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LeAsha Bryant
Ms.Cuddihy
Period 1
17 May 2021

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Migration is a behavior where animals travel from one habitat to another in search of food, living conditions, or reproductive needs.

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 What is Migration for Monarch Butterflies?

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 What is Migration for Monarch Butterflies?

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In the fall Monarch Butterflies leave the northeastern U.S. and Canada and travel for 3,000 miles to reach their winter home grounds in southwestern Mexico.

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Image

Greater Snow Geese ( Chen caerulescens) flying over the Saint Francis river. Geese go through a migration every year to escape the harsh northern winters.

Photograph by David Doubilet

Encyclopedic Entry Vocabulary

Migration is a pattern of behavior in which animals travel from one habitat to another in search of food, better conditions, or reproductive needs. There are two important factors that make migration different from other types of animal movement: First, migration happens seasonally, and second, migration involves a return journey. This makes it different from emigration, when animals travel to find a new, permanent place to live. Many animal species migrate, including species of fish, crustaceans, amphibians, reptiles, insects, and mammals. These animals might journey by land, sea, or air to reach their destination, often crossing vast distances and in large numbers.

One of the main reasons animals migrate is to find food. In Tanzania, wildebeests ( Connochaetes taurinus), zebras ( Equus quagga), and gazelles ( Eudorcas thomsonii) migrate in huge herds. They roam the Serengeti looking for fresh grass and water, which are hard to find during the dry season. Humpback whales ( Megaptera novaeangliae) migrate for food as well. In the summer, they travel to feeding grounds near the polar ice, where the water is full of krill and small fish. In the winter, they migrate back to warmer waters to raise their calves.

Other animals migrate because of the climate or seasons. For example, monarch butterflies ( Danaus plexippus) migrate to avoid cold temperatures in the winter. These butterflies cannot survive freezing temperatures, so they fly from Canada all the way to Mexico, where they gather to keep warm over winter. They make the return journey over many generations, stopping to lay eggs on milkweed plants along the way. The caterpillars eat the milkweed and then finish the journey as butterflies.

Finally, some animals migrate for reproductive reasons: either to find a mate, raise their young, or to spawn. For example, salmon start life in rivers and migrate to the sea to feed and grow. After spending up to seven years in the ocean, they migrate back to the rivers they were born in so that they can spawn. Christmas Island red crabs ( Gecarcoidea natalis) migrate for similar reasons. They spend most of their life in the forest but migrate to the ocean to reproduce.

Recent improvements in technology have helped us understand migration better, but there is still a lot we do not know. Scientists are not yet entirely sure how animals know where to go and when to leave, especially when they have never made the journey before. Some researchers suggest that these animals use a mix of stimuli, such as sunlight, the Earth's magnetic field, and chemical cues, to find their way.

environment where an organism lives throughout the year or for shorter periods of time.

movement of a group of people or animals from one place to another.

period of the year distinguished by special climatic conditions.

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Director

Tyson Brown, National Geographic Society

Author

National Geographic Society

Production Managers

Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society

Program Specialists

Sarah Appleton, National Geographic Society
Margot Willis, National Geographic Society

Specialist, Content Production

Clint Parks

Producer

Andre Gabrielli, National Geographic Society

Last Updated

June 28, 2019

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Director

Tyson Brown, National Geographic Society

Author

National Geographic Society

Production Managers

Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society

Program Specialists

Sarah Appleton, National Geographic Society
Margot Willis, National Geographic Society

Specialist, Content Production

Clint Parks

Producer

Andre Gabrielli, National Geographic Society

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Image Greater Snow Geese ( Chen caerulescens) flying over the Saint Francis river. Geese go through a migration every year to escape the harsh northern winters. Photograph by David Doubilet Encyclopedic Entry Vocabulary Migration is a pattern of behavior in which animals travel from one habitat to another in search of food, better conditions, or reproductive needs.

