Lesson 4: Food Chains

• What organisms eat helps to define roles within an ecosystem.
• Organisms in an ecosystem interact to form complex relationships, which can be meaningfully discussed and examined using a food web.
• One path of consumption can be depicted using a food chain.

• How are organisms within the same ecosystem related to each other?
• How does what an organism eats affect its role in an ecosystem?

• Students will be able to discuss and understand that food chains can be a useful way to focus on one section of an ecosystem.
• Students will understand that food chains are one part of a food web.
• Students will be able to classify organisms based on what they eat.

NGSS:

• Disciplinary Core Ideas: LS1.C (K-2) (3-5) , LS2.A (3-5) (6-8), LS4.C (K-2)
• Science and Engineering Practices: 1, 2, 6, 7
• Crosscutting Concepts: 1, 2, 4, 5, 7

CCSS: ELA/Literacy:

• Writing: W.2.K, W.2.1
• Speaking and Listening: SL.1.K, SL.3.K, SL.5.K, SL.6.K, SL.1., SL.3.1, SL.5.1, SL.6.1, SL.1.2, SL.3.2, SL.5.2, SL.6.2, SL.1.3, SL.3.3, SL.6.3, SL.1.4, SL.3.4, SL.6.4, SL.1.5, SL.3.5, SL.6.5, SL.1.6, SL2.6, SL.5.6, SL.1.7, SL2.7, SL.5.7, SL.1.8, SL2.8, SL.5.8

National Geography Standards: 2

Organisms living in an ecosystem are all connected and intertwined. Some organisms work together, with shared benefits (like honey bees aiding pollination of some flowers). More common, organisms rely on each other as a food source. A food chain shows how each living thing gets its food. Some animals eat plants and some animals eat other animals. For example, a simple food chain links the shrubs, the rabbits that eat shrubs, and the bears that eat the rabbits. Each link in this chain is food for the next link. All food chains start with energy from the sun. This energy is captured by plants. Thus the living part of a food chain always starts with plant life and ends with an animal.

Plants are called producers because they are able to use light energy from the sun to produce food (sugar) from carbon dioxide and water. Animals cannot make their own food, so they must eat plants and/or other animals. They are called consumers. There are three groups of consumers: primary, secondary, and tertiary. Animals that eat ONLY PLANTS are called herbivores and are primary consumers. Animals that eat OTHER ANIMALS are called carnivores. Carnivores that eat herbivores are called secondary consumers, and carnivores that eat other carnivores are calledtertiary consumers. Animals—including people—that eat BOTH animals and plants are called omnivores. Then there are decomposers (bacteria, fungi, and even some worms), which feed on decaying matter. These decomposers speed up the decaying process, which releases mineral salts backinto the food chain for absorption by plants as nutrients.

In a food chain, energy is passed from one link to another. When a herbivore eats, only a fraction of the energy that it gets from the plant food becomes new body mass; the rest of the energy is lost as waste or used up by the herbivore to carry out its life processes (i.e., movement, digestion, reproduction). Therefore, when the herbivore is eaten by a carnivore, it passes only a small amount of the total energy it has received to the carnivore. Of the energy transferred from the herbivore to the carnivore, some energy will be “wasted” or “used up” by the carnivore. So the carnivore has to eat many herbivores to get enough energy to grow. Because of the large amount of energy that is lost at each link in the food chain, the amount of energy that is transferred up the chain gets less and less. The higher up the food chain you go, the less food—and hence energy—remains available.

Food chains can be represented as a pyramid.

The above energy pyramid shows many trees and shrubs providing food and energy to giraffes. As we go up the food chain, there are fewer giraffes than trees and shrubs and a lot fewer lions than giraffes. This shows that as we go higher up a food chain, there are fewer and fewer consumers. In other words, a large mass of living things at the base of a food chain is required to support a few at the top. Many herbivores are needed to support a few carnivores. Most food chains have no more than four or five links. There cannot be too many links in a single food chain because the animals at the end (top) of the chain would not get enough food—and hence energy—to stay alive.

Food chains can get complicated because animals usually eat a variety of food. Most animals are part of more than one food chain and eat more than one kind of food in order to meet their food and energy requirements. When many food chains weave together, they create a food web.

• food chain: a sequence of organisms arranged in such a way that each feeds on the organism below it in the chain and serves as a source of food for the organism above it
• consumers: organisms that digest other organisms as an energy source
• herbivores: organisms that only eat plant life
• carnivores: organisms that only eat meat
• omnivores: organisms that eat both meat and plant life
• decomposers: organisms that feed on carcasses of dead organisms
• food web: all the connected or linked food chains within an ecological community

Ask students to draw a diagram of what an organism that lives nearby eats. Have students answer the following questions:

• How important is this particular organism to the ecosystem?
• What do you think would happen if this organism disappeared from the ecosystem?

Have students repeat this activity at the end of the lesson.

• Food chains are the only way to talk about predator-prey relationships.
• It is all right to focus on one food source when talking about relationships between organisms.