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 the gray seals and harbor seals of Sable Island. They will conclude by illustrating maps of the island to show the seals' feeding behaviors and the shark's predatory activities, and they'll write paragraphs describing their maps.

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 nature;
• locate Sable Island on a map;
• view pictures of Sable Island and its seals;
• read and answer questions about the seals of Sable Island;
• discuss competition between gray and harbor seals on Sable Island;
• discuss how Crittercam helped scientists learn about competition between the seals;
• draw and illustrate maps of Sable Island; and
• write paragraphs describing what their maps show.

Write the word "competition" on the board, and ask students if they can think of examples of competition in nature. Perhaps they've learned about competition between species of a particular ecosystem for food or to avoid predators. Ask them what characteristics animals have developed to maximize their potential to successfully compete in an ecosystem.

For example, animals with sharp teeth or claws are often at an advantage over animals without such predatory or defensive assets. List students' ideas on the board.

Tell the class that they will be focusing on Sable Island, a remote island off the eastern Canadian coast where two seal species compete against each other in the same ecosystem.

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 and its seals. Explain that the island is an excellent seal habitat, particularly for gray and harbor seals.

Sable Island: An Introduction
Harbor Seals
Gray Seals

Have students read this Introduction to Sable Island.

Ask them to click on the left-hand link "Gray and Harbor Seals" and read this information. Discuss these questions as a class:

• Which seals are thriving, and which are having trouble?
• When do gray and harbor seals mate and give birth?

Have students click the left-hand link "Research" and look at the picture. Explain that this gray seal has a Crittercam on its back. While this page describes research on gray and harbor seal mating, scientists have also used Crittercam to study the seals' eating habits and to learn about how these two seal species compete with each other in their ecosystem.

Share the following information with the class. As an option, print and copy these paragraphs so students can refer to the information on their own papers.

• 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.

Have students work in groups, in pairs, or as a class to discuss and list the ways that each of the above paragraphs relates to competition between gray and harbor seals on Sable Island. Ask them to answer these questions:

• How does the presence of sharks affect competition between the seals?
• How are the seals competing for food? Which food-seeking strategy seems to be the most effective?

Discuss the ways that Crittercam could be used to study competition between the gray and harbor seals' eating behaviors. How do students think this technology has helped scientists learn about this competition?

Have students draw maps of Sable Island. Their maps should take up most of one page, but they should leave some room for the offshore areas.

Ask students to illustrate their maps with the following items:

• Representatives from both seal species
• Seals looking for and finding food (sand lances)
• A shark looking for juvenile or adult seals to eat

Have students write paragraphs describing what's going on in their map/diagrams. Their paragraphs should explain the relationships between the different species they've drawn and the ways that the two seal species compete for food and to avoid predators.

• Have students research other examples of competition in marine ecosystems, including competition involving human activities. Examples include green sea turtles and loggerhead sea turtles in Shark Bay, Australia (the subject of another Crittercam episode) and the perceived competition between people, sea lions, and other marine mammals for fish along the West Coast.
  
  Have students write paragraphs describing the situation they've researched and explaining the "prognosis" for the species involved in the competitions.
  
  
• Students can learn more about Sable Island at Sable Island: A Story of Survival or at The Seals of Sable Island.

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.

Gray Seals
Harbor Seals
National Geographic: Crittercam Chronicles
National Geographic: Xpeditions Atlas
Sable Island: An Introduction
Smithsonian National Zoological Park: Gray and Harbor Seals at Sable Island