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Activity Guide
Predators: Aquatic Connections


  • Select one part of the overall activity to conduct during the field trip. The two remaining parts can be done as pre- or post-visit activities.


In this series of interrelated activities, students participate in simulations that demonstrate three ecological concepts: carrying capacity, food webs, and the role of top predators.


Students will:

  • Explain how the availability of food may be a limiting factor for herbivores within an ecosystem.
  • Identify feeding relationships and the transfer of energy among plants and animals.
  • Discuss human activities that affect the food web in an aquatic environment.


Science, Mathematics


carnivore, carrying capacity, food chain, food web, herbivore, limiting factor, omnivore, phytoplankton, predator, prey, watershed, zooplankton


3 sessions (one session per part)


  • Tokens (poker chips, beans, pennies or other non-destructible small items, approximately 15 per student)
  • Small plastic bag or cup (one per student)
  • Colored cloth armbands (three different colors are required, enough of each color for one-third the number of students, plus enough of the third color for half the number of students)
  • Copies of handouts: Aquatic Connections Worksheet (one per student), optional; Data Sheets including: Data Sheet, Part 1 (one copy for instructor, plus one copy of completed worksheet per student optional); Data Sheet, Part 2 (1 copy for instructor plus one copy of completed sheet per student, optional); and Data Sheet, Part 3 (one copy for instructor)
  • Clipboard
  • Pencil
  • Whistle


This activity supports the following National Academy of Sciences Science Education Standards (Grades 5-8):

  • Unifying Concepts and Processes—Systems, order, and organization
  • Unifying Concepts and Processes—Evidence, models, and explanation
  • Standard A: Science as Inquiry—Abilities necessary to do scientific inquiry
  • Standard C: Life Science—Regulation and behavior
  • Standard C: Life Science—Populations and ecosystems
  • Standard F: Science in Personal and Social Perspectives—Populations, resources, and environments


The transfer of food energy from one species to the next links the components of every ecosystem. This is called the food chain or, more accurately, the food web, since the connections are complex and varied. Producers, or plants and some kinds of bacteria that convert the sun’s energy to food, form the first level of the food web. They make the food that supports the other species in the web. Consumers must eat other living things to obtain food energy. There are three kinds of consumers: those that eat plants are called herbivores (Latin for “grass-eating”), those that only eat other animals are called carnivores (Latin for “meat-eating”), and those that eat both plants and animals are called omnivores (Latin for “all-eating”).

A predator is a carnivore and usually refers to animals that catch and kill their food. The animals they eat are called prey. A given animal might be a predator of one species and the prey of another.

At each stage of the transfer, food energy is used for many different things. Some is used for growth; some is stored as fat or oil for later use as food. Some is used for reproduction. Some goes undigested and is lost in feces or the bodies of dead organisms. Much of the food that an animal eats is broken down in a process called respiration which takes place inside body cells.

Human activities may affect food webs in many ways. Pollution and habitat destruction cause obvious changes that can be easy to see. However, more subtle changes may result from extensive harvesting or removal of specific levels of the food web. When some fish are selectively removed, it may have significant impact on other animals and on the plants in a watershed. Very careful planning is necessary to be able to either remove or add animals to a system without disturbing the overall functioning of the entire system.


This exercise works best in an open space. Plan the location for the activity. If you are planning to conduct the activity outside, make sure you have an alternate site in case of rain.


Part I: Carrying Capacity—The relationship between food availability and the number of herbivores an area can support.

1. Review the terms zooplankton and phytoplankton. Zooplankton includes herbivores that feed on the tiny phytoplankton (plants) that drift through the surface water in an aquatic environment. What happens when herbivores have to compete for food? Have students predict what might happen if the herbivores cannot get enough to eat. (Answer: They might starve and die or may not be able to reproduce.)

2. In this exercise, the playing field is a “pond” that produces a limited number of phytoplankton and has only a few zooplankton. The tokens (beans or poker chips) represent phytoplankton. The food supply stays the same from one generation to the next.

3. Round 1: Designate two-thirds of the class to act as zooplankton. The rest of the class is reserved for the next generation. Give each zooplankton a plastic bag to collect phytoplankton.

