8.11A: Relations in a Food Web
Organisms and Environments
Part I: The Chain vs. the Web
Fundamental Question: How does energy flow in a food chain and food
web?
A Food Chain
The example on the right is a food chain. The arrows show how energy flows
through the food chain by indicating what organism is eaten by another
organism.
The base of the food chain is the producer, grass. The grass obtained great
energy from the sun using it for photosynthesis. Once the grasshopper eats
the grass, some of the grass’s energy is transferred to the grasshopper. But
the energy is not as great as it originally was in the grass. Once the frog eats
the grasshopper, the grasshopper’s energy is transferred to the frog. The
energy the frog obtains is not as great as it was in the grasshopper. Therefore,
as you move up the food chain, energy decreases. So the hawk in the food
chain would not obtain as much energy as the grass did, even though it is at
the top of this particular food chain.
Food chains can be linked together to create a food web. A food web shows
how all organisms in a particular ecosystem are related, and how they depend
upon each other in order to survive
Try to create a food web using 24 organisms:
1. Your group has been assigned a food web to construct. Cut out all 24
pictures from the two pages given and spread across the desk.
2. As a group, arrange the pictures into food chains. Then proceed to group
the food chains together to create a food web.
A Food Web
3. Glue the food web onto the poster board.
4. Draw in arrows within the food web showing
how energy flows as one organism is eaten
by another organism.
5. Complete Part I of the
Student Journal.
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8.11A: Relations in a Food Web
Organisms and Environments
Part II: Producers, Consumers, Decomposers, Oh My!
Fundamental Question: How does energy flow in a food chain and food web?
Producers, or autotrophs, are organisms that produce their own food. We’re talking about plants!
Any plant, tree, bush, etc. creates its own food through photosynthesis.
Consumers, or heterotrophs, are organisms that cannot make their own food. They have to eat other
organisms to obtain energy and to live. Consumers are broken down into three categories: herbivores,
carnivores, and omnivores.
Herbivores are organisms that eat only plants.
HINT: The prefix “herb” in the word herbivore means plant.
Carnivores are organisms that eat other animals, not plants.
HINT: The prefix “carn” in the word carnivore means meat.
Omnivores are organisms that eat both plants and animals.
HINT: The prefix “omni” in the word omnivore means all.
Decomposers are organisms that break down dead plants or animals.
Web Role Identification
1. Also on that page, define each of the vocabulary words.
2. In Part II of your Student Journal, use your group’s food web to fill in the table with examples of
producers (autotrophs), consumers (heterotrophs), and decomposers.
Part III: Energy Pyramid
Fundamental Question: How does energy flow in a food chain and food web?
An Energy Pyramid places organisms into trophic levels. As you travel from the bottom of the
pyramid on up, energy decreases. The bottom of the pyramid is where the primary producers or
autotrophs are identified. Energy is the greatest at this level because of the sun supplying energy to
plants for photosynthesis to occur.
The remaining levels up the pyramid are for the consumers or heterotrophs. To break it down further,
the first level of consumers are the primary consumers. Primary consumers are herbivores. The
second level of consumers are the secondary consumers. Secondary consumers are either carnivores
or omnivores. The third level of consumers are the tertiary consumers. Tertiary consumers are either
carnivores or omnivores and are at the top of the food web. But remember, just because these guys
are at the top of the food web doesn’t mean they have the most energy. They have the least amount
of energy.
Fill in the Flow
1. In Part III of the Student Journal, use your group’s food web to complete the Trophic Levels
within the Energy Pyramid. Use the names of the plants and animals within the food web.
2. Also at the bottom of that page, complete the question.
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8.11A: Relations in a Food Web
Organisms and Environments
Part IV: So Many Relationships!
Fundamental Question: Can you describe the predator/prey relationships and parasite/host
relationships within a food web?
A predator is a carnivorous animal that hunts, kills, and eats other animals in order to survive. There
are a variety of ways for predators to obtain their kill; it depends on the type of animal. Predators may
hunt and attack actively for their prey, or they may hide and wait patiently as their prey approaches
closer to them before attacking. Once the prey is obtained, the predator may slowly chew it or
swallow it whole. Some predators may use a venom to paralyze its prey. Other predators may
squeeze their victim to death. Some eat thousands of prey at once, such as a whale eating plankton.
Whatever method a predator uses, the goal is one in the same. To kill and eat so to resolve hunger.
The prey is the underdog in the predator-prey relationship. The prey is the predator’s meal. The prey
may try to run away or fight back against its predator and sometimes this might allow it an escape
from being eaten. Life is tough when you’re at the bottom of the food chain. Prey are always having
to be on the look out for their predators. It’s not an easy life. They lack the agility and sometimes
intelligence of their predator. The prey usually is the victim and cannot defeat its predator.
In Part IV of the Student Journal, use your group’s food web to identify two or three predator-prey
relationships.
Other types of relationships that exist between two organisms include commensalism, mutualism, and
parasitism.
In commensalism, one organism benefits from the relationship while the other organism does not
benefit and is not harmed. An example of commensalism is a bird living in a tree. The bird benefits by
having its shelter in the tree. The tree does not benefit from the bird living in it and the tree is not
harmed from the bird living in it.
In mutualism, both organisms benefit from each other. An example is a bird that rides on the back of a
rhino. The bird benefits by picking off the insects that are on the rhino and eating them as food. The
rhino benefits by having the irritating insects removed off its back by the bird.
In parasitism, one organism (the parasite) benefits but the other organism (the host) does not benefit.
It is harmed. An example is a flea on a dog. The flea benefits by feeding off the blood of the dog. The
dog is being harmed by the flea and can contract diseases from the flea that could be life-threatening.
The parasite-host relationship can be deadly. An example of a parasite-host relationship in a marine
environment is a nematode inside a fish. Nematodes can live within the flesh of a fish. This infestation
may cause health problems for the fish, and lead to death, but also nematodes can be transferred to
and harm humans if consumed.
In Part IV of the Student Journal, use your group’s food web to identify one of each type of
relationship within your food web.
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8.11A: Relations in a Food Web
Organisms and Environments
Part V: Food Web Gallery Walk
Fundamental Question: How does energy flow in a food chain and food web?
Take time to browse through every group's food webs. For each one, complete the question in Part V
of your Student Journal.
Finish up this exploration with the Reflections and Conclusions section of the Student Journal.
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