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Decoding the Secrets of the Monarch Butterfly Migration with Isotopes and IAEA Data - World Wildlife Day 2020


Monarch butterfly (Danaus plexippus). (Photo: Bernardo Roca-Rey Ross)

Migratory animals, their long-distance movements and life cycles have eluded scientists for centuries, particularly for hard-to-track and tiny species like the iconic monarch butterfly. But thanks to the help of isotopic techniques, experts worldwide can better understand the flows and patterns of animal migration where traditional techniques have shown their limits.

“Isotopic techniques offer an incomparable advantage over the traditional tracking methods, as they are non-invasive and do not necessitate the recapture of the same animals,” said Leonard Wassenaar, Head of the IAEA’s Isotope Hydrology Laboratory.

For over a century, conventional mark-and-recapture approaches used to track animal movement have relied on external markers, such as tags and radio and satellite tracking, which are inappropriate for small and short-lived animals. Then, in 1996, research by Leonard Wassenaar and Keith Hobson, who at the time were isotope scientists for Environment Canada, demonstrated that isotopic techniques can be used to determine the origin of individual animals.

How is the isotopes research conducted and what did it reveal about the monarch butterfly migration path?

Isotopic research is based on measuring deuterium — a rare isotope of hydrogen — in rainwater, which is directly absorbed by plants or ingested by animals and humans. As rainwater and its deuterium composition are unique to the area where it rains, rainwater deuterium content serves as a direct marker that scientists can use to identify the origin of individual animals that were grown in different areas by measuring the amount of deuterium in hair, wings, claws, feathers or bones. For monarchs, the deuterium contents show the area where the insect was born.

The study of the deuterium measured in the wings of the monarch butterfly (Danaus plexippus) helped to establish their migration travel. (Photo: Bernardo Roca-Rey Ross)

This discovery has helped to unlock new doors into the mysterious lives of monarch butterflies. These insects play a crucial role as pollinators for countless wildflower species along their migration path.

What do we know about monarch butterfly migration?

Monarch butterfly migration map (Infographic: H. Boening/IAEA).

Every October, millions of monarch butterflies travel for two months from the United States and Canada to the mountains of central Mexico where they spend the winter. In March, they begin their return ‘home’, but their northbound return journey takes four to five months to cover 3500 kilometers and is usually completed over several generations of butterflies. This longer travel time is because the generation born in the north lives 8 to 10 times longer than those born in the south, which allows the north-born to complete the southbound journey within two months.

“While under undisturbed conditions monarch butterflies could easily survive high altitude forested hibernation, now some are dying from the cold winds released due to illegal logging in the mountains of Mexico,” said Wassenaar. Illegal avocado plantations are also replacing the butterflies’ few-hectare winter habitats leaving them stranded and dying. Many are also dying from pesticides used to destroy the larval milkweed host plant on the monarch butterflies' breeding ground, mainly affecting the Corn Belt of the United States.

The Science

Wassenaar and Hobson captured in 1996, for the first time, the complete migration cycle of North American monarch butterflies. They collected 1200 specimens from 13 hibernation colonies and across the continent, checked the patterns of deuterium concentration in the wings and compared them with the IAEA’s Global Network of Isotopes in Precipitation (GNIP) database to determine the butterflies’ origin and deduct their migration routes.

Today, GNIP and its deuterium isotope measurements are widely used to study the migration of many animals, ranging from bats, birds, insects and fish.

What is the Global Network of Isotopes in Precipitation (GNIP)?

GNIP is a worldwide network for collecting hydrogen and oxygen isotope data on precipitation. Initiated in 1960 by the IAEA and the World Meteorological Organization (WMO), GNIP helps scientists to study the global water cycle, the origin, movement and history of water. With hundreds of monitoring sites in over 90 countries generating more than 130,000 monthly isotope records, the GNIP’s database is today a precious resource for many environmental scientists.

“It took us one year to complete the study, but with traditional methods we would have needed decades to get to our conclusions,” said Wassenaar. Since then, many studies on monarch migration and conservation have been using isotopes. “The IAEA, through its global isotope database, plays a foundational role in enabling the application of isotopes to study animal migration. The use of isotopes to easily and cost-effectively link all geographical areas used by migrating species has proven to be fundamental in helping scientists trying to link patterns of species population declines to habitat destruction, land use change, infection and pests, and climatic factors.”

About the World Wildlife Day

World Wildlife Day 2020 celebrates "sustaining all life on earth" at a time when, more than ever, conservation plans are needed all around the world. Climate change, destruction of habitats for agriculture, illegal poaching and logging, pollution and use of pesticides are threats to animal species around the globe.

“The rate of species loss is exponentially higher than at any time in the past 10 million years. […] One million species are in near-term danger of extinction,” said Antonio Guterres, United Nations Secretary-General, to the General Assembly on 22 January 2020, pushing for the adoption of the post-2020 global biodiversity framework.

To preserve the lives of migratory animals, isotopic techniques significantly contribute to identifying these animals’ origin, their breeding grounds, and wintering and intermediate stopover sites. Based on this scientific data, policy makers can develop better conservation methods for all sorts of animals, such as fish, birds, mammals or insects.

Global map of animal migration. (Photo: CMS)

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