VCE Environmental Science (Year 11) · Unit 1 — Earth's Systems and Ecosystems · 16m read

Ecosystems — Structure, Function and Interactions

Ever wondered why plants need sunlight but animals don't? Ecosystems are communities where living things and their physical environment constantly interact and depend on each other. Energy flows from the sun through plants to animals, while nutrients cycle back to the soil—it's like nature's recycling system. Mastering this helps you understand climate change, food webs, and why protecting habitats matters for your own survival.

Learning objective: Explain ecosystem structure and the flow of energy and matter through ecosystems.

By the end of this lesson you can:

• I can define biotic and abiotic components of ecosystems.
• I can trace energy flow through food webs and explain efficiency losses.
• I can describe nutrient cycling using the carbon and nitrogen cycles.

An ecosystem is a spider web. Every species is a strand; tug one and the whole web vibrates.

Ecosystem structure and energy flow

### Ecosystem components

Biotic factors: Living components — producers, consumers, decomposers.
Abiotic factors: Non-living components — temperature, light, water, soil, pH, salinity.

Ecosystem: All biotic and abiotic components interacting in a defined area.
Community: All living organisms in an ecosystem.
Population: All individuals of one species in an area.
Niche: The functional role of a species — what it eats, where it lives, what eats it.

### Trophic levels and energy flow

Producer (Trophic Level 1): Plants, algae, cyanobacteria — convert solar energy to chemical energy through photosynthesis.

Primary consumer (TL2): Herbivores — eat producers.

Secondary consumer (TL3): Carnivores — eat primary consumers.

Tertiary consumer (TL4): Top predators.

Decomposers: Bacteria and fungi — break down dead organic matter, returning nutrients to the system.

### The 10% rule

Only approximately 10% of energy is transferred from one trophic level to the next. The remaining 90% is lost as:
- Heat (metabolic processes, respiration)
- Incomplete digestion (faeces)
- Respiration (cellular metabolism)

$$\text{Energy at TL}_n = \text{Energy at TL}_{n-1} \times 0.10$$

Implication: Top predators need large amounts of primary production to support them. This is why apex predators are always rare.

Ecological efficiency = Energy at higher TL ÷ Energy at lower TL × 100 = ~10%

### Nutrient cycling

Unlike energy (which flows through and is lost), nutrients cycle.

Carbon cycle:
- Photosynthesis: CO₂ + H₂O → glucose + O₂ (carbon fixed)
- Respiration: glucose + O₂ → CO₂ + H₂O (carbon released)
- Decomposition: releases carbon from dead organic matter
- Fossil fuels: burning releases long-stored carbon

Nitrogen cycle:
- Nitrogen fixation: N₂ → NH₃ (bacteria in soil/root nodules)
- Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (by bacteria)
- Assimilation: plants absorb NO₃⁻
- Denitrification: NO₃⁻ → N₂ (returned to atmosphere)

AARI TIP: Ask Aari to explain why top predators are always rare using the 10% rule with a calculation.

Ecosystem Energy Formulas

• Energy transfer: E_{n+1} = E_n \times 0.10 (10% rule: only 10% transfers up each trophic level)
• Ecological efficiency: \eta = \dfrac{\text{Energy at higher TL}}{\text{Energy at lower TL}} \times 100\% (Typically ~10%)

Quick reference

| Component | Example |
|---|---|
| Biotic | Plants, animals, microbes |
| Abiotic | Temperature, water, soil |
| Producers | Photosynthesise |
| Consumers | Eat others |
| Decomposers | Recycle nutrients |

Key ideas at a glance

• Energy flows: One way.
• Matter cycles: Carbon, nitrogen.
• Interactions: Predation, competition, symbiosis.
• Keystone species: Disproportionate impact.

Worked example (Easy)

A grassland ecosystem has 100,000 kJ of energy at the producer level. How much energy would be available at the tertiary consumer level?

1. Producer → Primary consumer. 100{,}000 \times 0.10 = 10{,}000 \text{ kJ}
2. Primary → Secondary consumer. 10{,}000 \times 0.10 = 1{,}000 \text{ kJ}
3. Secondary → Tertiary consumer. 1{,}000 \times 0.10 = 100 \text{ kJ}

Answer: Only 100 kJ (0.1% of original) is available to tertiary consumers. This is why food chains rarely exceed 4-5 levels.

Worked example (Medium)

Identify the trophic levels in a coastal kelp ecosystem and explain one cascading effect when otters (a top predator) are removed.

1. Producers. Kelp performs photosynthesis using sunlight.
2. Primary consumers. Sea urchins graze on kelp.
3. Secondary consumers. Sea otters predate urchins.
4. Cascade if otters removed. Urchin numbers explode, kelp is overgrazed, kelp forest collapses, fish nursery habitat is lost.

Answer: Removing a keystone predator like otters propagates downward through urchins to producers and then outward to dependent species — a classic trophic cascade.

Common mistakes

• Saying energy is "recycled" through an ecosystem Students confuse energy flow with nutrient cycling. Energy flows one-way and is lost as heat at each trophic level. Fix: Always distinguish: energy FLOWS (one-way, lost as heat) while nutrients CYCLE (reused). Only matter is recycled in ecosystems.
• Applying the 10% rule as an exact figure Students treat 10% as a fixed constant rather than an approximation that varies between ecosystems. Fix: Use "approximately 10%" and note that actual transfer ranges from 5-20%. Use 10% in calculations unless told otherwise, but acknowledge it is an average.
• Confusing nitrogen fixation with nitrification Both involve nitrogen transformation by bacteria, but they convert different compounds. Fix: Nitrogen fixation: N₂ → NH₃ (atmospheric nitrogen to ammonia, by Rhizobium). Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (ammonia to nitrate, by Nitrosomonas and Nitrobacter).

Trophic cascades — when top predators reshape ecosystems

A trophic cascade occurs when changes at the top of a food web ripple downward, dramatically altering the entire ecosystem. The most famous example is the reintroduction of wolves to Yellowstone in 1995.

After wolves were eliminated in the 1920s, elk populations exploded and overgrazed riverbank vegetation. This caused erosion, reduced habitat for songbirds and beavers, and even changed the physical course of rivers.

When wolves were reintroduced, elk numbers decreased and — critically — elk *behaviour* changed. Elk avoided open river valleys where they were vulnerable. Vegetation recovered, beavers returned and rebuilt dams, songbird diversity increased, and rivers stabilised.

This demonstrates a key concept: ecosystem function depends on species interactions, not just species presence. Removing or adding a single species — especially a keystone predator — can transform an entire ecosystem through indirect effects across multiple trophic levels.

In Australian ecosystems, the decline of dingoes (through baiting programs) has allowed kangaroo and feral cat populations to increase, with cascading effects on smaller native mammals and vegetation. Understanding trophic cascades is essential for evaluating conservation strategies.

Practice questions

1. The approximate percentage of energy transferred between trophic levels in most ecosystems is:

   1. 50%
   2. 90%
   3. 10%
   4. 1%

   (1 marks)

2. Describe the role of decomposers in ecosystem nutrient cycling.

   (3 marks)

Summary

• Biotic: living. Abiotic: non-living. Both interact to form an ecosystem.
• Trophic levels: producers → primary → secondary → tertiary consumers → decomposers.
• 10% rule: only 10% energy transferred between levels. 90% lost as heat.
• Energy FLOWS (lost); nutrients CYCLE (reused).
• Carbon cycle: photosynthesis (fix), respiration/decomposition (release). Nitrogen cycle: fixation, nitrification, denitrification.