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CONTENTS
Where Do They Live? . . . iv
Introduction What are Keystone Species? . . . 1
Chapter 1
Shaking Up Sharks . . . 12
Chapter 2
Gearing Up for Gray Wolves . . . 25
Chapter 3 Brilliant Beavers . . . 35
Chapter 4 All About Alligators . . . 45
Chapter 5 The Buzz About Bees . . . 57
Chapter 6 Fantastic Flying Foxes . . . 68
Chapter 7 Wild About Wildebeests . . . 80
Chapter 8
Burly Bison . . . 90
Chapter 9
Crazy About Corals . . . 101
Glossary • Metric Conversions
Essential Questions • Resources • Index
Interested in Primary Sources? Look for this icon.
Some of the QR codes in this book link to primary sources that offer firsthand information about the topic. Many photos are considered primary sources because a photograph takes a picture at the moment something happens. Use a smartphone or tablet app to scan the QR code and explore more! You can find a list of the URLs on the Resources page. You can also use the suggested keywords to find other helpful sources.
species
keystone
KEYSTONE
WHAT ARE KEYSTONE SPECIES?
Imagine an archway made of stone or bricks. At the very top of the arch is a single piece that locks all the other pieces into place. It holds the arch together. The arch would collapse without it. That piece is called the keystone.
ESSENTIAL QUESTION
Why is it important to challenge scientific assumptions?
You are probably wondering what an archway has to do with ecology. Keep reading!
KEYSTONE SPECIES
WORDS TO KNOW
keystone: a central stone at the top of an arch that locks the whole together. Also, the central part of a system.
ecology: the study of the patterns of relationship between species in an ecosystem.
ecosystem: an interdependent community of living and nonliving things and their environment.
species: a group of organisms that share common traits and can reproduce offspring of their own kind.
organism: any living thing, such as a plant or animal.
keystone species: a species that plays an essential role in an ecosystem and without which the ecosystem would be greatly altered.
ecologist: a scientist who studies the interaction between organisms and their environment.
food chain: a community of plants and animals where each is eaten by another higher up on the chain.
food web: a network of connected food chains.
producer: a part of the food chain that includes plants, which make their food through photosynthesis.
herbivore: an animal that eats only plants.
predator: an animal or plant that kills and eats other animals.
hypothesis: an unproven idea that tries to explain certain facts or observations.
photosynthesis: the process a plant goes through to make its food. The plant uses water and carbon dioxide in the presence of sunlight to make oxygen and sugar.
Now, imagine an ecosystem. Within every ecosystem in the world, there are living and nonliving things. Together, they create a community. In that community, all the plants and animals have roles to play to keep the ecosystem healthy.
Yet some species are more important than others—they have the same role in their ecosystems that a keystone has in an arch. Without these organisms , the ecosystem would collapse. That is why they are called keystone species.
DISCOVERY!
Ecologists are scientists who study the relationships between organisms and their environment within an ecosystem. Part of their job is to study food chains and food webs.
Before the 1960s, scientists thought that the number of producers in an ecosystem limited the number of herbivores. And, in turn, the number of herbivores limited how many predators there were. In other words, they thought everything in the ecosystem was regulated by how much food was available, starting at the bottom of the food chain.
But some scientists wondered: Why didn’t the herbivores just devour all the plants? Why were any plants left in the ecosystem?
WHAT ARE KEYSTONE SPECIES?
Scientists came up with a new hypothesis. They proposed that the number of herbivores was controlled by the number of producers and by the number of predators. This was a switch from what scientists had assumed, that predators didn’t play any significant role in the overall ecosystem. The idea that predators helped regulate herbivore populations was an entirely new way to think of food webs. After all, how could predators help to keep the world green?
The energy being passed around in a food web originally comes from the sun. Through photosynthesis, green plants get their energy from the sun. The animals eating those plants are benefitting from the sun’s energy, too.
Scientists faced a problem: How could they run a controlled experiment on a natural ecosystem? Enter Robert Paine (1933–2016), a zoology professor at the University of Washington. In the early 1960s, he wanted to find a system he could control and manipulate to test the hypothesis. He discovered the tidepools on the Pacific Coast of Washington—each one was its own tiny ecosystem with more than a dozen species living there. It was the perfect place to run the experiment.
A food web
KEYSTONE SPECIES
WORDS TO KNOW
filter feeder: an animal that gets its food by filtering food particles or tiny living things from water.
scavenger: an animal, bird, or insect that eats rotting food or animals that are already dead.
algae: a plant-like organism that lives in water and grows by converting energy from the sun into food.
carnivorous: describes a carnivore, a plant or animal that eats only animals.
data: facts and observations about something.
Paine started by observing and recording all of the species that lived in the tidepools. There were filter feeders and scavengers and algae and other plants. At the very top of the food chain was a carnivorous starfish.
