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Food Chains and Food Webs

Science, Grade 6

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Table Of Contents: Food Chains and Food Webs

1. Energy Flow

2.1. What Is an Ecosystem?
An ecosystem is a group of organisms and their physical environment. All organisms within an ecosystem need energy to survive.
2.2. Organisms and Energy
Organisms obtain energy from different sources. Each organism plays a role in the flow of energy through the ecosystem.
2.3. Producers
Producers make their own food through the process of photosynthesis. Plants, algae and some bacteria are producers that use the sun's energy to make food.
2.4. Consumers
Consumers get energy by eating other organisms. They are often classified by what they eat. Herbivores only eat plants. Carnivores eat animals, and omnivores eat both plants and animals.
2.5. Example of Energy Flow
Energy flows from producers to the different types of consumers. For example, plants are eaten by herbivores such as grasshoppers. A grasshopper is eaten by a mouse, which is then eaten by a rattlesnake.
2.6. Decomposers
Decomposers obtain energy by eating or absorbing nutrients found in animal waste and dead bodies. Bacteria, fungi and worms are decomposers that help return raw materials to an ecosystem.

2. Food Chains

3.1. What Is a Food Chain?
Movement of energy can be shown in a food chain. A food chain is a series of events in which one organism eats another organism to obtain energy.
3.2. Snake Food Chain
Food chains are linear and look at one path of energy flow. This is a food chain for a snake.
3.3. Trophic Levels
The levels of a food chain are referred to as trophic levels. Each level indicates one more energy-transferring step away from the sun.
3.4. Parts of a Food Chain
A food chain is made of producers and multiple levels of consumers. Top-level consumers are typically carnivores that do not have any natural enemies.

3. Pause and Interact

4.1. Review
Use the whiteboard tools to complete the activity.

4. Food Webs

5.1. What Is a Food Web?
Energy flows through an entire ecosystem in a complex network of feeding relationships called food webs. Many different food chains are found within a food web.
5.2. Producers and Herbivores
Producers are at the base of the food web. First-level consumers are primarily herbivores.
5.3. Omnivores and Carnivores
Higher-level consumers are typically omnivores and carnivores.
5.4. Decomposers
Decomposers such as worms, fungi and bacteria are also included in a food web.
5.5. Feeding from Different Trophic Levels
Many animals feed on organisms from different trophic levels. For example, the raccoon eats organisms from both the producer and consumer levels.

5. Owl Food Web

6.1. Owl Food Web Example
This food web shows a barn owl and great horned owl as the top-level consumers. Owls feed on rodents, insects, reptiles, amphibians and small birds.
6.2. Owl Pellet Formation
An owl generally swallows its prey whole. Some parts of the prey, such as bones, are indigestible and form an oval-shaped mass called a pellet in the owl’s gizzard.
6.3. Pellet Composition
The pellet is regurgitated through the owl’s mouth. A pellet may contain the remains of more than one prey if the owl has eaten multiple meals within several hours.
6.4. Examining an Owl Pellet
Scientists can learn about an owl’s diet by examining its pellets. Bones, fur and feathers provide evidence of what the bird has eaten.
6.5. Identifying Bones in an Owl Pellet
The bones can be specifically identified as part of the skeletal system of a particular animal.

6. Pause and Interact

7.1. Review
Use the whiteboard tools to complete the activity.

7. Energy Pyramid

8.1. Energy in a Food Web
Because organisms use food energy for movement, growth and daily activities, there is only a portion of energy available for the next organism in the food web.
8.2. What Is an Energy Pyramid?
An energy pyramid is a way to show the different levels of stored energy within an ecosystem. As you move up the pyramid, less energy is available.
8.3. Energy Transfer between Levels
Approximately 10% of the energy from one level is transferred to the next higher level. In this example, the top-level consumer receives only a small amount of energy that originated in the producer level.
8.4. Ecosystems and Number of Organisms
An ecosystem can only support a limited number of trophic feeding levels. There are many more organisms at the producer and first-level consumer levels than at the higher levels of the energy pyramid.

8. Food Web Balance

9.1. Changes in Population Size
If the population size of a species changes within an ecosystem, the balance of the food web can be disrupted. Over time, all levels of the food web can be affected.
9.2. Alaskan Marine Ecosystem Example
This food web shows the feeding relationships of organisms in an Alaskan marine ecosystem. If the population of seals and sea lions dramatically decreased, what impact would it have on this food web?
9.3. Killer Whales Feed on the Otters
Without as many seals and sea lions to eat, killer whales might start feeding on more sea otters, causing a decline in their population.
9.4. Sea Urchin Overpopulation
With fewer otters as predators, the number of sea urchins would grow and eat more of the kelp forests.
9.5. Loss of Kelp Habitat
The kelp forests are the habitat for many fish larvae. Loss of this habitat would cause a decline in the fish population.
9.6. Marine Ecosystem Is Altered
This example demonstrates that the original change in the population of the seals and sea lions could cause a chain reaction of events that would alter the balance of the food web.

9. Pause and Interact

10.1. Review
Use the whiteboard tools to complete the activity.

10. Pause and Interact

11.1. Food Webs
Follow the onscreen directions.

11. Vocabulary Review

12.1. Food Chains Vocabulary Matching
Use the whiteboard tools to complete the activity.

12. Virtual Investigation

13.1. Owl Pellet Dissection
Owls are typically top-level consumers in their food web. They eat a variety of organisms including rodents, birds, reptiles and amphibians. Owls usually swallow their prey whole. The indigestible parts of the prey (fur, bones, feathers, claws, teeth) form a pellet in the owl's gizzard. The pellet is then regurgitated, containing the remains of what was eaten. In this virtual investigation you will dissect barn owl pellets. You will sort the bones found in the pellets and identify which type of prey the owl has eaten.

13. Assessment

14.1. Food Chains and Webs
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