A keystone species is a species that has a major influence on the structure of an ecosystem. Its presence impacts many other members of the ecosystem, and if its population dwindles or disappears, there can be far-reaching consequences for the ecosystem.

Tiger sharks inhabit temperate and tropical waters worldwide and are a keystone species in some ecosystems, including Shark Bay in Western Australia. Tiger sharks in Shark Bay are extremely important in regulating populations of green sea turtles and dugongs, both of which feed on the rich sea grass in this region. Tiger sharks thus indirectly keep sea grass from being overgrazed and help maintain the grass at levels necessary for the success of the ecosystem as a whole, which relies on the sea grass for much of its nourishment.

Students will learn about Tiger Sharks through Web sites such as National Geographic: Filming Tiger Sharks. Students will draw food chains and conduct a simulation to see the relationship between members of the Shark Bay ecosystem and to understand why the tiger shark is considered a keystone species. They will conclude by writing paragraphs describing these relationships.

Geography, life sciences

Standard 8: "The characteristics and spatial distribution of ecosystems on Earth's surface"

Three hours

• Computer with Internet access

Students will

• use the Internet to view pictures of animals that live in the Shark Bay ecosystem of Australia and sketch these animals;
• draw food chains that include tiger sharks;
• discuss what might happen if tiger sharks were removed from the food chain or if their populations diminished;
• do a simulation to see the relationships between members of the Shark Bay ecosystem;
• discuss the tiger shark as a keystone species;
• discuss how scientists would go about trying to determine whether other shark species are keystone species; and
• write paragraphs describing the relationships between species shown by the food chain and explaining why the tiger shark is a keystone species in this ecosystem.

Write the words "ecosystem" and "food chain" on the board and ask if students can define these concepts.

Define ecosystem as "a system made up of all the animals and plants in an ecological community (in an area)."

Define food chain as "a group of animals and plants in which each member eats the next member of the 'chain'." For example, wolves eat elk, and elk eat grass and seedlings, making up one food chain in the Yellowstone National Park ecosystem. Each ecosystem has many food chains. When they are combined, the food chains are called food webs.

As an option, have the class help to construct, briefly, a food chain of local animals and draw the food chain on the board. The chain might include foxes, squirrels, snakes, owls, grasshoppers, or any other species that live in the area.

Have students use all or some of the following Web pages to find pictures of tiger sharks, bottlenose dolphins, green sea turtles, dugongs, and sea grass. These are all animals and plants that live in Shark Bay, Western Australia. As students look at the pictures, ask them to sketch the animals on blank paper and write the animals' names next to their pictures.

National Geographic: Filming Tiger Sharks
National Geographic: Filming Tiger Sharks—Location Photos (see photos of sea grass and bottlenose dolphins)
National Geographic Postcards (click on the bottlenose dolphin)
Green Sea Turtle
BBC Nature Programmes—Wild Down Under: Shark Bay (see pictures of bottlenose dolphins and dugongs, and watch videos if you have fast computers)

Have students look at a map of Shark Bay. Explain that all of the animals they have seen pictures of are important members of the Shark Bay ecosystem.

Ask students to take out new blank pieces of paper and follow these instructions:

• Hold the page horizontally.
• Draw one box at the top of the page (the box should be no more than two inches square—you might want to show them an example).
• Draw three boxes across the middle of the page.
• Draw two boxes across the bottom of the page.

Explain to the class that they are about to draw a very simple and partial food chain of the Shark Bay ecosystem that includes tiger sharks.

Read the following facts to the class. As you read, ask students to write the names of the animals and plants you mention in the boxes where they belong. The top box will be for the top predator, and the bottom boxes will be for the lowest species on the food chain.

• Tiger sharks eat almost anything, alive or dead.
• Bottlenose dolphins will abandon the rich waters where they like to feed if there are too many tiger sharks around.
• Dugongs and green sea turtles are some of the tiger shark's favorite foods.
• Dolphins eat fish.
• Dugongs and green sea turtles eat sea grass.

Once students have filled in the boxes, have them draw arrows from the lowest members to the highest members of the food chain.

