AC9S7U02 • Homework
Food Webs, Energy Flow and Ecosystem Change
Use food-web evidence and correct scientific vocabulary. For prediction questions, identify the changed factor, trace direct effects before indirect effects, and avoid claiming that one outcome is guaranteed when the model allows alternatives.
Part A — Short response
- State the role of a producer and the role of a decomposer in an ecosystem. (2)
- Explain why less usable energy is available at each successive trophic level. (2)
- Describe one likely effect of removing a predator from a simple food chain. (2)
- Name one abiotic factor and explain one way it could change a population. (2)
- Explain how decomposer microorganisms contribute to matter cycling. (2)
Part B — Extended response
grass → grasshopper → frog → snake
grass → rabbit → fox
grasshopper → bird → fox
dead material and wastes from all organisms → bacteria and fungi → mineral nutrients → grass
- Analyse the model. Trace one pathway of energy from the Sun to a predator, explain the arrow direction, and describe how matter returns to the producer pathway through microorganisms. (5)
- Pollinators disappear from an ecosystem containing flowering plants, herbivores and predators. Predict a direct effect on plants and two possible indirect effects on consumers. Explain your reasoning. (5)
- Choose one change — seasonal drought, habitat clearing or introduction of a competing species. Explain how it changes at least one abiotic or biotic factor and trace a three-step population response through a food web. (5)
- Use an energy-flow model to explain why ecosystems usually support fewer top predators than producers. Include the approximate “10% transfer” idea and one limitation of that simplified model. (5)
- Explain how a community-led, place-based response to an invasive species could use knowledge of Country together with population monitoring and food-web evidence. State why a specific cultural example must be accurately sourced rather than invented. (5)
Part C — Enrichment
11. Ecosystem-change analysis. A woodland experiences lower rainfall, a decline in a pollinator, an introduced herbivore and a change in fire regime. Develop a reasoned explanation of how these abiotic and biotic changes could affect producers, consumers, decomposers, matter cycling, energy pathways and population stability. Include at least one trophic cascade, state two limitations of a food-web model, and explain how accurately sourced First Nations Australian fire-management knowledge could contribute to a place-specific management plan.
Answers and marking guide
1
Producer: makes organic matter using an energy source, usually sunlight. Decomposer: breaks down dead matter/waste and releases nutrients.
2
Organisms use energy for metabolism, movement and maintenance, and much is dissipated as heat; therefore less usable energy is transferred onward.
3
Prey or herbivore numbers may rise because predation pressure is reduced; later effects can include increased grazing and producer decline.
4
Examples: rainfall, temperature, water availability, light, fire or soil conditions. The response must link the factor to resources, survival or reproduction.
5
Many bacteria and fungi break down dead organic material, returning mineral nutrients to soil or water so producers can use them again.
6
Example energy path: Sun → grass → grasshopper → frog → snake. Food-web arrows point from food to consumer. Dead material from every level is decomposed; nutrients return to the environment and can be taken up by grass. Matter cycles, but energy is dissipated and requires ongoing input.
7
Direct: reduced reproduction/seed production in pollinator-dependent plants. Indirect: less plant food for herbivores and later less prey/energy for predators. Alternative plants or pollinators make the size of change uncertain.
8
Responses vary but must identify the factor correctly, trace at least three linked effects and distinguish direct from indirect changes. Example drought: less water → lower plant growth → fewer herbivores → later predator decline.
9
Energy decreases through transfers, so less is available to support biomass/populations at higher levels. A simplified model may use about 10% transfer per level, but actual efficiencies vary among organisms and ecosystems.
10
A strong response includes community authority, local seasonal/ecological knowledge, monitoring of native and invasive populations, and food-web evidence to judge effects. Practices and knowledge vary by People and Country/Place; invented examples are scientifically and culturally unreliable.
11
Full-credit responses trace several connected pathways, distinguish matter from energy, include a justified trophic cascade, acknowledge alternative foods/time delays/interaction strengths or omitted habitat factors, and treat fire management as diverse, place-based knowledge requiring reliable community-approved sources.