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Saving Monarch Butterflies

These butterflies travel long distances and face serious threats along the way

Joel Sartore

About 60 miles northwest of Mexico City, Cuauhtemoc Sáenz-Romero is moving a forest 1,000 feet up a mountain. As the climate warms, scientists predict conditions might become too hot and dry for the forest’s sacred fir trees to survive in their current location. “The climate that is good for the sacred fir is now occurring at higher altitudes,” says Sáenz-Romero, a forest geneticist at the Michoacan University of St. Nicholas of Hidalgo.

So he’s planting fir seedlings higher up the mountain, where it’s cooler and wetter. Since 2015, Sáenz-Romero has planted 1,000 sacred fir trees, and he plans to plant 1,000 more. These fir trees are crucial to the life cycle of monarch butterflies, which undertake an epic migration that crosses North America. Each fall, 100 million monarchs fly 2,500 miles from the U.S. and Canada to reach these sacred fir forests, where they spend the winter. Sáenz-Romero wants to save the sacred fir forests, which will help save the butterflies.

About 60 miles northwest of Mexico City is a forest of sacred fir trees. Cuauhtemoc Sáenz-Romero is moving this forest 1,000 feet up a mountain. As the climate warms, scientists predict conditions might become too hot and dry for the trees to survive in their current location. “The climate that is good for the sacred fir is now occurring at higher altitudes,” says Sáenz-Romero. He’s a forest geneticist at the Michoacan University of St. Nicholas of Hidalgo.

So he’s planting fir seedlings higher up the mountain, where it’s cooler and wetter. Since 2015, Sáenz-Romero has planted 1,000 sacred fir trees. He plans to plant 1,000 more. These fir trees are crucial to the life cycle of monarch butterflies. Every year, 100 million monarchs make an epic migration across North America. In the fall, the butterflies leave the U.S. and Canada and fly 2,500 miles south. Their destination is this sacred fir forest. They spend the winter there. Sáenz-Romero is working to save the sacred fir forests. This will also help save the butterflies.

IN THE WOODS

As days shorten in mid-August, monarchs head south. Though the butterflies stop to feed on the nectar from flowers, they don’t reproduce on this leg of their journey. The monarchs arrive in Mexico in early November and spend the winter “basically hanging out in trees,” says Karen Oberhauser, a butterfly researcher and director of the University of Wisconsin-Madison Arboretum. Sacred fir trees give the monarchs warmth and protection from wind and rain. At the end of winter, the butterflies finish their development so they can reproduce along their journey north (see The Monarch Migration Relay, below).

Researchers monitor the health of monarch populations by measuring the forest area that they occupy in the winter. Over the past two years, the size of this area has dropped by 26 percent. This is part of a nearly 30-year downward trend. From 1993 to 2000, the monarchs covered an average of 8.6 hectares every winter, but from 2011 to 2020, that average dropped to 2.6 hectares. (One hectare is equal to 2.47 acres.)

In 2002, a storm wiped out about 80 percent of the overwintering monarchs. The species recovered, but “if that happened again, to an even smaller population,” Oberhauser says, “you wouldn’t have very many left.”

When the days shorten in mid-August, monarchs head south. The butterflies stop to feed on the nectar from flowers along their journey. But they don’t reproduce at this time. The monarchs arrive in Mexico in early November. They then spend the winter “basically hanging out in trees,” says Karen Oberhauser. She’s a butterfly researcher and director of the University of Wisconsin-Madison Arboretum. Sacred fir trees give the monarchs warmth and protection from wind and rain. At the end of winter, the butterflies finish their development. Now they are mature adults that can reproduce as they head north (see The Monarch Migration Relay, below).

Researchers monitor the health of monarch populations every year. They do this by measuring the forest area that the butterflies occupy in the winter. Over the past two years, the size of this area has dropped by 26 percent. This is part of a 30-year downward trend. From 1993 to 2000, the monarchs covered an average of 8.6 hectares every winter. But from 2011 to 2020, that dropped to 2.6 hectares. (One hectare is equal to 2.47 acres.)

In 2002, a storm wiped out about 80 percent of the monarchs spending the winter in Mexico. The species recovered, but “if that happened again, to an even smaller population,” Oberhauser says, “you wouldn’t have very many left.”

Illustration by Kate Francis

MONARCH LIFE CYCLE:

1. Female butterflies lay eggs on milkweed plants.
2. Each egg hatches, and the caterpillar grows as it eats milkweed.
3. After two weeks, the caterpillar forms a silky sac, called a chrysalis, in which it transforms into a butterfly.
4. After another two weeks, the butterfly emerges from the chrysalis.

GROWING THREATS

Monarchs face threats at every phase of their journey. In 1983, the International Union for Conservation of Nature listed monarch migration as an “endangered phenomenon.” The butterflies themselves, while not officially endangered, are in decline.

Climate change is a major threat to monarchs, says Oberhauser. Scientific models predict that as the planet warms, storms that can slow migration and kill overwintering populations will happen more often. Milkweed is the only plant monarch caterpillars eat. The plant’s range is expected to move northward into Canada. If that occurs, the monarchs will need to travel farther north to their summer breeding grounds. In turn, their offspring—the migratory generation—will have to fly greater distances to reach the sacred fir forests in Mexico. There, a drought and warming climate are already affecting the sacred fir trees.

