Mexico City - Illegal logging more than tripled in the monarch butterfly's wintering grounds in central Mexico, reversing several years of steady improvements, investigators announced Tuesday.

Almost all of the loss occurred in just one rural hamlet in the state of Michoacan. Loggers cut down 47 acres (19 hectares) of trees in San Felipe de los Alzati since last year's gathering of butterflies. A total of 52 acres (21 hectares) of forest in the reserve were lost overall, including losses due to drought or pests.

That's the highest figure since 2009, well above the 20 acres (8 hectares) lost in 2014, according to the announcement by the World Wildlife Fund and the Institute of Biology of Mexico's National Autonomous University. The 2014 loss was about 12 acres (5 hectares) due to logging and 8 acres (3 hectares) to drought.

Illegal logging fell to almost zero in 2012, and experts stressed that 31 of the 32 communities in the reserve had kept logging down to very, very low levels.

The forest canopy is a sort of blanket against cold for the masses of orange-and-black butterflies that form huge clumps on tree branches during their winter stay in Mexico.

Loss of that habitat is just one of the threats to the butterflies' amazing migration across Canada and the United States to Mexico. The migration is an inherited trait: No butterfly lives to make the full round trip, and it is unclear how they find the route back to the same patch of pine forest each year. Some scientists suggest the butterflies may release chemicals marking the migratory path and fear that if their numbers fall too low, the chemical traces will not be strong enough for others to follow.

This year butterflies that reached the wintering grounds covered 2.79 acres (1.13 hectares), a 69 percent rebound from last February's 1.65 acres (0.67 hectare), which was the lowest since record-keeping began in 1993. Butterflies cluster so closely together that they are counted by the area they cover, rather than by the number of individuals.

At their peak in 1996, the monarchs covered more than 44.5 acres (18 hectares) in the mountains west of Mexico City. But the overall tendency since then has been a steep, progressive decline. Each time the Monarchs rebound, they do so at lower levels. The species is found in many countries and is not in danger of extinction, but experts fear the migration could be disrupted if very few butterflies make the 3,400-mile (5,470-km) trip.

Largely Indian farm communities in the mountain reserve have received government development funds in return for preserving the 139,000-acre (56,259 hectare) reserve in the mountains west of Mexico City that UNESCO has declared a World Heritage site. Some of the communities earn income from tourist operations or reforestation nurseries to grow and plant saplings. Funding for the hamlet of San Felipe de los Alzati has temporarily been suspended due to the logging there.

The fact that most of last year's loss also occurred in San Felipe indicates a growing problem there, said Omar Vidal, head of the World Wildlife Fund in Mexico.

"The government has to step up enforcement and start talking more seriously with this community, to find out the causes" behind the logging, Vidal said. Some communities have complained that outside loggers - sometimes armed - invade local forests without the consent of the community. Other logging, however, has been the work of locals who have few other job opportunities."

After illegal logging felled hundreds of acres of trees in the reserve between 2003 and 2006, authorities cracked down on illegal sawmills and stepped up incentives to encourage communities to preserve the woods.

"The main problem in Mexico is the lack of protection," said writer and activist Homero Aridjis, who noted that some officials at the reserve were replaced and that President Enrique Peña Nieto recently appointed his cousin, Alejandro del Mazo, to head the agency that oversees Mexico's nature reserves.

Del Mazo's office did not immediately respond to requests for comment.