4. Scatter 10 food items (tokens) per zooplankton. Tell students that they are to try to eat (collect) as much as they can without encroaching on another zooplankton’s food supply. Begin the food search and let students pick up all the food. It will be over pretty fast.

5. Have everyone sit down. Did they all get the same amount of food? (Answer: No, some individuals are more efficient searchers than others.) Record the results on the instructor’s Data Sheet for Part 1 , using this guide to the outcomes:

Note that some of the zooplankton did not get enough phytoplankton to reproduce while others did. Those that got more food leave more offspring. Some could not survive with the limited amount of phytoplankton.

6. Round 2: After reproducing in the first round, all the parents die, leaving behind the number of offspring indicated on the data sheet. Change the number of players to match the number of offspring and repeat the game. Recruit from the reserve and allow substitutions for tired zooplankton. Scatter the same number of food items as used in the first round regardless of whether the number of players went up or down. (Assume the same number of phytoplankton will be produced.) The amount of food is limited.

7. Record the results for Round 2 on the data sheet. Continue to repeat the game for four or five rounds, changing the number of zooplankton and recording the results each time. You should find that as long as the food supply remains the same, approximately the same number of animals is produced in each generation.

8. Return to classroom and post the simulation results on the board or distribute copies of the completed data sheet. Discuss what happened during the simulation and the results of the worksheet. Consider the following questions:

a. Did all the zooplankton get enough to eat? (Answer: No. Even though there were food items enough for everyone to survive, some were better at competing for food than others.)

b. What happened to those that got more food? (Answer: They used their extra food to reproduce. The best competitors had the most offspring.)

c. Did anyone find a particular trick to help them compete more successfully for food? If this trick or adaptation was one that was inherited, would their offspring also be better at finding food? Would more animals in the next generation be better at catching food? (Answer: Yes. Those that got the most food left the most offspring. At least some are likely to inherit the food-finding adaptation.)

9. Review and discuss the results. Note that animals compete for food. Those that do not get enough to eat die or are caught by predators because they are weak or diseased. Those that compete most successfully leave more offspring. If the limit to the number of herbivores in an area is the food supply and that food supply remains relatively constant, the number of herbivores remains more or less the same from one reproductive period to the next. This average number of animals is the carrying capacity for that habitat.

10. To reinforce the interpretation of the data and practice math and graphing skills, distribute the Aquatic Connections Worksheet and ask students to complete the questions. (Optional)

Part II: Predator-Prey—The feeding relationships among animals that live in an ecosystem.

1. In this part of the activity, the class is going to represent the animals in a pond ecosystem. Have students name some animals that might live in a pond. Possible answers include large and small fish, frogs, crayfish, tiny zooplankton, insects, beavers, and raccoons. You might want to have pictures of pond animals on hand to show the class. Explain that this simulation is going to use groups of animals based on predator-prey relationships.

2. In the pond, where does the food come from? (Answer: From plants and algae that use the sun’s light to photosynthesize. Some of these organisms are tiny, free-floating phytoplankton while others are rooted green plants that grow under the water or along the edge of the pond.) Write the words phytoplankton and green plants at the bottom of the board. Introduce the word producers for those species that make their own food.

3. Who might eat these plants and algae? The tiny animals called zooplankton eat phytoplankton as the students learned in the previous activity section. Many insects, crayfish, and beavers are among the pond animals that feed on plants. Write insects, crayfish, beavers, and zooplankton above the plants on the board and draw an arrow up to them. Explain you are drawing a diagram of the path food takes in the pond. Animals that eat plants and algae are called herbivores.

4. Who eats the herbivores? (Answer: Small fish, frogs, and some bigger fish) Add them to the next level along with an arrow. Animals that eat other animals are called carnivores. The animals that are eaten are called prey.

5. Finally, who eats the carnivores? (Answer: The big fish and the raccoon) These are the top carnivores. They do not just eat the level below them; they also eat animals from the lower levels. Add the top carnivores to the diagram and make sure the students see the food web or food chain forming as you draw the lines between the levels.

