Food Webs, Energy Flow and Ecosystem Change

An ecosystem includes living organisms and the non-living environment interacting. A food web is a scientific model that connects feeding relationships so we can trace the transfer of matter and energy and make reasoned predictions about population change.

By the end of this topic, you should be able to:

• identify producers, consumers, decomposers and microorganisms in food webs
• read food-web arrows in the correct direction
• explain why matter cycles but energy flows through and leaves ecosystems as heat
• trace direct and indirect population effects
• distinguish abiotic from biotic factors
• predict effects of pollinator loss, predator loss, drought, habitat change and introduced species
• state strengths and limitations of food-web and trophic-energy models
• discuss First Nations Australian ecosystem-management knowledge respectfully and in place-specific context

A food chain shows one feeding pathway. A food web links several pathways because most organisms eat, or are eaten by, more than one species.

Arrow direction

In grass → rabbit, the arrow points from the organism eaten to the consumer. It shows matter and chemical energy moving from grass to rabbit. It does not simply show which organism chases another.

Producers capture incoming energy and store some of it as chemical energy in organic matter. Consumers receive matter and energy by feeding. Decomposers break down dead material and release nutrients that can be taken up again by producers.

Why less energy reaches higher trophic levels

Organisms use energy for respiration, movement, growth, repair and maintaining life. Much of this energy is dissipated as heat, and not all biomass is eaten or digested. Therefore less usable energy is available to the next trophic level.

A simplified model sometimes uses an approximate 10% transfer between trophic levels. This is useful for estimating why top-predator populations are relatively small, but it is not an exact rule: transfer efficiency varies among organisms and ecosystems.

Start with the changed organism, identify its direct interactions, then trace indirect effects through the web.

A chain of effects across several trophic levels is called a trophic cascade. Strong answers use words such as “may”, “likely” and “depending on” because alternative foods, migration, disease, feedback and time delays can change the outcome.

Abiotic factors are non-living conditions: rainfall, temperature, water availability, light, fire, soil nutrients and habitat structure. Biotic factors involve living organisms and interactions: predators, competitors, disease, pollinators and introduced species.

ecosystem, population, producer, consumer, decomposer, microorganism, trophic level, food chain, food web, matter cycle, energy flow, abiotic factor, biotic factor, competition, invasive species, trophic cascade, resilience, model limitation

Aboriginal and Torres Strait Islander ecological knowledge is diverse and connected to particular Peoples, Countries/Places, languages and cultural authority. Community-led management can combine long-term observation of Country, seasonal knowledge, local species relationships and contemporary monitoring to understand invasive-species effects on food webs and resources.

Specific examples must be accurately sourced. Do not invent a “traditional” response or write as though every community uses the same practice. A responsible case study identifies the place, species, source and community authority where that information is publicly available.

First Nations Australian fire management includes diverse, place-specific practices developed over long relationships with Country. Fire timing, frequency, intensity, season, weather, fuel, purpose and landscape position can influence vegetation structure, shelter, food availability and the distribution of fauna.

Patchy or mosaic fire histories can create varied habitat conditions, but ecological outcomes are not identical everywhere. Avoid the claim that one type of fire always increases biodiversity or prevents intense fire. Use community-approved, place-aware evidence for specific claims.

Food webs are useful because they make interconnected pathways visible and help identify direct and indirect effects. However, a basic food web usually does not show:

• how strong each interaction is
• population sizes or rates of change
• seasonal diet shifts and migration
• disease, shelter or breeding-site needs
• time delays, feedback loops or changing behaviour
• all abiotic conditions

A model-based prediction should therefore be treated as a reasoned, testable expectation rather than a guaranteed forecast. Field observations and population data can test and improve the model.

• “Arrows point to what is eaten.” They point from food to the consumer receiving matter and energy.
• “Energy is recycled by decomposers.” Decomposers recycle matter; energy is dissipated as heat.
• “Decomposers belong only at the end.” They act on dead material and waste from all trophic levels.
• “Removing one species has one effect.” Food webs contain indirect pathways, alternatives and feedback.
• “A food web predicts exactly.” It is a simplified model with limitations.
• “The 10% rule is exact.” It is an approximate teaching model.
• “More complex always means stable.” Alternative pathways may help resilience, but disturbance, interaction strength and species roles also matter.
• “First Nations fire management is one national method.” Knowledge and practices are diverse and place-based.

1. A drought reduces grass biomass. Predict two direct and two indirect food-web effects.
2. A snake eats frogs and mice. Frog numbers fall. Explain two possible snake responses and identify the data needed to choose between them.
3. An introduced plant replaces native grasses but provides poor shelter. Predict why herbivores and small ground animals may respond differently.
4. Explain why a food-web model should be combined with field evidence when planning invasive-species management.
5. Evaluate the claim “one successful local burn proves fire always increases biodiversity”.

Content description — AC9S7U02: use models, including food webs, to represent matter and energy flow in ecosystems and predict the impact of changing abiotic and biotic factors on populations.

Elaboration breadth reviewed:

• E1: food-web feeding relationships and microorganisms
• E2: energy pathways into and out of ecosystems
• E3: effects of removing pollinators or predators
• E4: seasonal change, habitat destruction and introduced species
• E5: place-aware First Nations Australian responses to invasive species
• E6: place-aware consideration of fire management and species distribution

Elaborations are teaching examples and breadth checks; the content description is the required learning target.

Comparable middle-years science internationally includes food chains and food webs, transfer of energy and matter, decomposers, ecological interactions, population responses to environmental change and evaluating ecosystem models. Exact terminology and year placement vary; AC9S7U02 is the authoritative alignment for this resource.

1. What does a food web show?
2. Which direction do arrows point?
3. What roles do producers, consumers and decomposers have?
4. How do microorganisms contribute to matter cycling?
5. Why does matter cycle but energy not cycle?
6. Why is less energy available at higher trophic levels?
7. What is a trophic cascade?
8. How can predator removal affect producers?
9. How can pollinator loss affect higher consumers?
10. Distinguish abiotic and biotic factors.
11. How can drought affect a food web?
12. How can an introduced species alter populations?
13. What is one strength and two limitations of a food web?
14. Why must First Nations ecological examples be place-aware and accurately sourced?
15. Why are ecosystem predictions usually conditional rather than certain?

• AC9S7U01 — Biological classification and dichotomous keys
• AC9S7H01 — Evidence and changes in scientific knowledge
• AC9S7H02 — Cultural perspectives and scientific knowledge
• AC9S7H03 — Scientific responses and contemporary issues
• AC9S7I01 — Questions and predictions
• AC9S7I02 — Planning reproducible investigations