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Each fall, millions of monarch butterflies leave their summer breeding grounds in the northeastern U.S. and Canada and travel upwards of 3,000 miles to reach overwintering grounds in southwestern Mexico.

But unlike birds or wildebeest that also embark on epic migrations, these individual butterflies will never return. ( See National Geographic's amazing photos of monarchs.)

Why won't they make it all the way back? How do they know where to go in the first place?

How Does Monarch Butterfly Migration Work?

As the days get shorter and the temperatures drop off, monarchs begin to abandon breeding and feeding territories in search of a safe place to spend the winter.

For monarchs, that overwintering ground is found high up on just a few mountains in central Mexico. Once there, the monarchs huddle together by the millions on the branches of oyamel fir trees.

These trees, also known as sacred firs, create a microclimate that protects the insects, says Pablo Jaramillo-López, a research scientist at the National Autonomous University of Mexico.

"The tree canopy and ecosystem provide a blanket effect for the monarchs, so the temperatures don't go too high or too low," says López.

After waiting out the winter, these individuals head part of the way back north to warmer climes such as Texas, where they mate and lay eggs on milkweed plants. In just a few days, the eggs hatch into brilliantly striped caterpillars of black, gold, and white. These monarch larvae consume vast amounts of milkweed before forming a chrysalis and transforming into adult butterflies. ( Read how the monarch's decline is linked to milkweed.)

At this point in the cycle, the new butterflies take to the skies again and fly another few hundred miles north before finding another patch of milkweed and repeating the process.

It might take the monarchs as many as four to five generations to complete the journey all the way back up to Canada, says Sarina Jepsen, who directs the endangered species program for the Xerces Society, an international nonprofit focused on invertebrate conservation.

Then, when fall rolls around again, the monarchs make use of air currents to wing all the way back to Mexico-a seemingly impossible feat for such a delicate-looking insect.

"Interestingly, the waves of monarchs heading north will complete their entire life cycles in just five to seven weeks each," saysJaramillo-López.

" But when fall rolls around again, a special 'super generation' of monarchs that can live up to eight months will make use of air currents to wing all the way back to Mexico-a seemingly impossible feat for such a delicate-looking insect."

"This makes the migrating monarchs so unique as they are the same species but for some reason live much longer."

Why Do Monarchs Migrate, and How Do They Know Where to Go?

This part is still very much up for debate, says Jepsen. But there are a couple of theories.

One explanation might be that when monarchs march north, they are essentially following the bloom of their primary food source-milkweed plants. And then when winter comes and those plants die back, the animals retreat to a place with conditions that protect them from the weather.

Another idea involves their immune systems. "Migration also allows monarchs to escape habitats where parasites have accumulated at the end of the summer," says Sonia Altizer, an ecologist at the University of Georgia. (Read: " Imperiled Monarch Butterflies Get $3.2 Million From U.S. Government.")

Altizer has found that adult butterflies infected with a protozoan parasite called Ophryocystis elektroscirrha, or OE, can't fly as well in lab tests and travel shorter distances in the wild. (Read m

"We think that migration weeds out the most heavily infected monarchs, removing them from the populations," says Altizer.

As for how the monarchs know where to go, a study published in 2016 suggests that the critters navigate based on their relative position to the sun. But scientists are still ironing out exactly how this works.

Why Are Monarch Butterflies Important?

While monarchs may seem small and insignificant, the creatures play a crucial role in the ecosystems they inhabit.

As adults, monarch butterflies visit countless numbers of wildflowers each year as they seek out nutrient-rich nectar. In doing so, the monarchs transfer pollen from one plant to another and assist in those species' reproduction. ( Learn the key to the monarch's long-distance migration.)

And even though monarch caterpillars and adults are poisonous to most predators, thanks to toxins they acquire from milkweed, some animals are still able to stomach them. Orioles and grosbeaks in particular make a feast of monarchs over the winter, and ants, wasps, flies, and spiders have been known to prey on the caterpillars when they get the chance.

Are Monarch Butterflies Endangered?

