Note: Please see the "For Teachers" tab for downloadable handouts.
Keystone species are those that, although small in numbers, play a large role in maintaining the balance between predator and prey in an ecosystem.
Children will be able to describe how animals and plants in an ecosystem rely on each other for food and shelter.
Children will understand the role of a keystone species within an ecosystem.
Cyberchase Video: "An Urchin Matter"
Grades: 3rd - 5th grade
Index cards (one per student)
Name tags (one per student)
Ball of yarn
"Story of the Gray Wolf" handout, one per child (download from the "For Teachers" tab)
Divide the name tags into 5 piles, depending on how many children are in your class. One pile should contain more name tags than the others, and one pile should contain only 2 or 3 name tags.
Write one species on each pile of name tags: kelp, fish, birds, sea otters, and sea urchins. Write kelp on the larger pile of name tags and sea otters on the pile only containing 2 or 3 name tags.
Cue up the provided video "An Urchin Matter."
Before Watching the Video:
Explain that the plants and animals that live in an ecosystem are connected and rely on each other for survival. Sometimes, one particular species is so important that, if removed, its absence will have a big impact on the entire ecosystem. A living thing this important is referred to as a "keystone species."
Watch the Video:
Something has gone terribly wrong in Big Bay. The kelp plants that live there are dying. In this video, the CyberSquad journeys to a nearby bay to find clues to the interrelationships among bay species. Who is eating what, and with what result?
After Watching the Video:
Give each child a name tag with one of the following written on it: kelp, fish, birds, sea otters, or sea urchins.
Have students stick their name tag onto their shirts and stand in a circle. Give the ball of yarn to a student.
Explain that each name tag represents something that lives in (or flies above) the bay. The students’ job is to explain how each living thing connects to another.
Provide several examples. Ask students how a sea urchin and an otter are connected. (The otter eats the sea urchin.) Identify a question that’s more difficult: How are an otter and kelp connected? (The otter eats the sea urchin, which eats the kelp.)
Explain how the game is played. One person holds onto the end of the yarn, tosses the ball of yarn to another person, and explains the connection. (Demonstrate.) Then that person holds on to the yarn and throws the ball to the next person and explains the connection, and so on.
Continue to do this until every child is holding a portion of the yarn. The circle of children should show a web of yarn as each species interacts with each other.
Once each child is holding a portion of the yarn, ask the students to take a step back to tighten the web.
Describe a scenario in which the sea otters leave the ecosystem. For example, from the mid-1700’s to the early 1900’s, they were hunted for their fur.
The children wearing the “sea otter” name tags should drop the yarn and step out of the circle.
Have the “sea otter” students trace the dropped portion of the yarn back to the other students and explain how the absence of sea otters will affect the other animals.
Ask students: “With the sea otters gone, what will happen to the other animals and plants?” (Ask them to consider separately the effect on each species in the ecosystem.)
Ask students: “What evidence can you provide to show that sea otters are a keystone species?”
Remind students that the ocean is only one kind of ecosystem. In the next activity, they’ll learn about a keystone species that lives in a different ecosystem – the forests of Yellowstone National Park. The keystone species is the gray wolf. The students’ mission is to find out what happened when the gray wolf disappeared.
Divide the students into small groups, and distribute a copy of “The Story of the Gray Wolf” to each student.
Have students read paragraphs 1 - 5 and record the following information:
What happened to the elk after the gray wolf disappeared?
What happened to the aspen trees after the gray wolf disappeared?
Discuss students’ answers to these questions. Then ask students to predict what might happen if the wolves returned to Yellowstone Park. Record their responses.
Ask students to read paragraphs 6 - 8 to see if their predictions were correct. Discuss.
In order to check students’ mastery of the objectives and summarize their learning, you can ask them to complete a short written task at the conclusion of the lesson, often called an "exit pass."
Distribute index cards to students, and ask them to define a keystone species, then write a sentence that provides evidence that the sea otter or the gray wolf is a keystone species.
Collect the index cards as students leave the room or as you move on to the next subject.
The kelp is struggling to survive in Big Bay--but it's thriving in Little Bay. So, Digit goes diving in Little Bay to try to uncover why there's a difference in the bays. What does he discover as soon as he hits the ocean floor?
Digit has to compare the species numbers in Big Bay to Little Bay. What does he use to collect his sample? What were the results of the comparison?
What does the Cybersquad discover about sea otters? How are they important and how do they impact the overall health of the bays?Use specific information from the video to support your answer.
Assign directly to your students using the code or link above, without having them log in. Simply tell your students to go to
www.pbsstudents.org and enter the Assignment Code, or click on the Assignment URL to share the assignment as a link.