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.

Scientists have already determined that tiger sharks are keystone species in some ecosystems, including Shark Bay, Western Australia. Scientists are now trying to figure out whether bull and hammerhead sharks are keystone species in the Florida Keys. They are using Crittercam and satellite transmitters to learn as much as possible about the sharks' feeding, swimming, and other behaviors.

Students will first learn what a keystone species is, and then they will draw a simple food chain to see why tiger sharks are considered a keystone species in Shark Bay. Next, they will turn their attention to hammerheads, reading about the hammerhead diet and the reasons sharks (including hammerheads) are threatened by fishing practices. They will consider how scientists might go about trying to determine if hammerheads are keystone species, and they will conclude by creating public education brochures or reports describing why this would be such a significant discovery.

Geography, life sciences

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

Three hours

• Computer with Internet access

Students will

• read and discuss an article defining keystone species;
• draw food chains that include tiger sharks and discuss tiger sharks' role as a keystone species;
• read a Web page describing how sharks are threatened by fishing and discuss questions about these fishing practices;
• read about and list hammerhead shark food sources;
• discuss the significance of hammerheads' role as a keystone species;
• discuss how scientists might go about trying to determine if hammerheads are keystone species; and
• prepare public education brochures or reports describing the significance of hammerheads' role as a keystone species.

Have students read In the Wild—Spotlight: Keystone Species and discuss these questions:

• What is a keystone species?
• Are keystone species always top predators? At what levels of the food chain can keystone species be found?
• What makes sea otters keystone species?

Inform the class that tiger sharks are keystone species in some of the ecosystems where they are found, including in Shark Bay, Western Australia. Have students look at a map of Shark Bay or point out its location on a wall map in the classroom.

Have students go to the following Web pages to see pictures of some animals that live in Shark Bay:

National Geographic: Filming Tiger Sharks (see the picture of the tiger shark)
National Geographic: Filming Tiger Sharks: Location Photos (see photos of sea grass and bottlenose dolphins)

Ask students to take out blank pages 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 about 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 simple and partial food chain that exists in the Shark Bay ecosystem.

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? If tiger sharks were not around to eat sea turtles and dugongs, what might happen to the sea grass population? How would this impact the ecosystem, which has many members that depend on the sea grass as a primary food source?

Students should realize that the removal of a keystone species, in this case the tiger shark, would have far-reaching implications for the entire ecosystem. In particular, they should understand that if tiger sharks were not there to eat the dugongs and sea turtles, those animals might overgraze the sea grass. If too much sea grass were eaten, much of the life the sea grass supports would be in trouble, including green sea turtles, dugongs, and, ultimately, tiger sharks (thus causing a "downward spiral"). 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 it at just the right level by eating the animals that eat the sea grass.

Explain 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. One of the reasons they are interested in studying hammerheads is that this species is declining due to fishing. People fish directly for hammerheads, and many hammerheads are also killed as "bycatch" when fishermen are really trying to catch other types of fish in their nets.

Have students research the answers to these questions:

• In what ways do fishing practices harm sharks?
• About how many sharks do people kill worldwide each year?
• Why are most of these sharks killed?

Have students go to the Great Hammerhead Biological Profile and scroll down to the section entitled "Food Habits" right under the photograph of the stingray. Ask them to read this paragraph and list the hammerhead's main food sources.

Discuss what hammerheads' possible role as a keystone species would mean for the coastal Florida ecosystem that they inhabit (and other ecosystems where they can be found). What might happen if current trends continue and hammerhead populations decline?

Also discuss how students think scientists would go about figuring out whether hammerheads 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 prepare public education brochures or reports for residents of Florida and any other interested individuals, explaining the significance of studies currently being conducted to determine whether hammerhead sharks are keystone species off the coast of Florida. Their reports should include the following components:

• a food chain that includes hammerheads;
• a paragraph explaining why it is important to find out if hammerheads are a keystone species;
• a paragraph explaining what might happen if hammerheads are indeed a keystone species and their populations continue to decline; and
• a paragraph explaining how scientists should go about investigating whether hammerheads are a keystone species.

To clarify why keystone species have this interesting name, 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.

Have students read the National Geographic News article Are People Eating Sharks Out of Existence?, at least up to the paragraph that begins "We believe there is a 90 percent probability that the large coastal shark stocks." Ask students to write paragraphs answering these questions:

• What do you think can be done to protect shark populations, considering their popularity in Asia?
• Do people in Asia have to be convinced to change their tastes, should fishermen be banned from catching sharks just for their fins, or should something else be done?"

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.

Great Hammerhead Biological Profile
In the Wild—Spotlight: Keystone Species
National Geographic: Filming Tiger Sharks