Monarch butterflies in the El Rosario Monarch Butterfly Biosphere Reserve in Michoacan Mexico.
Photograph: Sylvain Cordier/Getty Images
every year, at the beginning of November, one of the most impressive natural spectacles in the world takes place in Michoacán, Mexico. Hundreds of millions of migrating monarch butterflies settle in the forested massifs of the country’s Monarch Butterfly Biosphere Reserve, roughly 100 kilometers west of Mexico City. Having flown south for eight months, beginning their journey in the northern United States or southern Canada, they hibernate here for the winter before mating in the spring.
After flying for more than 4,000 kilometers, the butterflies land in the oyamel fir trees of the Ejido el Rosario region, where for weeks they congregate, protecting themselves from the wind and the cold nights. Without these trees, the butterflies would not be able to survive their exhausting journey.
The oyamel fir grows in a very small climatic space, one that is humid yet cold. “Its distribution is very limited to the highest mountains in central Mexico,” says Cuauhtémoc Sáenz Romero, a professor at the Universidad Michoacana de San Nicolás de Hidalgo. Sáenz Romero is the lead author of a recent study that anticipates that this forest will gradually deteriorate to the point of disappearance as a result of climate change, endangering the butterflies.
For the roosting monarchs, the oyamel canopy acts as a buffer to the local temperature and humidity, Sáenz Romero explains. “During the day, under the shade of the oyamel, the environment stays 5 degrees Celsius cooler than outside. It is a protection against high temperatures. At night it is the other way around, resulting in a 5 degree Celsius warmer environment.” The density of the canopy also protects against winter rain. “If the temperature drops below zero and the butterflies get their wings wet, they can freeze. That’s why these trees represent such a particular habitat,” says Sáenz Romero.
After awaking from hibernation and mating in central Mexico, the insects fly north to Texas in the United States, where they lay their eggs. “For all this, they need energy reserves to return, which they don’t have to spend on fighting the cold in the wintering sites,” he explains.
This fine balance for their survival is provided only by the oyamel firs. However, some models indicate that a climate conducive to them will have disappeared in this area by 2090. “Due to rising temperatures, we are observing a process of forest decline,” says Sáenz Romero, who is leading an initiative to establish new overwintering sites for the monarchs, which are on the red list of threatened species.
Sáenz Romero has more than 20 years’ experience conserving Michoacán’s coniferous forests. Since 2017, he’s been demonstrating the feasibility of planting new oyamel firs on a volcano in the Nevado de Toluca, a higher-altitude region to the east that is also a Protected Natural Area. “As temperatures and dryness increase, the trees become stressed and weakened. Then, the bark beetle, a natural and common pest for these trees, has a more potent impact, attacking more trees with more success,” he says. “This is a death foretold in two stages.”
Faced with this dramatic situation, Sáenz Romero’s team has been working with the indigenous community of Calimaya in the neighboring State of Mexico—the federal region that contains Mexico City—to create new forests outside their current geographic area. The idea is “to use assisted migration to establish new forests of fir trees on the colder slopes of the mountains, where they could offer the monarch butterflies a much-needed refuge,” the researcher explains.
This project would not have got off the ground without the indigenous community being involved. “They have a great sense of attachment to their territory and a lot of ecological knowledge. They know where and when to collect the seeds, how to get the plant out of the root ball, make the stock, tighten the soil, says Sáenz Romero. “This knowledge is the product of accumulated wisdom, of a close relationship with the territory.”
Seven years ago, the team began transplanting pine seedlings collected from eight spots in the Monarch Butterfly Biosphere Reserve into a community nursery in Ejido la Mesa, in the State of Mexico, before then planting them at four sites at different altitudes in Nevado de Toluca: at 3,400 meters, 3,600 meters, 3,800 meters, and 4,000 meters. Since the project began, scientists and local foresters from the indigenous Matlatzinca people have planted nearly 1,000 seedlings. This highest site is the upper limit of tree vegetation and was included to find the highest elevation at which the oyamel firs can survive in the current climate.
It is expected that the planted sites will receive migrating monarchs in the not too distant future. During the winter of 2023, numbers of butterflies in the usual sites decreased, with new colonies establishing themselves in colder locations within the Nevado de Toluca. “Not exactly where we planted, which is on the northwest slope, but on the southwest side of the same complex,” explains Sáenz Romero. It’s a sign that the butterflies are looking for new places to spend the winter, he believes. “Hopefully, once our seedlings have grown, they will discover our site as well.”
The current problem is the disparity between the speed at which climate change is occurring and the speed at which nature is able to adjust. “The forest is shifting, but not at the speed it needs to. Thats why human intervention is needed. If at 3,000 meters the oyamel trees had an average temperature of 10 degrees, now they will find that temperature at 3,300 meters. In other words, the climate is moving upwards, but the trees cannot move,” Sáenz Romero says. Forests move naturally through seeds dispersing, germinating, successfully growing into trees, and dispersing again. Getting a forest to move up a hillside is “a very slow process.”
Not everyone is on board with the experiment. For some conservation groups, taking the forest to new locations is a too radical a solution. But trying to replenish nature without human intervention only made sense before climate change, Sáenz Romero believes. “After the scale of disruption of ecosystems that we are observing at the planetary level, we have to respond and evolve our idea of conservation,” he says. Plus, the creation of new areas for the butterflies is not mutually exclusive with trying to conserve their current habitat in the Monarch Butterfly Biosphere Reserve. In Sáenz Romero’s opinion, the approaches should be complementary and have the same priority.
To carry out the trial, his team has not only created experimental oyamel plantations, but also shrub nurseries. To survive, the young oyamel firs have to be planted under “nurse plants” that protect them from excessive sunlight and extreme cold.
The ultimate challenge is that the projected future climate is not here yet. “We don’t know how far we can move the oyamel firs without them dying in the present but surviving into the future,” Sáenz Romero says, hence the need to transplant the firs to sites at different altitudes. To save the monarch's habitat, “we need human intervention,” he says, “and we need to experiment.”
This story originally appeared on WIRED en Español and has been translated from Spanish.