butterfly reserve on the U.S. World Heritage List in 2008, said the Monarch reserve should be moved to the "world heritage in danger" list, because of ongoing forest cover loss, logging and building.
The butterflies depend on the mountaintop pine forests in western Michoacan state which serve as "blankets" to protect the insects against winter rain and cold.
Aridjis says logging has continued and noted that less than half of the 193,000-acre (56,259-hectare) reserve is now forested.
The butterflies also depend on milkweed to breed and feed. The plant has grown less common as farmers increasingly use herbicide-tolerant corn and soybeans, and droughts further reduce the plant's availability.
Luis Fueyo, the head of Mexico's National Commission on Protected Natural Areas, said the United States has to do more to stem the effect of herbicides on milkweed.
In October, Chip Taylor, director of the Lawrence, Kansas-based Monarch Watch, said the Monarchs faced especially dire conditions as they passed through the Midwest on their annual migration to Mexico.
The University of Kansas ecology and evolutionary biology professor said the migratory path through Texas was difficult because much vegetation has dried up in the hottest summer on record. Taylor said many surviving plants were then burned in wildfires that blackened millions of acres.