Ecology and Ecosystems
Grade 9 · Science · Study Guide
Discover how living organisms interact with each other and their environment in the complex web of life.
Ecology is the scientific study of how organisms interact with each other and with their physical environment. The levels of ecological organization start with individual organisms, then populations (all members of one species in an area), communities (all populations in an area), ecosystems (a community plus its abiotic — non-living — environment), and finally the biosphere (all ecosystems on Earth).
Energy flows through ecosystems in one direction. Producers (autotrophs), primarily plants, capture energy from the sun through photosynthesis and convert it into organic matter. Primary consumers (herbivores) eat producers; secondary consumers eat primary consumers; tertiary consumers are at the top. These feeding relationships form food chains and more complex food webs. At each trophic level, only about 10% of energy is passed on — the rest is lost as heat. This is why food chains rarely exceed 4-5 levels.
Matter, unlike energy, cycles through ecosystems. The carbon cycle, nitrogen cycle, and water cycle are critical biogeochemical cycles. Decomposers (bacteria and fungi) play a vital role by breaking down dead organic matter and returning nutrients to the soil. Symbiotic relationships between species include mutualism (both benefit, like bees and flowers), commensalism (one benefits, one is unaffected), and parasitism (one benefits, one is harmed). Human activities — deforestation, pollution, habitat destruction — disrupt ecosystems and reduce biodiversity, threatening ecosystem stability.
| Trophic Level | Organism Type | Example | Energy Available |
|---|---|---|---|
| Level 1 | Producers | Grass | 10,000 units |
| Level 2 | Primary Consumers | Grasshopper | 1,000 units |
| Level 3 | Secondary Consumers | Frog | 100 units |
| Level 4 | Tertiary Consumers | Hawk | 10 units |
| Type | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | Benefits (+) | Benefits (+) | Bee and flower |
| Commensalism | Benefits (+) | Unaffected (0) | Barnacle on whale |
| Parasitism | Benefits (+) | Harmed (-) | Tapeworm and host |
Key terms
- Ecosystem
- A community of living organisms interacting with each other and their non-living physical environment.
- Producer (Autotroph)
- An organism that produces its own food using energy from the sun or chemicals (e.g., plants, algae).
- Consumer (Heterotroph)
- An organism that obtains energy by eating other organisms.
- Decomposer
- An organism (bacteria, fungi) that breaks down dead organic matter, recycling nutrients.
- Food Web
- A complex network of interconnected food chains showing feeding relationships in an ecosystem.
- Trophic Level
- A feeding level in a food chain; producers are level 1, primary consumers level 2, etc.
- Biodiversity
- The variety of life in an area; greater biodiversity generally means a more stable and resilient ecosystem.
- Mutualism
- A symbiotic relationship where both species benefit (e.g., bees pollinating flowers).
Flashcards
- Producer
- Organism that makes its own food via photosynthesis; base of food chain.
- Consumer
- Organism that eats other organisms for energy.
- Decomposer
- Breaks down dead matter and recycles nutrients (bacteria, fungi).
- Food Web
- Interconnected network of feeding relationships in an ecosystem.
- 10% Rule
- Only ~10% of energy passes from one trophic level to the next; rest lost as heat.
- Mutualism
- Symbiosis where both organisms benefit (e.g., bees and flowers).
Practice questions
Which organisms form the base of most food chains?
- Decomposers
- Consumers
- Producers
- Carnivores
Show answer
Answer: Producers
Producers (plants, algae, some bacteria) convert solar energy into chemical energy via photosynthesis — they are the foundation of all food chains.
About 90% of energy is transferred from one trophic level to the next.
Show answer
Answer: False
Only about 10% of energy transfers between trophic levels — the other 90% is lost as heat, waste, and metabolic processes. This is why large predators are rare.
A symbiotic relationship where both species benefit is called ___.
Show answer
Answer: mutualism
In mutualism, both species benefit. Bees get nectar while the flower gets pollinated. Compare to commensalism (0/+) and parasitism (-/+).
Which term describes all the populations of different species living in the same area?
- Ecosystem
- Population
- Community
- Biosphere
Show answer
Answer: Community
A community includes all the different species (populations) living together in the same area. Add the abiotic (non-living) components and you have an ecosystem.
Decomposers are critical to ecosystems because they:
- Produce energy from sunlight
- Recycle nutrients back into the environment
- Eat primary consumers
- Create oxygen
Show answer
Answer: Recycle nutrients back into the environment
Decomposers break down dead organisms and waste, releasing nutrients (nitrogen, carbon) back into the soil and atmosphere for producers to use — they close the nutrient cycle.
Fun facts
- A single teaspoon of healthy soil contains more microorganisms than there are people on Earth.
- The Amazon rainforest produces about 20% of the world's oxygen and is home to an estimated 10% of all species on Earth.
- Wolves reintroduced to Yellowstone in 1995 triggered a 'trophic cascade' — changing the behavior of deer, allowing riverbanks to revegetate, which changed the course of rivers.
Get a personalized course on Ecology and Ecosystems built around your child.