Ecosystems, food webs & energy flow

What you will learn

the difference between a habitat, a population, a community and an ecosystem,
roles in an ecosystem: producer, consumer (primary/secondary/tertiary), decomposer,
how to read and build a food web, and why arrows point in the direction of energy flow,
the “10% rule” for energy transfer and why food chains are usually short,
how biotic and abiotic changes (habitat loss, climate, introduced species) disturb ecosystems.

Worked example 0 Real-world example: what happened when wolves returned to Yellowstone

Wolves were extinct in Yellowstone National Park by 1926. Without their top predator, elk populations exploded, overgrazing young willow and aspen trees. Beavers (who need willow) declined; riverbanks eroded.

In 1995 wolves were reintroduced.
Elk avoided open valleys where wolves hunted; willows regrew.
Beavers returned; their dams restored wetlands.
Songbirds, fish and even river courses stabilised.

Key idea: a “trophic cascade” — a change at the top of a food web can reshape vegetation, rivers, and dozens of species. Ecosystems are connected.

1. Levels of organisation

Individual — one organism (e.g. one kangaroo).
Population — all the organisms of one species in an area (e.g. all kangaroos in a valley).
Community — all the populations of all species in that area (kangaroos + grass + hawks + beetles).
Ecosystem — the community plus the non-living environment (soil, water, air, sunlight).

Biotic factors are living: plants, animals, fungi, bacteria. Abiotic factors are non-living: temperature, water, light, pH, minerals.

2. Feeding roles

Producer (autotroph) — makes its own food using sunlight (green plants, algae) or chemicals (some bacteria).
Primary consumer — eats producers (herbivore).
Secondary consumer — eats primary consumers (often a carnivore).
Tertiary consumer — eats secondary consumers (top predator).
Decomposer — breaks down dead matter and returns nutrients to the soil (fungi, bacteria, worms).

Worked example 1 Classifying organisms in an ecosystem

In an Australian grassland: grass, grasshopper, small lizard, wedge-tailed eagle, dung beetle. Assign each to a role.

Grass → producer.
Grasshopper → primary consumer.
Small lizard → secondary consumer.
Wedge-tailed eagle → tertiary consumer (top of the chain shown).
Dung beetle → decomposer (breaks down animal waste).

Key idea: the same species can be a primary consumer in one food chain and a secondary consumer in another, depending on what it is eating at the time.

3. Food chains and food webs

An arrow in a food chain means “is eaten by” and shows the direction energy flows:

\text{grass} \to \text{grasshopper} \to \text{lizard} \to \text{eagle}

Most organisms eat more than one thing and are eaten by more than one thing, so a food web is a more realistic picture.

Simplified food web for an Australian woodland. Arrows point from food to feeder.

Worked example 2 Reading a food web

In the web above, what happens if a disease wipes out mice?

Kookaburras lose a major food source → they eat more grasshoppers, or their population drops.
Lizards face more competition from kookaburras for grasshoppers.
Seed plants are no longer eaten as heavily → seed stocks rise.
Eagles may switch to lizards and kookaburras more.

Key idea: removing a single species ripples across many links, usually in ways that are hard to predict from a single food chain.

4. Energy pyramids and the 10% rule

Only about 10% of the energy at one level is captured by the next. The rest is lost as heat, movement, or undigested waste. That is why:

Food chains are rarely more than 4 or 5 links long — there is not enough energy at the top.
There are always far more producers than top predators in terms of total mass.

Worked example 3 Using the 10% rule

A grassland holds $10\,000$ kg of grass (producers). Estimate the mass of top predators (tertiary consumers) it can support.

Producers: $10\,000$ kg.
Primary consumers ( $10\%$ ): $1000$ kg.
Secondary consumers ( $10\%$ ): $100$ kg.
Tertiary consumers ( $10\%$ ): $10$ kg.

So $10\,000$ kg of grass supports only $10$ kg of eagle — about one adult wedge-tailed eagle.

Key idea: the more links in a chain, the less energy at the top. This limits how many top predators an ecosystem can hold.

5. Disturbance: how ecosystems change

Biotic changes: a new species arrives, a disease spreads, a predator is lost. Abiotic changes: temperature, rainfall, fire, pollution.

Worked example 4 Cane toads in Queensland

Cane toads were introduced in 1935 to control sugar-cane beetles. They failed to control the beetles but thrived themselves.

Native predators (quolls, goannas, snakes) ate the toads and were poisoned by their skin toxin.
Predator populations crashed.
With fewer predators, populations of small reptiles and rodents shifted.

Outcome: the “solution” to one problem created a larger cascade of problems.

Key idea: introducing a species without an existing predator or competitor often causes rapid, often permanent, ecosystem change.

Practice: Year 7

Fluency

Tier 1: recall and identify

Define: habitat, population, community, ecosystem.
Give two biotic and two abiotic factors in a pond.
What is a producer? Give two examples.
What does a decomposer do? Name one.
In the chain grass → rabbit → fox, identify the producer, primary consumer and secondary consumer.
In a food web diagram, what does an arrow between two organisms mean?
State the 10% rule.
Why are food chains rarely longer than 4 or 5 links?
Name three biotic and three abiotic factors that affect an Australian forest.
Give one example of an introduced species causing harm to an Australian ecosystem.

Reasoning

Tier 2: explain and reason

An ecosystem has 5000 kg of grass. Using the 10% rule, estimate the mass of grasshoppers, lizards, and eagles it can support.
Explain why there are usually more producers than top predators in any ecosystem.
A farmer sprays pesticide that kills insects. Predict two knock-on effects on birds that eat insects.
Explain why drawing arrows the wrong way round in a food web changes its meaning.
Why can an ecosystem recover from a short drought but rarely from the loss of a keystone species?
Decomposers are sometimes called the “recyclers” of an ecosystem. Explain what they recycle and why the ecosystem would fail without them.

Problem solving

Tier 3: apply to a novel context

Draw a food web for a suburban backyard using at least 6 organisms, with arrows in the correct direction.
A river’s water temperature rises 4°C due to a nearby power station. List two abiotic changes and two likely biotic effects.
An island has rabbits, grass, foxes and eagles. Foxes are removed for hunting. Predict in order the short-term and long-term effects.
Use the 10% rule to explain why feeding people directly with grain is more energy-efficient than feeding grain to cattle and then eating beef.

Challenge

Reasoning

Harder reasoning

A lake is “eutrophic” — excess fertiliser from farms caused an algal bloom. Explain step by step why the fish later died, even though algae are producers and increased food should help the ecosystem.
Design a careful experiment to test whether an introduced species of fish reduces native fish numbers in a pond, identifying independent, dependent and controlled variables.
Bioaccumulation: mercury concentrates up a food chain. If small fish hold 0.1 mg/kg, larger fish that eat 10 of them hold roughly 1 mg/kg, and top predators that eat 10 of those hold roughly 10 mg/kg. Explain using the 10% rule why energy thins but toxins concentrate.
A forest loses all its decomposers overnight. Describe the ecosystem after one year, after ten years, and after a hundred years.