Imagine a Christmas tree so covered in ornaments that you can barely make out the foliage underneath. Now, instead of candy canes, snowflakes and Santa Claus figurines, picture millions of small black, white and orange butterflies dangling from the branches, leaves and trunks of trees in central Mexico.

Every year, between November and March, oyamel, oak and pine forests on the mountainous border of Michoacán and Mexico state house most of the monarch butterflies that migrate from the United States and Canada by flying south to Mexico for the winter.

"Sometimes you can barely make the silhouette of the tree; all you see is butterflies," said biologist Eduardo Rendón, coordinator of the World Wildlife Fund-Mexico's Monarch Butterfly Program.

Aside from the ecological significance of these migrations -- monarch butterflies are the only insects known to migrate to warmer climates more than 2,500 miles away -- the butterflies' five-month layover in Mexico before returning to the United States has become one of the region's main tourist attractions and economic drivers.

More than 120,000 people visit the rural areas of the Monarch Butterfly Biosphere Reserve each year to witness the event, according to reports by the WWF in Mexico.

But over the past couple of years, the number of monarch butterflies that reach the Mexican sanctuaries has been declining, generating concern among rural communities that rely on spillovers of butterfly tourism activities, as well as entomologists, biologists, ecologists and monarch aficionados around the world.

According to Mexico's annual report on monarch populations in the reserve region, released in March, the number of hectares occupied by the butterflies -- used as an indicator of population density -- in the 2012-2013 season dropped by 59 percent compared with the year before.

At barely 1.19 hectares (2.94 acres) occupied, it is the smallest monarch population registered in almost two decades.

A 'thermometer' of climate variability
Monarch butterflies are widely distributed across North America, particularly near corn and soybean fields rich in milkweed, their food of choice, Rendón said. Monarchs lay their eggs on the milkweed plants, and caterpillars feed on the weed during their larva stage. As adults, butterflies feed on the nectar of flowers.

Over the course of one summer, monarchs can produce four generations. The first three generations have an average life span of four to six weeks. But near the end of summer, something happens to the fourth generation.

As days grow shorter, these new butterflies mature without developing their reproductive organs, allowing them to outlive the others. In fact, according to Rendón, the fourth generation -- the one that sets out to make the trip all the way to Mexico and back -- can live up to nine months.

To determine what's devastating Mexico's monarch colonies means looking at what's been happening to the butterflies in the United States before they head south, Rendón said.

"Historically, it's been said that monarchs could be a thermometer of what's going on in terms of climate variability, in this case, in North America," he said.

Declining along with their favorite food
The probable causes for the decline in butterflies during the migration and hibernation between 2012 and 2013 are the reduction in milkweed availability in feeding and reproduction sites throughout the United States and Canada, and extreme weather events affecting the reproductive generations in the United States during spring and summer 2012, the report said.

"Milkweed is regarded as just a plain 'weed' that reduces crop yields and is toxic for cattle," Rendón said.

Since the 1970s, farmers in the U.S. Midwest have been trying different herbicides to fix their milkweed infestation problem. But it wasn't until the introduction of glyphosate herbicide in the '90s, together with increased planting of genetically modified glyphosate-tolerant corn and soybeans, that monarch populations began to drop sharply.

Because of the use of this herbicide, there has been a 58 percent decline in milkweed availability and an 81 percent decline in monarch production in the Midwest from 1999 to 2010, Rendón said.

But that's only part of the problem.

According to Rendón, climate change is also reducing the spread of places where butterflies can feed and reproduce, with rising temperatures and severe droughts affecting crops and milkweed at the same time. Furthermore, it is known that hotter and drier conditions can be lethal during the larva stage and can affect the survival and reproductive capacity of the adults.

"Climate is, initially, hitting the plants and, later, the butterflies directly," Rendón said. "It's very likely that the decline in the population density of monarchs in Mexico during December 2012 is because of the extremely hot and dry climate conditions experienced during the summer."

Grave threat to local tourism
While monarchs are not an endangered species yet, due to their remarkable adaptability, the International Union for Conservation of Nature considers the annual migration a threatened phenomenon.

"What's really in jeopardy is the migration and hibernation event in the numbers we are used to," Rendón explained. While migration will still occur, according to Rendón, it is likely it will happen in smaller numbers.

Which brings us back to tourism in communities near, or within, the monarch reserve in Mexico.

Situated in the mountains in the Trans-Mexican Volcanic Belt, the reserve covers terrains of 10 municipalities that belong to "ejidos" -- communal land used for agriculture, on which community members individually possess and farm a specific parcel -- indigenous communities and small properties.

Because it's mostly a rural region, communities are very poor and depend on the subsistence farming of corn. So the tourists who come to see the butterflies have become vital for the ejidos.

"Tourism has become the conservation flag of the monarch butterfly -- and, consequently, their habitat -- because it is generating economic revenues to the owners of these lands," Rendón said.

It that sense, Mexico has come a long way to help preserve the butterfly's wintering grounds. For one, it has almost stopped illegal logging in the reserve, he said.

But the country's efforts toward habitat conservation can only go so far. At the end of the day, Mexico has no control over the number of butterflies that actually reach the reserve.

Following the report on monarch populations, WWF-Mexico issued a statement highlighting the need for cross-country action. According to branch General Director Omar Vidal, the conservation of the monarch butterfly is a shared responsibility among Mexico, the United States and Canada, and just as Mexico is doing its part by protecting these sanctuaries, so should the United States and Canada protect their habitats as well.

Otherwise, the large-scale monarch migration could potentially end.

"For us, it is a real concern," Rendón said. "Not only are we losing this beautiful biological phenomenon of migration and hibernation, but we are also losing one of the main incentives and economic inputs in the region."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500