Ecosystem Structure
The Cast of Characters🌿 What Is an Ecosystem?
An ecosystem is a community of living organisms (biotic factors) interacting with each other and with their non-living environment (abiotic factors) in a defined area. Energy flows through ecosystems (one-way) and matter cycles within them (recycled). Every ecosystem has producers that capture energy, consumers that feed on them, and decomposers that break down dead matter and return nutrients to the soil.
🌡️ Abiotic Factors (Non-living)
- Temperature — affects metabolic rates of all organisms
- Light intensity — drives photosynthesis; determines plant zone distribution
- Water availability — limiting factor in most terrestrial ecosystems
- Soil pH and mineral content — determines plant species composition
- Salinity — key in aquatic ecosystems
- Atmospheric gases — O₂ for respiration; CO₂ for photosynthesis
- Wind and currents — affect temperature, moisture, seed dispersal
🦁 Biotic Factors (Living)
- Producers (autotrophs) — make their own food via photosynthesis or chemosynthesis
- Primary consumers (herbivores) — eat producers
- Secondary consumers — eat primary consumers
- Tertiary consumers — eat secondary consumers
- Decomposers (saprotrophs) — bacteria and fungi; break down dead organic matter
- Detritivores — animals that eat dead matter (earthworms, dung beetles)
| Term | Definition | Example |
|---|---|---|
| Habitat | The physical place where an organism lives | A rock pool; a forest canopy; a river bed |
| Niche | The role an organism plays in its ecosystem — what it eats, when it is active, how it reproduces, what eats it | A barn owl's niche: nocturnal predator of small rodents, nests in buildings/trees, prey of larger raptors |
| Population | All individuals of one species in a defined area at a given time | All lions in the Kruger National Park |
| Community | All populations of all species in a defined area — all living organisms together | All the plants, animals, fungi, and microbes in a grassland |
| Ecosystem | A community plus all the abiotic factors it interacts with | The grassland community + soil, rainfall, temperature, light |
| Biome | A large geographic area with characteristic climate and vegetation | Savanna, fynbos, tropical rainforest, desert, tundra |
Food Chains & Food Webs
Who Eats Whom🍽️ Energy Pathways Through the Ecosystem
A food chain shows a single linear pathway of energy transfer from producer to top consumer. A food web shows ALL the feeding relationships in an ecosystem simultaneously — a more realistic picture, since most animals eat more than one thing and are eaten by more than one predator. Arrows in food chains and webs point in the direction of energy flow (from eaten to eater).
A Simple Food Web — Savanna Example
| Term | Definition |
|---|---|
| Trophic level | A feeding level in a food chain. Producers = T1; primary consumers = T2; secondary consumers = T3; tertiary = T4 |
| Producer (autotroph) | Makes own food from sunlight (photosynthesis) or chemicals (chemosynthesis). Always Trophic Level 1 |
| Primary consumer (herbivore) | Eats producers directly. Trophic Level 2 |
| Secondary consumer | Eats primary consumers. Trophic Level 3. May be carnivore or omnivore |
| Tertiary consumer | Eats secondary consumers. Trophic Level 4. Often apex predator |
| Omnivore | Eats both plants and animals — occupies multiple trophic levels simultaneously |
| Decomposer | Breaks down dead organic matter; releases inorganic nutrients back into the soil/water. Not on the main food chain trophic levels |
| Apex predator | Top predator with no natural predators. Removal causes trophic cascade |
Energy Flow
The 10% Rule⚡ Energy Flows One Way — It Is Never Recycled
Unlike matter (which cycles), energy flows through ecosystems in one direction only: from the sun through producers to consumers, and is lost as heat at each step. This is why food chains rarely have more than 4–5 trophic levels — there simply isn't enough energy left to support more. Understanding energy transfer efficiency is essential for exam calculations.
The Ecological Pyramid of Energy — 10% Transfer Rule
1 kJ (1%)
10 kJ (10%)
100 kJ (100%)
1 000 kJ (1 000%)
Only ~10% of energy at each level is transferred to the next. The rest is lost as heat via cellular respiration.
Rule: Only 10% of energy at one trophic level passes to the next. 90% is lost as heat (via cellular respiration, movement, maintaining body temperature).
Example 1 — Moving UP the pyramid
Producers fix 50 000 kJ. How much energy is available to secondary consumers?
T1 (producers) = 50 000 kJ
T2 (primary consumers) = 50 000 × 10% = 5 000 kJ
T3 (secondary consumers) = 5 000 × 10% = 500 kJ
Example 2 — Moving DOWN the pyramid
Tertiary consumers have 80 kJ available. How much energy was in the producers?
