Ecosystems: Energy Flow and Cycles of Matter: Grade 7 Study Guide
An ecosystem is a community of living things and their environment, linked by energy and matter.
An ecosystem includes biotic (living) factors such as plants, animals and microbes, and abiotic (non-living) factors such as sunlight, water, temperature and soil. Organisms compete for limited resources, and a population is limited by its carrying capacity, the number the environment can support.
Producers capture the Sun's energy by photosynthesis and form the base of food webs. Consumers eat producers or other consumers, and decomposers break down dead matter, returning nutrients to the soil. Only about 10% of energy passes up each level, so there are fewer top predators than plants.
Energy pyramid
| Level | Example | Energy available |
|---|---|---|
| Producers | grass | 100% |
| Primary consumers | grasshopper | about 10% |
| Secondary consumers | frog | about 1% |
| Tertiary consumers | hawk | about 0.1% |
Cycling of matter
Matter is recycled. In photosynthesis, plants turn carbon dioxide and water into sugar and oxygen. In respiration, organisms use sugar and oxygen and release carbon dioxide and water. The same atoms of carbon, oxygen, hydrogen and nitrogen are used again and again. Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂.
Photosynthesis.
Energy is transferred.
Energy for life; CO₂ is released.
Nutrients return to the soil.
Interactions and change
Species interact through competition, predation and symbiosis (mutualism, where both benefit; parasitism, where one is harmed). Removing a species can change the whole web. Biodiversity, the variety of living things, makes ecosystems more stable. Human actions such as habitat loss, pollution and invasive species reduce it, but conservation, protected areas and restoration can help. Engineers and scientists evaluate solutions with trade-offs.
Worked example and common misconceptions
Example: If wolves are removed, deer may increase, overgraze plants and change the habitat for other species. Common misconceptions: thinking energy is recycled (it flows one way and is lost as heat, while matter cycles), thinking decomposers are a separate group outside the web and thinking plants get their mass from the soil (most of it comes from carbon dioxide and water).
Check your understanding: model answers
1. For example, a tree (biotic) and sunlight (abiotic). 2. Energy is lost at each level, so less can support the next level. 3. It breaks down dead matter and returns nutrients to the soil.
✨ Key Takeaways
- Energy flows; matter cycles.
- About 10% of energy passes up each level.
- Biodiversity makes ecosystems stable.
- Humans can protect or harm ecosystems.
💬 Questions for Discussion & Reflection
- Name a biotic and an abiotic factor.
- Why are there fewer top predators than plants?
- What does a decomposer do?
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