Discuss students' food chains. Who eats whom? What is the top predator in Shark Bay? What might happen if this predator were not there any more or if its numbers declined?

Conduct a simulation to help students better understand the relationship between the members of the Shark Bay ecosystem. Give each student a slip of paper that assigns him to the role of one of the animals he has drawn on his food chain, except for the fish. There should be many fewer tiger sharks than dugongs, sea turtles, and dolphins. If you have a class of thirty students, have about four tiger sharks, six dolphins, ten turtles, and ten dugongs. Follow these directions to do the simulation:

• Designate one section of the room an area full of sea grass. Explain that the sea grass supports many animals and is very important to the ecosystem. Students will learn more about this in a few minutes.
• Have students stand up and ask the dugongs and turtles to go over to the sea grass. Explain that this is their favorite type of food.
• Ask the dolphins to stand in an area near the sea grass. Explain that they might be looking for fish to eat.
• Have the tiger sharks travel slowly over to the area where the dolphins, turtles, and dugongs are. Explain that the sharks might be hungry—what will they decide to eat today? Allow each shark to choose "prey" by selecting one of the other students to "eat." The students whom the sharks choose to eat should go to another area of the room and sit down to observe the rest of the simulation.
• Pause the simulation and explain that the dolphins are becoming alarmed at the number of tiger sharks swimming around. The dolphins are going to begin "swimming" off to another area, in deeper water away from the sea grass, to keep clear of the sharks.
• Each shark, meanwhile, should return to the sea grass and "eat" another turtle or dugong.

Ask students to describe how the tiger sharks have impacted the ecosystem. They should recognize that the sharks have altered the behavior of the bottlenose dolphins. They should also realize that the sharks, by eating green sea turtles and dugongs, are helping to keep these animals from eating too much sea grass.

Ask students what the consequences of overgrazing sea grass might be. They should understand that if the sea grass goes away, so will much of the life that it supports, including green sea turtles, dugongs, and perhaps tiger sharks. The ecosystem would be very different and much less diverse. The sea grass is therefore crucial to the health of the ecosystem, and the tiger sharks help keep the sea grass at just the right level by eating the animals that eat the sea grass.

Write the phrase "keystone species" on the board, and tell the class that tiger sharks and other animals that have a major impact on an ecosystem are called keystone species. To clarify this point, draw an arch on the board and point out the keystone. Alternately, have students look at a picture of an arch in England and explain that the keystone is the top stone where the emblem is. Explain that the keystone is the stone in the arch that carries the most weight and keeps the rest of the arch from collapsing. Likewise, a keystone species holds the rest of the ecosystem together and keeps it from dramatically changing or collapsing.

Tell the class that scientists are now trying to figure out whether other shark species, such as the bull and hammerhead sharks of Florida, are also keystone species. Ask students how they think scientists would go about figuring out whether these species are indeed keystone species. What would they need to look for? What other parts of the ecosystem would they want to study? Some good answers would be to study what the sharks eat, what their prey eat, and what would happen to the ecosystem if the sharks' prey are not eaten and their populations explode.

Have students illustrate their food chains with pictures of the animals and sea grass. They should include some pictures of the ecosystem members interacting, such as a tiger shark eating a dugong and a dugong eating sea grass.

Have students write paragraphs describing their illustrated food chains and the relationships the chains show and explaining why the tiger shark is a keystone species in this ecosystem.

Have the class create a large poster or mural illustrating the ecosystem they have learned about in this lesson. Or, have them create a poster or mural illustrating another ecosystem that contains a keystone species, such as the ecosystem of Monterey Bay, California, home to the California sea otter. If you choose to have them do this latter option, they will need to conduct research to find out why the sea otter (or another keystone species) is so important to the ecosystem.

This material is based on work supported by the National Science Foundation under Grant No. 0229817.

Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

BBC Nature Programmes—Wild Down Under: Shark Bay
Green Sea Turtle
National Geographic Postcards
National Geographic: Filming Tiger Shark—Location Photos
National Geographic: Filming Tiger Sharks