# Ecosystems, Food Webs and Energy Flow

> Biotic and abiotic factors, food chains and webs, trophic levels, energy and biomass pyramids, and the carbon and nitrogen cycles.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/ecosystems-and-energy-flow/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: When the dingoes came back
In parts of outback Australia, dingoes are kept out by a fence over 5000 km long. Scientists compared land on both sides. Where dingoes live, there are fewer foxes and feral cats, because dingoes chase and kill them. With fewer foxes and cats, small native mammals like hopping mice do better. Where dingoes are missing, kangaroo numbers explode and they eat the grass to the ground.

Removing one animal changed everything from grass to mice. That is the power of a food web: every organism is linked to many others.

**In short:** An ecosystem is a community of living things together with its non-living environment. Energy flows one way through food chains and webs, with only about 10% passing to each next level, while matter such as carbon and nitrogen is recycled by decomposers.

## Starting point: what you already know

- Producers make their own food by photosynthesis. Consumers eat other organisms (Year 7 science).
- Energy enters food chains from the sun and is released by respiration (see Photosynthesis and Cellular Respiration).
- A habitat is the place an organism lives.

## Parts of an ecosystem

An **ecosystem** is all the organisms in an area and the non-living environment they interact with.

| Term | Meaning |
|---|---|
| **Population** | All the members of one species in an area |
| **Community** | All the populations of different species in an area |
| **Biotic factors** | Living things and their effects: predators, competitors, disease, food |
| **Abiotic factors** | Non-living conditions: light, temperature, water, soil, pH, salinity, fire |

## Food chains and food webs

A **food chain** shows what eats what. Arrows point **in the direction energy flows**, from the eaten to the eater.

grass → kangaroo → dingo

A **food web** is many linked food chains, which is more realistic.

| Trophic level | Role | Example |
|---|---|---|
| Producer | Makes food by photosynthesis | Grass, eucalyptus, algae |
| Primary consumer | Herbivore, eats producers | Kangaroo, koala, grasshopper |
| Secondary consumer | Eats primary consumers | Kookaburra, fox |
| Tertiary consumer | Eats secondary consumers | Wedge-tailed eagle, dingo |
| Decomposers | Break down dead matter and return nutrients to soil | Bacteria, fungi |

## Energy flow and pyramids

Only about **10%** of the energy at one level passes to the next. The rest is used in respiration (lost as heat), passed out in faeces, or never eaten.

### Worked example: The 10% rule
Grass captures 10 000 kJ of energy. About how much reaches a dingo, two steps above the kangaroo?

grass 10 000 kJ → kangaroo 1000 kJ → dingo 100 kJ

This is why food chains rarely have more than four or five levels, and why top predators are always rare.

*Diagram: An energy pyramid with a wide producer level at the bottom, then primary consumers, secondary consumers and a narrow tertiary consumer level at the top*
*Each level of an energy pyramid has about one tenth of the energy of the level below.*

An **energy pyramid** is always this shape. A **biomass pyramid** (the dry mass of living things at each level) is usually this shape too. A **pyramid of numbers** can be odd: one large tree can feed thousands of insects.

## Cycling of matter

Energy **flows** through an ecosystem and is lost as heat. **Matter** is **recycled**.

**The carbon cycle.**

- Photosynthesis takes $\mathrm{CO_2}$ out of the air into plants.
- Feeding passes carbon to animals.
- Respiration by plants, animals and decomposers returns $\mathrm{CO_2}$ to the air.
- Burning fossil fuels and forests adds extra $\mathrm{CO_2}$.

**The nitrogen cycle.** Plants need nitrogen to make proteins, but cannot use nitrogen gas from the air.

- **Nitrogen-fixing bacteria** in soil and in root nodules of legumes (like wattles) turn $\mathrm{N_2}$ into compounds plants can use.
- Decomposers turn dead matter into ammonia, and **nitrifying bacteria** turn it into nitrates.
- Plants absorb nitrates. **Denitrifying bacteria** return $\mathrm{N_2}$ to the air.

**Mind map**

```text
Ecosystems
  Parts
    Population and community
    Biotic: living
    Abiotic: non-living
  Feeding
    Food chains and webs
    Producers and consumers
    Decomposers
  Energy
    About 10% passes on
    Lost as heat
    Pyramids
  Matter recycles
    Carbon cycle
    Nitrogen cycle
```

### Year 11: Relationships between species
| Relationship | Effect on each species | Example |
|---|---|---|
| **Predation** | + for predator, − for prey | Quoll eats a bandicoot |
| **Competition** | − for both | Foxes and quolls competing for prey |
| **Mutualism** | + for both | Clownfish and sea anemone. Corals and the algae in them |
| **Commensalism** | + for one, 0 for the other | Orchids growing on tree branches |
| **Parasitism** | + for parasite, − for host | Ticks on a dog |
| **Allelopathy** | One plant releases chemicals that harm others | Some eucalypts release chemicals that stop seeds near them germinating |

A **keystone species** has a much bigger effect on its ecosystem than its numbers suggest, like the dingo in the story. A **niche** is the role of a species: what it eats, where it lives and when it is active. Two species cannot share exactly the same niche for long (**competitive exclusion**).

### Year 12: Ecosystems and disease
In Module 7, disease spreads through populations and food webs. Bats are hosts for Hendra virus, which can pass to horses and then people when habitat loss pushes bats closer to farms. Understanding ecosystems helps predict and prevent outbreaks.

## Quick summary

- An ecosystem is a community of organisms plus its abiotic environment.
- Food chain arrows show the direction of energy flow. Food webs link many chains.
- About 10% of energy passes to the next trophic level. The rest is lost mostly as heat.
- Energy flows one way. Matter, like carbon and nitrogen, is recycled.
- Decomposers and bacteria are vital for recycling nutrients.

## Quiz

Try these questions from the chapter quiz.

1. Which is an abiotic factor?
   - A. Predators
   - B. Disease
   - C. Soil pH
   - D. Competition
   - Answer: C. Soil pH. Soil pH is a non-living condition.

2. In a food chain, the arrows show
   - A. who is bigger
   - B. the direction energy flows
   - C. the order animals were born
   - D. where animals live
   - Answer: B. the direction energy flows. Arrows point from the eaten to the eater.

3. A koala eats eucalyptus leaves. The koala is a
   - A. producer
   - B. primary consumer
   - C. secondary consumer
   - D. decomposer
   - Answer: B. primary consumer. It eats a producer.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/ecosystems-and-energy-flow/#quiz