🌿 Ecosystems

📚 Middle & High School Biology / Environmental Science 🎯 Key Concepts: Food Chains, Energy Flow, Nutrient Cycles, Biodiversity

Before You Begin

Cell biology basics — fundamental life processes
Organismal biology — individual organism level

📌 After this unit, you'll be able to

Identify producers, consumers, and decomposers and explain their roles
Trace energy through food chains and explain why only ~10% transfers between levels
Describe the carbon and nitrogen cycles using ecosystem components
Explain how biodiversity loss affects ecosystem stability

🌱 Why study ecosystems?

Think of it intuitively

An ecosystem is an energy flow network. Solar energy flows: plants (producers) to herbivores to carnivores. But about 90 percent of energy is lost as heat at each step. This is why there must be far more plants than herbivores. The shape of the food pyramid is inevitable, driven by energy loss.

Ecosystems show us how all living things are interconnected. Why would losing bees cause a food crisis? Why does deforestation change the climate? Why do invasive species collapse ecosystems? The answer is always the same: everything is connected. Understanding ecosystems is essential for addressing today's most urgent challenges — climate change, biodiversity loss, and sustainable development.

⚡ 30-second summary

Trophic levels: Producers → primary consumers → secondary consumers → decomposers
10% Rule: Only ~10% of energy passes to the next trophic level. Higher = less energy
Carbon cycle: CO₂ ↔ photosynthesis/respiration ↔ organic matter ↔ combustion
Nitrogen cycle: N₂ → nitrogen fixation → ammonium → nitrates → organisms
Biodiversity: Genetic + species + ecosystem diversity. Threats: habitat loss, invasive species, pollution

1. Ecosystem Components

An ecosystem is all the living organisms (biotic factors) and non-living components (abiotic factors — sunlight, water, soil, temperature) in an area, interacting together.

Producers: Make their own food via photosynthesis (plants, algae, cyanobacteria).
Consumers: Obtain energy by eating others.
- Primary consumers: Eat producers (herbivores — deer, rabbits)
- Secondary consumers: Eat primary consumers (omnivores/carnivores)
- Tertiary consumers: Top predators
Decomposers: Break down dead organic matter, returning nutrients to the soil (bacteria, fungi).

2. Food Chains & Food Webs AP Exam

A food chain shows a single path of energy transfer: Grass → Grasshopper → Frog → Snake → Hawk

A food web shows all overlapping food chains in an ecosystem — a more realistic picture of feeding relationships.

Arrows in a food chain/web point in the direction energy flows (from eaten to eater).

Example food chain: Phytoplankton → Zooplankton → Small fish → Large fish → Shark

3. Energy Flow — The 10% Rule AP Exam

Only about 10% of energy is transferred from one trophic level to the next. The other 90% is lost as heat (metabolism, movement).

Ecological pyramid: Energy decreases at each higher level → pyramids of energy/biomass/numbers all taper toward the top.

Example: If producers have 10,000 kcal of energy:
Primary consumers receive ~1,000 kcal → Secondary consumers ~100 kcal → Tertiary consumers ~10 kcal.

4. Nutrient Cycles

Unlike energy, matter is recycled through ecosystems.

Carbon cycle: Photosynthesis (CO₂ → organic carbon) ↔ Respiration/decomposition (organic carbon → CO₂). Burning fossil fuels adds extra CO₂.
Nitrogen cycle: Nitrogen gas (N₂) → fixed by bacteria → absorbed by plants → returned to atmosphere by denitrifying bacteria.
Water cycle: Evaporation → condensation (clouds) → precipitation → runoff/groundwater → evaporation again.

5. Biodiversity & Ecosystem Stability

Biodiversity: Variety of species in an ecosystem. Higher biodiversity = greater resilience.
Keystone species: Species that has a disproportionately large effect on its ecosystem (e.g., wolves in Yellowstone).
Threats: Habitat destruction, invasive species, pollution, climate change, overexploitation.

Practice Questions

Q1: In the food chain: Grass → Rabbit → Fox → Eagle, if the grass contains 100,000 kcal, how much energy reaches the eagle?
Answer: Rabbit gets 10,000 kcal, Fox gets 1,000 kcal, Eagle gets 100 kcal (10% rule applied three times).

Q2: Why are there fewer organisms at higher trophic levels?
Answer: Because energy is lost (~90%) at each transfer, there is less energy available to support organisms at higher levels.

💡 Study Tip: Remember that in food chains, arrows represent energy flow, not "who eats whom" (even though they're the same direction). The 10% rule is why top predators are rare — it takes enormous amounts of primary productivity to support even a single apex predator.

Pre-Test Checklist

Food chain: producers → primary consumers → secondary consumers → decomposers
Energy efficiency: ~10% of energy transfers to the next trophic level
Ecosystem balance: population changes trigger feedback mechanisms
Matter cycles (carbon, nitrogen); energy does NOT cycle — flows one-way

🧠

Spaced Repetition — Ebbinghaus Curve

Review this material at increasing intervals to commit it to long-term memory.

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✓ NGSS Standards aligned ✓ Reviewed Apr 2026 🔍 Accuracy verified Found an error? Let us know