To see what would happen to the ecosystem without its top predator, Paine removed the starfish from one tidepool. He left the starfish in the other tidepools. Any time a starfish reappeared in the experimental tidepool, it was removed.
Within 18 months, that tidepool had changed. The number of other species had dropped. After eight years, the tidepool contained only a single organism—mussels.
What happened? When starfish were present, they kept the mussels from taking over. Without the top predator, the mussels pushed out all other species.
Scientist Spotlight: Bob Paine
Robert Paine and the starfish changed the field of ecology forever. Before Paine, the field of ecology was mostly about describing patterns in nature. He changed it to one that included experiments and data. As a boy, Paine was always interested in nature. He spent much of his youth exploring the woods. In an interview, Paine said, “All my early childhood memories involve biology.” That passion eventually led him to the Pacific Northwest. He was a professor at the University of Washington for decades. That’s where he discovered the ecosystems of the tidepools. Throughout his life, he continued to do research in the field. He also mentored the next generation of ecologists.
Watch this short video about Bob Paine and his work. Why was his discovery of tidepools as complete ecosystems so important?
PBS keystone species Paine
WHAT
ARE KEYSTONE SPECIES?
A NEW WAY OF THINKING
Paine’s discovery changed how we understand ecosystems and food webs. It was groundbreaking! Paine coined the term “keystone species.” These species play an important role in their environment. Removing a keystone species from an ecosystem has enormous impact. Its removal causes the entire system to collapse, just like a keystone in an arch.
Paine did other experiments, too. He found that not all species have such a deep impact on their environment as keystone species.
Use the interactive map or the species tiles to discover various keystone species around the world. How do some of these keystone species affect humans in the region?
Tidal pools in the U.S. state of Washington
Media HHMI keystone map
KEYSTONE SPECIES
WORDS TO KNOW
biodiversity: the variety of life on Earth.
trophic cascade: the changes that occur after a keystone species is gone that affect every species on every level of the food chain in an ecosystem. resilient: describes the ability to recover quickly from setbacks.
invertebrate: an animal without a backbone.
prey: an animal hunted and eaten by other animals. regenerate: to form again, renewing or restoring something.
tundra: the treeless area between the ice of the Arctic and the forests of northern lands.
wetland: a low area filled with water, such as a marsh or swamp.
Keystone species maintain biodiversity in an ecosystem. They help determine which other species are in that ecosystem. They also affect how many of each species are there. Plus, no other species can take over the role of a keystone species if they disappear.
When a keystone species does disappear, ecosystems experience a trophic cascade. This means that after the keystone species is gone, the effects cascade throughout the whole ecosystem. Every species on every level of the food chain is affected. Some species are pushed out. Others take over.
This is especially concerning because the greater the biodiversity in an ecosystem, the stronger that ecosystem is. And even though not all species have a keystone role, biodiversity keeps ecosystems healthier and more resilient .
Trophic cascades have occurred throughout Earth’s history. They happen due to species loss, volcanic eruptions, severe weather events, and other ecosystem changes.
The Scientific Method
A scientific method worksheet is a useful tool for keeping your ideas and observations organized. The scientific method is the process scientists use to ask and answer questions. Use a notebook as a science journal to make a scientific method worksheet for each experiment you do.
Question: What are we trying to find out? What problem are we trying to solve?
Research: What is already known about this topic?
Hypothesis: What do we think the answer will be?
Equipment: What supplies are we using?
Method: What procedure are we following?
Results: What happened and why?
Species Spotlight: Purple Starfish
The starfish Robert Paine studied were purple starfish (Pisaster ochraceous). They are five-armed invertebrates that are ruthless, carnivorous predators. They eat snails, barnacles, mussels, limpets, and even crabs. Yet starfish do not have teeth or jaws. To eat, the starfish pries open shells with its arms. Then, the starfish pushes its stomach out of its mouth. Not only that, it pushes its stomach into the prey’s shell. It digests its prey before swallowing. Wow!
If this isn’t weird enough, starfish can regenerate an arm if they lose one. This is important because they use those arms for eating and to move around. Also, starfish do not have a brain. Instead, they have a simple nervous system. It connects with nerves that extend down each arm and signals them to move.
OTHER KEYSTONE SPECIES
After Paine made his great discovery, other biologists conducted more research. They found that keystone species are part of all major ecosystems. Forests, deserts, oceans, tundra , grasslands, and wetlands all have keystone species. They are also found all over the world at every level of food chains.
Use the American Museum of Natural History’s website to learn more about biodiversity and why it’s important. What different kinds of species live in your area?
AMNH what is biodiversity
KEYSTONE SPECIES
WORDS TO KNOW
ecosystem engineer: a species that greatly alters an ecosystem by creating, modifying, maintaining, or destroying it.
mutualists: two or more species in an ecosystem that interact in such a way that both benefit.
foundational species: a species that provides the base on which an entire ecosystem is built.
hyperkeystone species: a species that can have an ecological impact on the entire planet.
conservationist: a person who works to preserve nature.