6. Move to the activity location. Assign one-third of the class to be zooplankton, crayfish and insects. Give them the same colored strip of cloth to wear and a plastic bag or cup. They are all herbivores.

7. Unlike in Part 1, in this simulation food is not a limiting factor. Thus, there are more plants available than the herbivores can eat. Scatter 20 food tokens (poker chips or large white beans) for each herbivore (that is, 20 times one-third the number of students). The tokens represent plants in the pond. The herbivores must get their food by picking up the tokens. To survive and reproduce, they must get 10 tokens before the end of the game. If they do not get 10 pieces, they die of starvation.

8. Give another one-third of the class a second color armband and a plastic bag or cup. They are the carnivores, the frogs and small fish. To eat, they must tag an herbivore. Tagged herbivores give their food bags to the carnivores and sit down, as they have been “eaten” by the predator and are out of the game. The carnivores must collect 20 pieces of food from the herbivore food bags to survive and reproduce at the end of the game. They must stop eating (tagging herbivores) when they have 20 pieces. If they do not get 20 pieces, they die of starvation.

9. The remaining students are the top carnivores. They get a third color armband and a plastic bag or cup. They feed by tagging either the herbivores or the carnivores, who give up their bags and sit down when tagged. The top carnivores require 40 pieces of food to survive and reproduce at the end of the game. They must stop eating when they pass 40 food items.

10. Caution the students about rowdy behavior and running into each other. Give the students five minutes to find food then stop the game. Record the results on the Data Sheet, Part 2 .

11. Briefly discuss what happened:

a. What was the cause of death in most cases for the herbivores? For the carnivores?

b. Were the proportions of herbivores and carnivores the same at the end as at the beginning (indicating the ecosystem has its various feeding levels intact)?

c. Do the students think this worked like a real pond ecosystem? Why or why not? How might the proportions in a real ecosystem differ from the game?

(Note: The proportions in the first game (Part I) were intentionally wrong. They do not represent a working system and were chosen precisely because they do not work. The students should be able to see the problem. The top carnivores are going to eat all their prey. With no herbivores left, the top carnivores will starve to death. If all the herbivores are eaten, they will not leave any offspring, and the rest of the levels will be affected in the future. The second game should provide information to develop inferences about more appropriate numbers at different levels.)

12. Change the number of students in each level. Assign one-half of the class members to be herbivores, four or five students to be top carnivores and remaining students to be carnivores. Scatter 20 food tokens for each herbivore. Redistribute the colored armbands and repeat the game for five minutes. Record the results on the data sheet.

Alternatively, you can allow students to make one change, other than changing the amount of phytoplankton, which should remain constant at 20 pieces per herbivore. You can give them some possibilities, such as:

a. changing the number of herbivores

b. changing the number of top carnivores

c. giving herbivores a safe refuge where they can hide between running out to gather food tokens (keeping in mind that they must gather 10 pieces of food to avoid starvation), or

d. adding 30-second safe times when the top carnivores rest and stop hunting.

Students should select one change at a time so results from each round can be attributed to a specific variable. Run multiple rounds, changing one variable at a time and analyzing the results. The ultimate goal is to have some individuals from each level remaining at the end of the simulation so that all levels can reproduce.

13. Return to the classroom. Post the results from the data sheet on the board or distribute copies of the completed data sheet. Compare the rounds of play and discuss. In order for the food web to be realistic, the students must have some individuals from each level remaining at the end of the simulation. These animals will be the ones that reproduce, creating the next generation. Ecosystems normally support very few top predators (perhaps one), a few carnivores and many herbivores. This exercise illustrates that there are fewer animals in each succeeding level of the food web.

Part III: Human Impact—Human activities that change the feeding relationships, and thus the ecological structure of a food web.

1. Part 3 of this activity combines the effects of competition and predation from the prior two sections and explores the impact humans can have on the workings of an ecosystem food web. Ask students to name some ways people might affect the animals in the pond food web. (Answer: Pollution, habitat destruction, removal of selective levels of the food web) Some of the impacts are obvious; it is easy to understand the consequences of filling a pond or marsh in order to build a shopping center or add a field to a farm. More subtle changes may result from removing too many fish (or other species) from a habitat and altering the structure of the entire ecosystem.

