Leopard seals are top predators in the Antarctic; the only animal that preys upon them is the killer whale. Leopard seals favorite food is penguin, which comprises 87 percent of their diet. Under normal circumstances, a balance is maintained between penguin and leopard seal populations, so that penguin populations remain steady despite the numbers that are eaten by leopard seals. As with any ecosystem, however, factors could arise to tip the balance and cause the seals, the penguins, or both to reduce in numbers.

The Crittercam leopard seal video describes the delicate relationship between leopard seals and penguins. Students will consider the factors that could contribute to the disruption of this balance. They will brainstorm their ideas about this topic, but feel free to provide them with hints or add your own ideas. The primary goals of the lesson are for students to learn some basic information about leopard seals and the Antarctic ecosystem and to think carefully about the fragility of the ecological balance. This concept can naturally be extended to any ecosystem. Students will conclude the lesson by drawing diagrams of the Antarctic food web and factors that could potentially change its structure and by writing paragraphs describing their drawings.

Geography, life sciences

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

One to two hours

• Computer with Internet access
• Writing materials, including blank paper for students to draw diagrams (large white construction paper is ideal)

Students will

• discuss the meaning of the term "ecological balance";
• read and discuss a Web page about leopard seals;
• discuss reasons why an ecological balance is maintained between leopard seals and penguins;
• list factors that might jeopardize this balance;
• draw diagrams showing leopard seals, penguins, and other members of the Antarctic food web and including factors that might affect the ecological balance of this food web; and
• write paragraphs describing their drawings.

Write the term "ecological balance" on the board and ask students what they think this term means. Have they learned about balance between parts of an ecosystem in previous lessons or classes? Perhaps they have studied the tropical rainforest and the balance between the trees and other plants, the birds, and the other animals that live there. If too many trees are cut down, the habitat for birds and other arboreal animals is diminished, which in turn affects other inhabitants of the forest.

Explain that in a few moments students will be focusing on the balance between two important types of animals in the Antarctic ecosystem.

Introduce students to leopard seals by having them read the text at the Australian Antarctic Division Web page. You might want to clarify the following points for the class before they read:

• Sub-Antarctic islands are a little bit north of the Antarctic continent.
• Tasmania is the island just south of Australia (students can find it on a map).
• Taronga Park Zoo is in Sydney, Australia.

After the class has read the Web page, discuss these questions. Students may return to the text to find some of the answers:

• What type of habitat do leopard seals prefer?
• What do they eat, in addition to penguins?
• How do leopard seals capture their prey?
• Do leopard seals live in packs or alone?
• Do they have any predators? If so, who eats the leopard seal?

Explain that penguins (the leopard seal's favorite food) live on the ice but must go into the water to feed on fish. It is at this ice/water boundary where they are in the most danger from leopard seals. The seals frequently wait for penguins in the water. Students can see pictures of penguins near the water's edge and jumping into the water at the Antarctic Photographic Archive.

Describe these additional facts about leopard seal hunting strategies to the class:

• Penguins comprise 87 percent of a leopard seal's diet.
• Leopard seals hide under the ice edge when they hunt.
• They take short, shallow dives before coming up to breathe.
• Leopard seals hunt alone.
• A single seal tends to stay around the same area and penguin colony rather than wandering throughout the Antarctic seas.

As students might suspect, leopard seals and penguins maintain a balance in which, barring any problems, neither population declines in number. Ask students to think about why this would be the case and discuss these questions as a class:

• A leopard seal eats an average of five penguins each day. At this rate, why don't penguin populations decline?
• On the other hand, penguins are hard to catch, requiring the seals to work hard for their prey. Could this create problems for leopard seal populations?

Ask students to work in groups or as a class to list as many factors as they can think of that might jeopardize the ecological balance between leopard seals and penguins. For example, they might realize that if fish supplies dwindled, penguins would have a hard time surviving, causing trouble for leopard seals. Other examples include factors that might prevent penguins from making it to the water (such as the development of huge ice barriers); disease epidemics in either species; or a decline in the populations of squid or other leopard seal prey.

Discuss students' ideas about factors that might alter the balance between leopard seal and penguin populations.

Have students draw food webs showing the relationship between leopard seals, penguins, fish, squid, and other Antarctic animals. They have already learned about leopard seal diets in this lesson, but you might want them to research the diets of penguins and their fish prey to find out specific species names. It would be ideal if students could draw their diagrams on large pieces of construction paper.

After they have drawn the food webs, ask students to add to their drawings some of the factors that could contribute to a disruption of the balance between leopard seal and penguin populations.

Have students write paragraphs describing the potentially balance-altering factors that they have placed on their drawings.

• Have students pretend that they are going to visit Antarctica and investigate whether any of the factors that they have cited as potentially destabilizing the balance between leopard seals and penguins might actually be occurring. Ask students to write paragraphs explaining which factors they would most like to study and describing the steps that they would take to figure out if those factors are really having an impact.
  
  
• Have students read the National Geographic News article Leopard Seal Kills Scientist in Antarctica. Discuss whether students think that studying leopard seals is worth the risk of attack. Would they get close to a leopard seal if they had the chance? What precautions might they take?

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.

Australian Antarctic Division: Emperor Penguins
Australian Antarctic Division: Leopard Seals
National Geographic News: Leopard Seal Kills Scientist in Antarctica
National Geographic: Crittercam Chronicles
PBS: Nature—Under Antarctic Ice Photo Essay