Ecosystems and Food Webs
Grade 6 · Science · Study Guide
Discover how living things interact with each other and their environment.
An ecosystem is a community of living organisms — plants, animals, fungi, and microbes — interacting with each other and with nonliving things like water, soil, sunlight, and air. Ecosystems can be as large as an ocean or as small as a puddle. Every organism in an ecosystem has a role to play.
Producers, also called autotrophs, make their own food using sunlight through photosynthesis. Plants, algae, and some bacteria are producers. Consumers, or heterotrophs, must eat other organisms to get energy. Herbivores eat only plants (primary consumers), carnivores eat animals (secondary or tertiary consumers), and omnivores eat both. Decomposers like fungi and bacteria break down dead organisms and return nutrients to the soil.
A food chain shows a single path of energy flow from producer to consumer. But in real ecosystems, feeding relationships are much more complex. A food web shows multiple overlapping food chains and gives a more accurate picture of who eats whom. Energy flows in one direction: from the sun, to producers, to consumers. At each step, about 90% of the energy is lost as heat, which is why there are always fewer top predators than prey animals.
Ecosystems can be disrupted when species are removed or introduced. If wolves are removed from a forest, deer populations explode and overeat vegetation, changing the whole ecosystem. This is called a trophic cascade. Healthy, biodiverse ecosystems are more resilient to disturbances.
| Type | What They Eat | Example |
|---|---|---|
| Herbivore | Plants only | Deer, rabbit, caterpillar |
| Carnivore | Animals only | Wolf, eagle, shark |
| Omnivore | Plants and animals | Bear, human, crow |
| Decomposer | Dead organisms | Mushroom, earthworm, bacteria |
Key terms
- Ecosystem
- A community of living things interacting with each other and their nonliving environment.
- Producer
- An organism that makes its own food through photosynthesis (e.g., plants).
- Consumer
- An organism that gets energy by eating other organisms.
- Decomposer
- An organism that breaks down dead matter and recycles nutrients (e.g., fungi, bacteria).
- Food web
- A diagram showing the feeding relationships among many organisms in an ecosystem.
- Trophic level
- A position in a food chain, from producers at the bottom to top predators at the top.
- Biodiversity
- The variety of different species living in an ecosystem.
- Predator
- An animal that hunts and eats other animals.
Flashcards
- Ecosystem
- A community of living and nonliving things interacting in an environment.
- Producer
- An organism that makes its own food through photosynthesis.
- Consumer
- An organism that obtains energy by eating other organisms.
- Decomposer
- Breaks down dead organisms and returns nutrients to the environment.
- Food web
- Multiple overlapping food chains showing the complex feeding relationships in an ecosystem.
- Trophic level
- Each step in a food chain; energy decreases by about 90% at each level.
Practice questions
Which organism is a producer in an ecosystem?
- Wolf
- Grasshopper
- Oak tree
- Mushroom
Show answer
Answer: Oak tree
Trees are producers — they make their own food through photosynthesis using sunlight, water, and CO2.
Energy flows from consumers back to producers in a food web.
Show answer
Answer: False
Energy flows in one direction only: from the sun to producers to consumers. It does not cycle back.
An organism that breaks down dead matter and recycles nutrients is called a ___.
Show answer
Answer: decomposer
Decomposers break down dead organic matter, releasing nutrients back into the soil for producers to use.
At each step up a food chain, approximately what percentage of energy is LOST as heat?
- 10%
- 50%
- 90%
- 100%
Show answer
Answer: 90%
The 10% rule states that only about 10% of energy passes from one trophic level to the next; the other 90% is lost as heat.
A food web shows more feeding relationships than a simple food chain.
Show answer
Answer: True
A food web connects many food chains and shows the true complexity of who eats whom in an ecosystem.
Fun facts
- Wolves were reintroduced to Yellowstone in 1995, and their presence changed the course of rivers by reducing deer overgrazing along riverbanks.
- A single oak tree can support over 500 species of insects, birds, and mammals.
- Decomposers are so efficient that they recycle nearly all nutrients — without them, Earth would be buried in dead matter.
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