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:
Most organisms eat more than one thing and are eaten by more than one thing, so a food web is a more realistic picture.
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 kg of grass (producers). Estimate the mass of top predators (tertiary consumers) it can support.
- Producers: kg.
- Primary consumers (): kg.
- Secondary consumers (): kg.
- Tertiary consumers (): kg.
So kg of grass supports only 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
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.
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.
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
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.
Answers
Answer key
Attempt the practice first. When you're ready to check, expand the answers below.
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Year 7 answers
Tier 1: recall and identify
- Habitat: where an organism lives. Population: all individuals of one species in an area. Community: all populations living together. Ecosystem: community plus abiotic environment.
- Biotic (pond): fish, algae, frogs, insects. Abiotic: water, temperature, light, dissolved oxygen, pH.
- A producer makes its own food from sunlight or chemicals. Examples: grass, algae, gum tree.
- Decomposers break down dead organic matter and return nutrients to the soil. Examples: fungi, bacteria, earthworms.
- Producer: grass. Primary consumer: rabbit. Secondary consumer: fox.
- The arrow shows the direction energy flows — from the food to the feeder (prey to predator).
- About 10% of the energy at one trophic level is transferred to the next; 90% is lost mostly as heat.
- Energy decreases by roughly 90% each link, so after 4 or 5 steps there is not enough to support another level.
- Biotic: trees, birds, insects. Abiotic: sunlight, rainfall, temperature, soil type.
- Examples: cane toads, rabbits, foxes, European carp, lantana.
Tier 2: explain and reason
- Grasshoppers: kg. Lizards: kg. Eagles: kg.
- Because only about 10% of energy transfers up each level, the total biomass shrinks by 90% per step. Top predators have very little energy to share.
- Birds lose insect food → population drops. Pesticide may also accumulate in birds’ bodies through the insects they do eat, causing further harm (bioaccumulation).
- Arrows represent energy flow from prey to predator. Reversed arrows would say the predator feeds the prey, which is biologically false and would mislead anyone trying to predict effects of change.
- Short droughts stress organisms but do not remove species. A keystone species — one whose role disproportionately affects the web — removes many connections at once, so recovery may be impossible without reintroduction.
- They recycle nutrients (carbon, nitrogen, phosphorus) locked in dead matter back into the soil for producers. Without them, dead material would pile up, nutrients would be trapped, and producers would starve.
Tier 3: apply to a novel context
- Accept any web with at least 6 organisms and arrows pointing prey → predator. Example: grass → cricket → magpie → cat; grass → snail → magpie; leaves → possum.
- Abiotic: warmer water holds less dissolved oxygen; evaporation increases. Biotic: cold-water fish die or migrate; algae grow faster, possibly causing blooms.
- Short term: rabbit population surges. Mid term: grass overgrazed, soil erosion, eagles switch to other prey. Long term: grass dies back, rabbits crash from starvation, whole system destabilised.
- 90% of energy is lost at each trophic step. Feeding grain to cattle first loses that 90% before it reaches humans, so only a small fraction of the original grain’s energy becomes beef.
Challenge
- Fertiliser runoff → algal bloom → algae die and are decomposed → bacteria using the dead algae consume dissolved oxygen → oxygen level in water crashes → fish suffocate. Extra food at the bottom does not help if decomposition strips the oxygen fish need.
- IV: presence/absence of introduced fish. DV: native fish population (count per week). Controlled: pond size, water temperature, food added, predators excluded, duration. Use replicate ponds with and without the introduced species; compare native numbers over time.
- Each link transfers only about 10% of the energy (producing the pyramid), but toxins like mercury are not broken down — they are stored in body tissue. A predator eats many prey, so it collects all their mercury, multiplying the concentration even as the energy thins.
- After 1 year: dead plant and animal matter accumulates, nutrient cycling stops, soil becomes impoverished. After 10 years: producers decline from lack of nutrients, herbivores starve, chains collapse upward. After 100 years: the forest is gone, replaced at best by pioneer species that can tolerate nutrient-poor soil; long term, some decomposition may restart from airborne microbes.
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