2. Ask students to identify the food web of the pond model from the previous simulation. Draw the food web on the board as students name the plants and animals. Which level of the food web is the most likely to be affected by humans? Since people like to catch big fish rather than tiny minnows, big fish are the likely candidates for human removal. Students have experienced an animal (like a raccoon) being killed by an automobile. Accidents often occur when people and animals share the same area. Use the fish and the raccoon examples for human disturbance of the pond food web.

3. Replay the simulation as in the first round of Part 2. This first round provides a baseline to see what happens before humans are involved. People will be added later. Assign three-fourths of the students to act as herbivores—the zooplankton, insects, beavers and crayfish. Make two students top carnivores—the big fish and raccoon. The remaining students will be carnivores that feed on the herbivores—the frogs and small fish. Distribute the colored flags for each feeding level and the plastic bags or cups to hold the food they eat.

4. Scatter 10 food items per herbivore. Each herbivore needs 10 to survive and reproduce. Each carnivore needs 20 pieces and each top carnivore needs 40 to survive and reproduce. When any animal has enough food, it can go to hide in a safe place. When an animal has been tagged by its predator, it has been “eaten” and must give up its food and sit down. Allow the students to play the game until everyone has either gotten enough to eat, run out of food to eat or been eaten. Record the results on the Data Sheet, Part 3. Was this system in reasonable condition? That is, did some members of each level survive?

5. Now repeat with human interaction. A motorist has killed the raccoon and an angler caught the big fish. The two top carnivores must sit out. Have the students predict the outcome. Play the game again and record the results.

6. What happened to the structure? Did more of the herbivores get eaten? What would happen in the next generation? Ask the students if there is a direct consequence of removing the top carnivores on the structure of the food web. (Answer: The secondary consumers are now able to eat almost all the herbivores, leaving few to reproduce in the following year. With the herbivores gone, the carnivores will starve in the following years.)

7. Discuss the conclusions. The feeding levels of natural food webs are generally in tune with one another over a period of years. When all of one level is removed, the other levels will be affected. Any human harvesting of natural populations must take into account the effect on the other levels of the food web.


Ask students to answer the following in writing:

  • Identify at least two limiting factors in a habitat. Explain how a limiting factor determines the carrying capacity of an area.
  • Explain the predator-prey relationship and how it plays a role in providing structure to the food web.
  • Discuss the importance of top carnivores. Explain the role that wildlife management plays in helping maintain the structure of the food web.

Needs Improvement—Student names one or no limiting factors. Explanations of carrying capacity, the role of predator-prey relationships in providing structure to food webs, the importance of top carnivores, and the role of wildlife management in maintaining food web structure are vague or incorrect.

Satisfactory—Student identifies two examples of limiting factors. At least two explanations (of carrying capacity, the role of predator-prey relationships in providing structure to food webs, the importance of top carnivores, and the role of wildlife management in maintaining food web structure) are complete and correct.

Excellent—Students identifies three or more examples of limiting factors. Explanations of three of four concepts are correct and detailed.


  • List types of wildlife that live in a pond ecosystem near your community. Draw a food web for this wildlife population. Determine how human activities influence this food web.
  • Research the strategies used by predators to catch their prey while avoiding being caught themselves.
  • Research your state fishing and hunting regulations. These management practices should be based on an understanding of food webs and ecological structure.


Food Chains and Food Webs, an interactive food web Web site sponsored by Parenting the Next Generation www.vtaide.com/png/foodchains.htm

For an interesting look at the importance of predators see Monster of God by David Quammen. Published by W. W. Norton and Company, Inc., New York, NY, www.wwnorton.com/catalog/fall04/032609.htm


From the American River Salmon Educator Activity Guide published by CA Department of Fish. Used with permission. Adapted from the Living in Water Activity Guide with permission from The National Aquarium in Baltimore, www.aqua.org .

Note to Teachers: This lesson and others relating to National Geographic’s Strange Days on Planet Earth can be found online at www.pbs.org/strangedays/

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