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FOCUS Book
Sometimes people try to use a nonnative species for a
specific purpose. Imagine that beetles are eating all the
wheat in a small country. The country’s leader wants
to introduce a beetle-eating bird to control the beetles.
You are an environmental engineer. First, create a
diagram of a natural food web in this country. Then
consider the impact of bringing in the new bird species.
How would various populations be affected? How will
you help the wheat farmers solve their problem without
creating new problems? Write up an action plan and
share it with your classmates. Discuss the pros and cons
of each person’s solution.
Beyond the Book
Identify an invasive species that is causing
a problem in the area where you live. What
can your community do to control this
invasive species?
Broken
Chains
Everything’s Connected
In the brief Alaskan summer, tiny plants use energy from
the Sun to quickly sprout and bloom. Busy little rodents
called lemmings gather up and eat these plants. Arctic
foxes hunt and eat the lemmings. This is an example of
a food chain—the movement of energy from one living
organism to another.
Arctic plants support more than just lemmings. These
plants also provide food for insects, hares, and caribou.
In addition to nourishing foxes, lemmings are food for
snowy owls, weasels, and wolves. These connected
food chains make up a food web.
FOCUS Question
How does a disrupted food chain affect
an ecosystem?
If lemmings disappear, what happens to the foxes, wolves,
plants, hares, and other organisms in the food web? If a
food chain breaks, the effects can reach far and wide.
Stability and Change
Photo Credits:
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© North Wind Picture Archives/Alamy; page 7 (bottom): © Rosemarie Stennull/Alamy; page 8 (top left): © Derek
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© Steve Byland/Dreamstime.com; page 9 (top): © Barrett Hedges/National Geographic Creative; page 9
(bottom): © S.Callahan/Photri Images/Alamy
ARCTIC FOOD WEB
lemming
saxifrage
Arctic fox
Broken Chains
© Learning A–Z
Written by Katherine Follett
Arctic wolf
All rights reserved.
caddisfly
caribou
These plants and animals are connected in a food web.
www.sciencea-z.com
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Natural Cycles
Missing Links
Lemmings in the Alaskan tundra make up a population—
all the members of a species living in one place.
Populations rise and fall naturally. For example, during
an especially warm Alaskan summer, more plants grow.
With this extra food, the lemmings have more offspring.
Now Arctic foxes, which prey on the lemmmings, are
able to have more young, and their population rises.
Species sometimes disappear for natural reasons.
Diseases and natural disasters can leave missing links
in food chains. But more often, people are the cause
of broken food chains.
In the 1800s, people hunted
sea otters for their warm,
waterproof fur. The animals
came close to extinction. Otters A sea otter eats
a sea urchin.
eat sea urchins, and sea urchins
eat kelp—a large seaweed. Without otters, the sea
urchin population exploded. Urchins devoured entire
kelp “forests.” These forests provided food and shelter
to hundreds of other species in the food web. These
animals now had nothing to eat and nowhere to go.
Before long, with too many predators and not enough
food to go around, the lemming population crashes.
Then many foxes starve or move to another location to
find food. Without hungry lemmings, the plants soon
recover. With more plants and fewer predators, the
lemming population explodes again.
While these populations go up and down, the food chain
stays in balance over the long term. However, if a species
disappears entirely, the food chain breaks.
Today, sea otters are protected. Their numbers have
rebounded enough to keep the sea urchin population
in check. Many kelp forests have recovered, too.
POPULATION CYCLE
more
lemmings
more
foxes
population of
lemmings and
arctic foxes
fewer lemmings
Kelp forests are
an important
underwater
ecosystem.
fewer foxes
time
The populations of predators and prey rise and fall regularly over time.
Food Chains • Broken Chains
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Invasive Links
When people from Europe started moving to Australia
in the late 1700s, pet cats began running wild. Native
prey animals had never seen cats. They had no defenses
against them. Even today, cats continue to overhunt
Australian birds and mammals.
The sea otter example shows what happens when a
species disappears. But food chains also break when new
species come in. Invasive species are organisms that move
into a new area and disrupt the natural food chain. These
new links in the chain show up in ecosystems all over the
world. But their effect may be greatest in Australia.
