Sable Island is a remote island about 180 miles (300 kilometers) east-southeast of Halifax, Nova Scotia. Operated by a nonprofit group called the Sable Island Preservation Trust, the island has very few human inhabitants but hosts scientists who come primarily to study wildlife. In part because of its inaccessibility to humans, the island is an attractive breeding ground for gray and harbor seals, among other animal species.

The gray seals of Sable Island have been thriving, but harbor seal populations have plummeted in recent years. Scientists, including those working with National Geographic's Crittercam project, have studied the seals' mating, lactating, and feeding behaviors in an attempt to find out why this is occurring. They've also investigated the presence of sharks in the island's offshore waters to learn about patterns of shark predation on the seals.

Crittercam has helped scientists discover that both gray and harbor seals feed on the same prey—tiny fish called sand lances. Their feeding behaviors, however, are different. While harbor seals follow schools of sand lances and eat fish that have strayed from the school, gray seals dig into the sandy ocean floor to find fish hiding there.

In this lesson, students will discuss the concept of competition in nature and will learn about competition between these two seal species on Sable Island. They'll conclude by drawing food chains showing sharks, seals, and sand lances and writing paragraphs explaining what their food chains show.

Geography, life sciences

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

Two hours

• Computer with Internet access

Students will

• discuss examples of competition in their lives and in nature;
• locate Sable Island on a map;
• view and discuss pictures of Sable Island and its seals;
• work in groups to determine how gray and harbor seals compete with each other on Sable Island;
• discuss what might happen to Sable Island's harbor seals;
• draw food chains of the Sable Island species they've learned about; and
• write paragraphs describing what they have learned about competition and what their food chains show.

Write the word "competition" on the board, and ask students to define it in their own words. They might think of examples from their own lives, such as competition between students, siblings, or sports teams.

Ask students if they can think of examples of competition between animals in nature. For example, what if two species of animal live in the same habitat, eat the same food, and like to sleep in the same type of shelter? Wouldn't they be competing for the same resources? What factors might determine which animal gets the better end of the deal? Can students think of specific examples they have learned about?

Explain that Sable Island, a remote island off the eastern coast of Canada, is one location where animals compete with each other for the same food resources. They also compete to avoid the same predator. Two of these animals, the gray seal and the harbor seal, at one time both had successful populations on Sable Island. Now, however, the gray seals are thriving while harbor seal populations are plummeting.

Have students go to National Geographic's Xpeditions Atlas. Ask them to select North America, Canada, and Nova Scotia. On the Nova Scotia map, they can find Sable Island to the southeast of Halifax. Have them figure out how far Sable Island is from Halifax.

Have students look at these pictures of Sable Island. Discuss the island's landscape as a class. What words come to their minds when they see these pictures? Do they think this looks like a good place for people to live? For wildlife? For marine animals such as seals?

Have students look at these two pictures of seals on Sable Island, and explain that the island provides an excellent habitat for seals, particularly the gray seal and the harbor seal.

Harbor Seals
Gray Seals

Have students look at pictures of great white sharks and sand lances at these Web pages:

National Geographic: Creature Feature—Great White Sharks
Puget Sound Shorelines: Sand Lance

Print and copy the following information about the current state of affairs for gray and harbor seals on Sable Island, and give the handouts to students. Have students take turns reading it out loud.

• Gray seals are doing very well on Sable Island. Harbor seals, however, are not doing as well. They seem to be having trouble reproducing and keeping their young pups alive. Only two to ten harbor seal pups are born on the island each year.
• Sharks, including the great white shark, like to eat both species of seal. Harbor seal pups and adults are smaller than gray seal pups and adults and are therefore easier for the sharks to catch and eat.
• Gray seals and harbor seals both eat sand lances, tiny fish that live just off the shores of Sable Island. To catch these fish, gray seals swim close to the ocean floor and dig their snouts into the sand to find hiding fish. Harbor seals, on the other hand, follow schools of sand lances and catch fish that wander from the school.

Divide the class into small groups. Assign each group to either the second or third paragraph above, making sure both paragraphs are distributed as evenly as possible throughout the classroom.

Focus students' attention once again on the word "competition" on the board. Ask groups to discuss the answer to the question "What does competition have to do with the facts you've read in this paragraph?" and have them list their ideas.

Discuss groups' lists as a class. In what ways is shark predation related to competition between these two seal species? In what ways is the seals' feeding behavior a sign of competition? In short, how are these two species competing on Sable Island?

Discuss what might happen in the future to the seals of Sable Island. Do students think there's any hope for the harbor seals here? Why or why not? Do they think the gray seals will continue to be more successful than the harbor seals? Why or why not?

Have students draw simple food chains showing the relationship between sharks, the two seal species, and sand lances.

Have them write paragraphs describing the relationships shown on their food chains, explaining what's happening to harbor seal populations, and predicting what will happen to both seal populations in the future.

Have students learn more about Sable Island at Sable Island: A Story of Survival. This site discusses aspects of Sable Island besides seals. Students can combine this information with the facts they've learned about Sable Island's seals to create multimedia presentations or posters showcasing the island's interesting features.

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.

National Geographic: Creature Feature—Great White Sharks
National Geographic: Crittercam Chronicles
National Geographic: Xpeditions Atlas
Picture of Gray Seals
Picture of Harbor Seals
Puget Sound Shorelines: Sand Lance
Sable Island: An Introduction