The monarch butterfly migration is one of nature’s greatest marvels. Each autumn, a new generation of butterflies hatches in the northern US and southern Canada, before embarking on an incredible 3,000-mile journey to the mountains of central Mexico.
Here, they spend the winter in high-altitude forests, rich with Abies religiosa – or sacred fir trees. These trees are vital to the monarch butterfly's survival, providing a unique habitat that shields them from harsh winter conditions.
However, the future of these forests is threatened by climate change. Experts predict that rising temperatures will push the fir trees higher up the slopes – and by 2090, they may run out of mountain.
On a mission to help, a team of scientists in Mexico are attempting to establish overwintering sites for the butterflies on one of the country's biggest mountains, a dormant volcano known as Nevado de Toluca. Their findings (so far) were published last month in Frontiers in Forests and Global Change.
Assisted migration
To safeguard the monarchs, conservationists from Mexico are exploring ways to establish new forests outside their current range, such as on higher peaks to the east – specifically, planting sacred firs on the Nevado de Toluca volcano, a stratovolcano in central Mexico, 50 miles west of Mexico City.
“We call this 'assisted migration’: planting seedlings grown from seeds from existing sacred fir populations to new sites whose climate by 2060 is predicted to become similar to that at today’s overwintering sites due to global warming,” says Dr Cuauhtémoc Sáenz-Romero, a professor at the Universidad Michoacana de San Nicolás de Hidalgo in Mexico, and the lead author of the study.
Planting on a volcano
The work began in 2017, when Sáenz-Romero and rest of the team collected seeds from sacred fir trees in the Monarch Butterfly Biosphere Reserve (MBBR). The seedlings were carefully nurtured for three years before being transplanted in July 2021 to four different altitudes on the Nevado de Toluca, ranging from 3,400 to 4,000 metres. This mountain, a protected natural area with a summit 1,130 meters higher than the firs’ current highest elevation, was chosen due to its proximity to the MBBR.
A total of 960 seedlings were planted under nurse plants, such as Senecio cinerarioides shrubs and Pinus hartwegii trees, to protect them from extreme weather.
Monitoring their growth, survival and health over the next two years, the researchers aimed to determine how well the seedlings could adapt to their new environment.
What was found
The study found that seedlings struggled more as the 'ecological distance' – the cumulative difference across a range of climate variables such as temperature, precipitation and dryness – between their original habitat and the new site increased.
Seedlings planted at the highest altitude of 4,000 metres showed almost no growth and suffered frost damage, while those between 3,600 and 3,800 metres experienced lower survival and growth rates compared to those at 3,400 metres. Still, the team considered the survival rates at these elevations to be promising.
“These planted stands could ultimately serve as overwintering sites for the monarch butterfly under warmer climates,” concludes Sáenz-Romero.
Creating new habitats for monarch butterflies does not replace efforts to conserve their current wintering grounds, Sáenz-Romero emphasised. Instead, both strategies must work hand in hand to ensure the survival of these iconic insects.
Find out more about the study Establishing monarch butterfly overwintering sites for future climates: Abies religiosa upper altitudinal limit expansion by assisted migration
Main image: Planting Abies religiosa on Nevado de Toluca volcano, central Mexico/Cuauhtémoc Sáenz-Romero, UMSNH
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