Another huge threat is loss of milkweed and other flowering plants, which adult butterflies depend on to fuel their flight. Growing caterpillars need a lot of milkweed. For every 30 milkweed plants, one monarch lives to adulthood. Unfortunately, modern agriculture is not friendly to the plant, which is considered a weed. Milkweed once grew in corn and soybean fields. But in the late 1990s, people began planting crops genetically modified to tolerate herbicides. “Farmers could spray their fields with herbicide that killed off the milkweed,” Oberhauser says.

Development, like building houses, roads, and farms, also destroys monarch habitat. In Mexico, illegal logging is shrinking the butterflies’ winter habitats.

Monarchs face threats at every part of their journey. In 1983, the International Union for Conservation of Nature listed monarch migration as an “endangered phenomenon.” The butterflies themselves are not officially endangered. But they are in decline.

Climate change is a major threat to monarchs. Scientific models predict that there will be more storms. The storms can slow migration and kill butterflies where they spend the winter in Mexico. Milkweed is the only plant monarch caterpillars eat. The plants are expected to move northward into Canada. If that happens, the monarchs will need to travel farther north to their summer breeding grounds. In turn, their offspring that migrate south to Mexico will have to fly greater distances to reach the sacred fir forests. There, a drought and warming climate are already affecting the sacred fir trees.

Another huge threat is loss of milkweed and other flowering plants. Adult butterflies drink the plants’ nectar as they fly back north. Growing caterpillars need a lot of milkweed. For every 30 milkweed plants, one monarch lives to adulthood. Unfortunately, modern agriculture is not friendly to the plant. That’s because it is considered a weed. Milkweed once grew in corn and soybean fields. But in the late 1990s, people began planting crops modified to resist herbicides. “Farmers could spray their fields with herbicide that killed off the milkweed,” Oberhauser says.

Development, like building houses, roads, and farms, also destroys monarch habitats. In Mexico, illegal logging is shrinking the butterflies’ winter habitats.

A HELPING HAND

Brian van der Brug/Los Angeles Times/Getty Images

Sáenz-Romero has already seen monarchs passing through the sacred fir trees he’s planted. It will be decades, though, before the trees are large enough to shelter the overwintering monarchs.

But you can help the butterflies right now. Planting milkweed and other flowering plants creates monarch habitat, says Katie-Lyn Bunney, education coordinator for the Monarch Joint Venture, a collaboration for monarch conservation. Just make sure that your plants have not been treated with pesticides, which can kill the butterflies. You can also participate in citizen-science projects by reporting monarch sightings to organizations like Journey North and the Monarch Larva Monitoring Project. “A lot of what we know about monarchs is because people are out collecting data,” says Oberhauser.

Sáenz-Romero has already seen monarchs in the sacred fir trees he’s planted. However, it will take years for the seedlings to grow into trees large enough to shelter the monarchs.

But you can help the butterflies right now. Planting milkweed and other flowering plants creates monarch habitat, says Katie-Lyn Bunney. She’s the education coordinator for Monarch Joint Venture, a group for monarch conservation. Just make sure that your plants have not been treated with pesticides first. Pesticides can kill the butterflies. You can also participate in citizen-science projects. You can report monarch sightings to organizations like Journey North and the Monarch Larva Monitoring Project. “A lot of what we know about monarchs is because people are out collecting data,” says Oberhauser.

Use percent models to visually represent information about monarch butterfly migration. Record your work and answers on our answer sheet.

Use percent models to visually represent information about monarch butterfly migration. Record your work and answers on our answer sheet.

The greatest distance any monarch has been known to travel in a day is 275 miles of its 2,500-mile journey. This percent is modeled below. What is that?

The greatest distance any monarch has been known to travel in a day is 275 miles of its 2,500-mile journey. This percent is modeled below. What is that?

These three models show the declines in winter butterfly populations. Match the percent fact with the model that represents the same percent.

These three models show the declines in winter butterfly populations. Match the percent fact with the model that represents the same percent.

 

A ________________

 

B ________________

 

C ________________

 

A.  Monarchs travel only during daylight hours. When they leave in August, daylight lasts 14 hours, or 58 percent of the day. When they reach Mexico in October, daylight lasts 11.5 hours, or 48 percent of a day. Shade in the percents using your blank answer sheet and label them to model the percent of the day monarchs can travel in August

A.  Monarchs travel only during daylight hours. When they leave in August, daylight lasts 14 hours, or 58 percent of the day. When they reach Mexico in October, daylight lasts 11.5 hours, or 48 percent of a day. Shade in the percents using your blank answer sheet and label them to model the percent of the day monarchs can travel in August

B. By how many percentage points is the number of daylight hours shortened between when the monarchs left in August and arrived in Mexico in October?

B. By how many percentage points is the number of daylight hours shortened between when the monarchs left in August and arrived in Mexico in October?

From 2001 to 2010, the butterflies covered an average of 5.8 hectares of forest every winter, but from 2011 to 2020, that dropped to 2.6 hectares—a decrease of 55 percent. Model this in the grid below.

From 2001 to 2010, the butterflies covered an average of 5.8 hectares of forest every winter, but from 2011 to 2020, that dropped to 2.6 hectares—a decrease of 55 percent. Model this in the grid below.

From 2014 to 2015, the forest area covered by monarchs in the winter increased by the percent modeled below.

From 2014 to 2015, the forest area covered by monarchs in the winter increased by the percent modeled below.

A. What percent is that?

A. What percent is that?

B. Why did it take more than one grid to model this percent?