Biology • Year 11 • Module 4 • Lesson 3
Food Chains and Food Webs
Apply food-web structure and trophic concepts to real Australian ecosystem data, population tables and a food-web construction task.
1. Construct a food web for a Kakadu billabong
The organisms below are found in a Kakadu National Park billabong. Use the feeding information provided to construct a food web by drawing arrows in the box below, then answer the questions. 10 marks total
Feeding information:
- Water lilies & algae (T1) — producers, consumed by zooplankton and small fish (herbivorous).
- Zooplankton (T2) — eat algae; eaten by small fish and aquatic insects.
- Aquatic insects (T2) — eat algae and zooplankton; eaten by barramundi and frogs.
- Small fish (T2/T3) — eat algae and zooplankton; eaten by barramundi, great egret and freshwater turtles.
- Frogs (T3) — eat aquatic insects; eaten by saltwater crocodile and file snakes.
- Barramundi (T3/T4) — eat aquatic insects and small fish; eaten by saltwater crocodile.
- Great egret (T4) — eats small fish and frogs; eaten by saltwater crocodile.
- Freshwater turtles (T3) — eat small fish and aquatic plants; eaten by saltwater crocodile.
- File snakes (T4) — eat frogs; eaten by saltwater crocodile.
- Saltwater crocodile (T4/T5) — eats barramundi, frogs, great egret, freshwater turtles, file snakes.
Draw your food web here (arrows from prey to predator):
List organisms and draw labelled arrows, or describe all feeding links systematically.
1.1 Identify two organisms that occupy more than one trophic level and explain why. 4 marks — 2 per organism
1.2 Write the longest food chain you can find in this web, starting from a producer. Label each trophic level. 3 marks — 1 per correct chain element beyond producer
1.3 Name the apex predator in this web and explain how you identified it. 2 marks
2. Interpret population change data
A research team monitored a grassland food web over five years. In Year 2 a disease wiped out 80% of the grasshopper population. The table below shows estimated population sizes. 8 marks total
| Year | Grasses (arbitary units) | Grasshoppers (thousands) | Frogs (hundreds) | Snakes (tens) | Hawks (individuals) |
|---|---|---|---|---|---|
| 1 | 100 | 50 | 30 | 20 | 12 |
| 2 | 100 | 10 | 30 | 20 | 12 |
| 3 | 130 | 12 | 16 | 18 | 11 |
| 4 | 145 | 15 | 11 | 14 | 9 |
| 5 | 160 | 18 | 10 | 11 | 8 |
Food chain: grasses → grasshoppers → frogs → snakes → hawks. This is a single-chain ecosystem (low connectivity).
2.1 Describe the trend in the grass population from Year 2 to Year 5. Explain this trend using trophic principles. 2 marks
2.2 Describe the trend in frog, snake and hawk populations from Year 2 to Year 5. Account for this trend by reference to the food chain. 3 marks
2.3 This ecosystem consists of a single food chain. Predict how the outcome for frogs, snakes and hawks would differ if the ecosystem had a food web with multiple herbivores at T2. Justify your prediction. 3 marks
3. Omnivory in a coastal wetland
A freshwater crab in a coastal wetland feeds on the following: phytoplankton (T1), water snails (T2) and small carnivorous fish (T3). 6 marks total
3.1 List all the trophic levels the crab occupies, one for each type of food it eats. Explain how you determined each trophic level. 3 marks
3.2 Explain why this crab’s feeding behaviour makes the food web more resilient to the loss of any single prey species. 2 marks
3.3 Could you accurately represent the crab’s role in this ecosystem using a single food chain? Explain why or why not. 1 mark
Q1 — Kakadu food web construction and questions
Food web: Correct webs will have arrows from: water lilies & algae → zooplankton; water lilies & algae → small fish; algae → aquatic insects; zooplankton → aquatic insects; zooplankton → small fish; aquatic insects → barramundi; aquatic insects → frogs; small fish → barramundi; small fish → great egret; small fish → freshwater turtles; frogs → saltwater crocodile; frogs → file snakes; barramundi → saltwater crocodile; great egret → saltwater crocodile; freshwater turtles → saltwater crocodile; file snakes → saltwater crocodile.
1.1 Example: Barramundi occupies T3 when eating aquatic insects (T2 → T3) and T4 when eating small fish that themselves ate zooplankton (T3 → T4). This is omnivory — feeding on prey from different trophic levels. Small fish similarly occupy T2 (eating algae) and T3 (eating zooplankton). Award 2 marks per organism: 1 mark for naming trophic levels, 1 mark for explaining why (they eat prey at different levels).
1.2 Longest chain: algae (T1) → zooplankton (T2) → small fish (T3) → barramundi (T4) → saltwater crocodile (T5). Accept any valid 5-level chain. Award 1 mark per correct element beyond the producer, up to 3.
1.3 Saltwater crocodile is the apex predator because no other organism in the web preys on it — there are no arrows pointing from the crocodile to any other organism. It occupies T4/T5.
Q2 — Population data interpretation
2.1 Grass population increased steadily from 100 to 160 units (Years 2–5). This occurred because grasshoppers (the primary consumer of grasses) declined dramatically due to disease, reducing herbivore pressure on grasses. Fewer grasshoppers meant less grass was consumed, so grass biomass increased — a bottom-up release from predation.
2.2 Frog, snake and hawk populations all declined steadily (frogs 30→10; snakes 20→11; hawks 12→8) from Year 2 to Year 5. This is a trophic cascade: the collapse of grasshoppers (the frogs’ primary prey) starved frogs, causing their populations to fall. Fewer frogs reduced food for snakes, which then fell, in turn reducing food for hawks. Each level declined sequentially because the single-chain ecosystem has no alternative prey at any level.
2.3 In a food web with multiple T2 herbivores (e.g. grasshoppers, caterpillars, mice), frogs could switch to eating caterpillars or other insects when grasshoppers declined. This alternative pathway would buffer the loss, so frog, snake and hawk populations would decline less sharply or stabilise rather than experiencing the cascading decline seen here. The food web’s connectivity provides resilience absent from a single chain.
Q3 — Omnivory in a coastal wetland
3.1 When eating phytoplankton (T1), the crab is a primary consumer — T2. When eating water snails (T2), the crab is a secondary consumer — T3. When eating small carnivorous fish (T3), the crab is a tertiary consumer — T4. Award 1 mark per trophic level correctly identified with clear reasoning (what the prey level is, so crab is one level higher).
3.2 If any one prey species declines (e.g. phytoplankton reduced by a bloom die-off), the crab can maintain its energy intake by switching to water snails or small fish. This prevents the crab population from crashing, which in turn prevents any predator of the crab from losing food. The crab acts as an alternative pathway, increasing food web resilience.
3.3 No. A single food chain can only show the crab at one trophic level, missing the multi-level feeding that defines omnivory. To accurately represent its role, a food web with three separate arrows (one per prey type) is required.