In 1859, a hunter released several rabbits into the
Australian countryside. With no predators, the rabbit
population exploded. They devoured native plants,
turning many areas into barren desert.
Australia is a continent surrounded by water. For millions
of years, food chains there had a healthy balance of native
species—organisms that evolved naturally in the area.
When people from Europe arrived in the
ASIA
1700s, they brought many invasive, or
PACIFIC
OCEAN
nonnative, species. These new plants
AUSTRALIA
and animals quickly disrupted many
food chains.
NATIVE
SPECIES
tree
python
red
kangaroo
Food Chains • Broken Chains
A cane toad eats
a native pygmy
possum. Cane toads
have disrupted
many natural food
chains in Australia.
NONNATIVE
SPECIES
Major
Mitchell’s
cockatoo
Many Australian animals
are found nowhere else.
They were adapted to their
isolated food chain, which was
disrupted by invasive species.
In 1935, people brought cane toads to Australia to eat
insects that damaged sugar-cane crops. Big mistake!
Instead of eating the insects, the toads ate native rodents,
snakes, and birds.
cane toad
In 1890, a man brought European starlings to New
York City. He wanted Central Park to have all the
birds mentioned in plays written by Shakespeare.
European starlings now live across the United States.
feral cat
rabbit
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Unexpected Consequences
Mending the Chain
People are part of food chains, too. When food chains
break, people are affected, along with other animals
and plants.
Can a broken food chain be fixed? Yes! Over many years,
nature can repair itself. In the 1900s, people hunted and
drove out all the wolves in the eastern United States.
Soon, the coyote population rose. The coyotes themselves
grew larger. Eastern coyotes now fill the wolf’s role in
some food chains.
Millions of bison
once roamed the
plains of North
America. Bison
were the main
artist’s drawing
European
settlers
killed
bison
for
their
hides
food source for
and to clear the land for railroads and farms.
many Native
American tribes. Thick bison hides provided warmth. In
the 1900s, people from Europe hunted the bison almost
to extinction. Without food to hunt, the native Plains
people could no longer live off the land as they once did.
eastern wolf
Invasive species can sometimes become naturalized, or
part of the local food chain. Some native predators learn
to eat the invasive species.
Some native plants adapt to
resist invasive herbivores.
Some native animals learn
to avoid invasive predators.
A native American robin eats
Cod was once the most popular fish in markets across
Europe and North America. In the 1980s, people simply
fished too many of them. The cod population in the North
Atlantic plummeted.
Fishermen in Canada,
New England, and
Great Britain had to
find another place
to fish, and many
lost their way of life.
a naturalized earthworm.
Many familiar species, such as honeybees and
earthworms, are not native to North America. But
these animals are now part of many food chains.
Due to overfishing, many former codfishing villages have few jobs left.
Food Chains • Broken Chains
eastern coyote
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Many countries now take steps
to protect food chains. They
limit or ban the hunting and
fishing of threatened and
endangered species. Some
species are reintroduced
into areas where they once
thrived. In 1995, scientists
began to reintroduce wolves
into Yellowstone National
Park. The new wolf population
quickly repaired Yellowstone’s
food chains.
Write your answers on separate paper. Use details from
the text as evidence.
1 Look at the graph on page 3. What causes the fox
population to increase?
2 What happened when sea otters were removed
from a food chain?
Wolves keep the Rocky
Mountain food web healthy
by controlling the number
of herbivores, such as deer
and elk.
3 What is one way that broken food chains have
affected people?
4 Identify two invasive species described in the book.
5 How are people trying to stop the spread of invasive
species?
People have also become careful about the spread
of invasive species. Many countries inspect animals,
fruits, vegetables, wood, and soil that come across
their borders to keep
out dangerous species.
FOCUS Question
How does a disrupted food chain affect an
ecosystem? Imagine that arctic foxes disappeared
from the food chain described at the beginning
of the book. How would the food
web change?
We’ve seen what happens
when food chains break.
Humans have become
the key to keeping natural
food chains and food
webs together.
Many invasive plants and animals,
such as shellfish and waterweeds,
can travel from place to place on
the sides of boats.
Food Chains • Broken Chains
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