Scientists further identified several types of keystone species. Predators are one type. Other types of keystone species are ecosystem engineers , mutualists , herbivores, and foundational species. Plants can even be keystone species!
Some keystone species are huge, such as elephants. And some are tiny. It might be hard to believe that tiny organisms can affect entire ecosystems. But they do! Some of the tiniest keystone species are bees (which we’ll talk about more in Chapter 6) and krill.
Krill are some of the smallest ocean animals. But they are the base of the marine food chain. They are an important source of food for some of the ocean’s largest animals, such as whales. Without krill, many marine animals would be very hungry.
Paine’s next study with fellow scientist Jim Estes found that sea otters are another keystone species.
Throughout the book, we’ll also take a look at the role of one other species: humans. Many years after his experiment in the tidepools, Paine considered the role of humans in the world. He saw how humans directly and indirectly affect all ecosystems and species. Thus he coined another term: hyperkeystone species.
Essential Questions
Each chapter of this book begins with an essential question to help guide your exploration of keystone species. Keep the question in your mind as you read the chapter. At the end of each chapter, use your science journal to record your thoughts and answers.
ESSENTIAL QUESTION
Why is it important to challenge scientific assumptions?
WHAT ARE KEYSTONE SPECIES?
As a hyperkeystone species, humans have a huge ecological impact that is felt everywhere on the planet. Can you think of some ways humans change the natural world?
In this book, you’ll meet keystone species of all kinds and discover the remarkable roles they play in their ecosystems. You’ll learn about threats to these species. You’ll also meet the scientists and conservationists working to protect them. Prepare to be amazed!
TEXT TO WORLD
What are some keystone species where you live?
Tropic of Cancer
Equator
Tropic of Capricorn
Tropical Rainforest Mountain
Deser t
Deciduous Forest
Tundra
Coniferous Forest Tropical Savanna
Temperate Grassland
NORTH AMERIC
SOUTH AMERICA
ATLANTIC OCEAN
INDIAN OCEAN
AFRICA
PACIFIC OCEAN
AUSTRALIA
ASIA
ARC TIC OCEAN
CENTRAL AMERICA
SOUTHERN OCEAN
Nor th Pole
South Pole
Explore the world map to discover where Earth’s major biomes are located. Notice how one biome rolls into another.
MAKE A SPECIES MAP
IDEAS FOR SUPPLIES
• note cards
• writing utensil
• research materials (books or internet)
You will meet a lot of scientists and conservationists in this book. Their work can include creating maps that show the species living in an area. This information tells them which animals or plants are found in different places and the number of each species. It also helps them determine places that need to be protected. Creating your own species map will reveal the species that live near you.
› Select a natural area you want to map. This could be a yard, garden, park, or forest. Pick an area that’s the size of a basketball court or smaller.
› Draw a map of the area. Include all important features of the area, such as buildings, sidewalks, and streets. Add a title, map key, and compass.
› Start your observation. You may need to sit quietly and wait for animals. Or you may want to walk around. Binoculars and a magnifying glass can be helpful. When looking for insects, be sure to get up close to plants and trees. Look under rocks and logs.
› Add all the species you see to your map. Include plants, flowers, bushes. Add birds, insects, and other animals. Put the species on the map in the location you observed them in. If you see more than one of certain species, record it.
› Trees and flowers can be tricky! Ask an adult to help you identify any species you don’t recognize.
» How many different species did you observe?
» Was there one part of your map that had more species?
» Did you observe anything that surprised you?
» What did you learn about your area?
Consider This!
Using the information you collected, go online and research the species you observed. Are these species native to your area? Are any of them rare? Are any of them migrating?
Consider doing your species map at a different time of year. Predict which animals you will see again and which you might not see.
Species Spotlight: Saguaro Cacti
The saguaro cactus is a plant. It is also a keystone species. These giants are found in the arid, rocky Sonoran Desert that spans parts of Arizona, California, and Mexico. They can be as tall as 50 feet and 10 feet around. Saguaro cacti live up to 200 years. During their lifetime, these cacti are part hotel, part buffet, and part water storage tank. They provide food and shelter for dozens of animals and insects. Birds such as Gila woodpeckers dig nest holes into the soft, fleshy part of the cactus. After they’ve moved out, other birds will move in and use the nest holes!
When the saguaro’s flowers bloom, they provide nectar and pollen for bats, birds, and insects. The fruit that ripens provides nutrients and moisture for desert animals. In especially dry times, some animals such as jackrabbits will even eat the flesh of the cactus for moisture.
WORDS TO KNOW
migrate: to move from one environment to another when seasons change. arid: very dry, receiving little rain. nutrients: substances in food, water, and soil that living things need to live and grow.