The short answer is no. Monarch butterflies are actually quite common across the world, with populations occurring as far away as North Africa and New Zealand.

However, the subspecies known as Danaus plexippus plexippus is the only one that performs the great North American migration-and these butterflies are increasingly under threat. (See " Monarch Butterflies Hit New Low; 'Worrisome' Trend.")

Jepsen says the Xerces Society has petitioned the U.S. Fish and Wildlife Service to declare the migratory subspecies of monarchs as threatened under the Endangered Species Act. That finding is currently under review.

What Threats Do Migrating Monarchs Face?

Because migratory monarchs have such a wide range, they also have many threats. These include declining milkweed populations across their range in the U.S. and Canada, parasites and diseases like those in Altizer's research, and even a growing demand for avocados sourced from Mexico. ( Read more about the world's great migrations.)

The avocado problem is a tricky one, says Jaramillo-López, because the people who live near the monarch wintering grounds need good ways to make a living. Unfortunately, when native forests are replaced with avocado plantations, it can have a ripple effect on monarchs higher up in the mountains.

Mexico has designated almost 140,000 acres of forest as a protected area for wintering monarchs, but Jaramillo-López says much of this area is really just a buffer zone that prevents strong winds from damaging the core area where the monarchs congregate. So even if the area being razed for avocado cultivation is at a lower elevation than butterflies prefer, it can still affect the forests on which they rely.

According to Jepsen, people can also do damage to wild populations by trying to raise monarch eggs bought from commercial suppliers. There's very little oversight, she says, which means that even if you're trying to help monarchs, you might be inadvertently spreading disease. (See " How Your Backyard Can Save Butterflies.")

"The best thing you can do," says Jepsen, "is to provide habitat for monarchs and just let them do their thing."

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Each fall, millions of monarch butterflies leave their summer breeding grounds in the northeastern U.S. and Canada and travel upwards of 3,000 miles to reach overwintering grounds in southwestern Mexico. But unlike birds or wildebeest that also embark on epic migrations, these individual butterflies will never return.

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I think that migration is very useful because it doesn't make sense for an animal to stay in a place where they can't thrive. If that means making a new home for the winter than I think that is cool.

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Migration can lead to something called "Migration load". Migration load is reduced fitness. This is what can cause natural selection. Also recurring adult migration in populations can also lead to selection.

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Abstract

Migration tends to oppose the effects of divergent natural selection among populations. Numerous theoretical and empirical studies have demonstrated that this migration-selection balance constrains genetic divergence among populations. In contrast, relatively few studies have examined immigration's effects on fitness and natural selection within recipient populations. By constraining local adaptation, migration can lead to reduced fitness, known as a "migration load," which in turn causes persistent natural selection. We develop a simple two-island model of migration-selection balance that, although very general, also reflects the natural history of Timema cristinae walking-stick insects that inhabit two host plant species that favor different cryptic color patterns. We derive theoretical predictions about how migration rates affect the level of maladaptation within populations (measured as the frequency of less-cryptic color-pattern morphs), which in turn determines the selection differential (the within-generation morph frequency change). Using data on color morph frequencies from 25 natural populations, we confirm previous results showing that maladaptation is higher in populations receiving more immigrants. We then present novel evidence that this increased maladaptation leads to larger selection differentials, consistent with our model. Our results provide comparative evidence that immigration elevates the variance in fitness, which in turn leads to larger selection differentials, consistent with Fisher's Theorem of Natural Selection. However, we also find evidence that recurrent adult migration between parapatric populations may tend to obscure the effects of selection.

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national center for biotechnology information

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Natural selection in populations subject to a migration load - PubMed

# data[].attributes.attachment_link.description
Migration tends to oppose the effects of divergent natural selection among populations. Numerous theoretical and empirical studies have demonstrated that this migration-selection balance constrains genetic divergence among populations. In contrast, relatively few studies have examined immigration's effects on fitness and natural selection within recipient populations.