T4 = 80 kJ → T3 = 80 ÷ 10% = 800 kJ → T2 = 800 ÷ 10% = 8 000 kJ → T1 = 8 000 ÷ 10% = 80 000 kJ
Example 3 — Efficiency calculation
Producers have 20 000 kJ; primary consumers have 1 600 kJ. What is the actual transfer efficiency?
Efficiency = (energy at next level ÷ energy at current level) × 100
= (1 600 ÷ 20 000) × 100 = 8%
(Real efficiency varies 5–20%; the 10% figure is an average approximation)
🔢 Pyramid of Numbers
- Shows the number of organisms at each trophic level
- CAN be inverted — e.g. one oak tree (T1) supports thousands of caterpillars (T2)
- Least useful pyramid — does not account for organism size
⚖️ Pyramid of Biomass
- Shows total dry mass of organisms at each trophic level
- Usually upright — more biomass at lower levels
- CAN be inverted in aquatic systems — phytoplankton reproduce so fast that large biomass of zooplankton is supported by a small standing crop of phytoplankton
⚡ Pyramid of Energy
- Shows energy available at each trophic level per unit area per unit time
- ALWAYS upright — energy is always lost at each level
- Most accurate and informative pyramid
- Cannot be inverted under any circumstances
Nutrient Cycles
Matter Is Recycled Forever♻️ Unlike Energy, Matter Cycles
Carbon, nitrogen, water, and other elements cycle continuously through living and non-living components of ecosystems. Decomposers are the key link — they break down dead organic matter and release nutrients back into the soil and atmosphere, making them available to producers again. Without decomposers, nutrients would become locked in dead organic matter and the ecosystem would collapse.
🌊 Carbon Sinks
- Oceans — dissolve huge amounts of CO₂ from atmosphere
- Forests — store carbon in wood and soil organic matter
- Peat bogs — partially decomposed organic matter stores carbon for thousands of years
- Fossil fuels — ancient carbon locked underground for millions of years
🔥 Human Impact on Carbon Cycle
- Burning fossil fuels releases ancient carbon rapidly → atmospheric CO₂ increase
- Deforestation removes carbon sinks AND releases stored carbon
- Increased CO₂ → enhanced greenhouse effect → global warming
- Ocean acidification as CO₂ dissolves to form carbonic acid → threatens marine life
| Process | What Happens | Who Does It |
|---|---|---|
| Nitrogen fixation | Atmospheric N₂ converted to ammonium (NH₄⁺) or nitrate (NO₃⁻) — forms plants can absorb | Nitrogen-fixing bacteria (Rhizobium in root nodules of legumes; free-living Azotobacter); lightning; industrial Haber process |
| Nitrification | Ammonium (NH₄⁺) converted to nitrite (NO₂⁻) then nitrate (NO₃⁻) in the soil | Nitrifying bacteria (Nitrosomonas, Nitrobacter) in soil |
| Assimilation | Plants absorb nitrates from soil through roots; use nitrogen to make proteins and DNA | Plants (producers) |
| Ammonification | Dead organic matter (proteins) broken down; nitrogen released as ammonium (NH₄⁺) | Decomposers (bacteria and fungi) |
| Denitrification | Nitrates (NO₃⁻) converted back to N₂ gas — returns nitrogen to atmosphere; completes the cycle | Denitrifying bacteria in waterlogged/anaerobic soils |
Ecological Succession
How Ecosystems Change Over Time🌱 Communities Don't Stay Still
Ecological succession is the gradual, directional change in the species composition of a community over time. It follows a predictable sequence — from simple pioneer communities to a complex, stable climax community. Each stage modifies the environment in ways that make it more suitable for the next community. Understanding succession explains how ecosystems recover from disturbance and how bare rock eventually becomes forest.
⚡ Key Differences from Primary
- Soil is already present — no lichen pioneer stage needed
- Seed bank in soil — dormant seeds germinate rapidly after disturbance
- Much faster — decades to centuries rather than thousands of years
- Triggered by: fire, flood, farming abandonment, storm damage, volcanic eruption that preserves soil
- Same endpoint: climax community
🌿 South African Example — Fynbos After Fire
- Fynbos is fire-adapted — many species require fire to germinate
- Within weeks: geophytes (bulbs) re-sprout; annual herbs germinate from seed bank
- Months: grasses and small shrubs re-establish
- Years: proteas, ericas, restios re-establish
- Without periodic fire: alien invasive plants overwhelm fynbos → biodiversity collapses. Fire IS part of the fynbos ecosystem
🎯 Ecosystem Assessment
Eight questions covering the full ecosystem topic.