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Biology · 7. Ecology and populations

Ecosystems, energy flow and population change

Analyse food webs, productivity and population data without confusing correlation with causation.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

Why do populations change over time, and what evidence can explain the cause?

Curriculum coverage

Vietnam Biology 7 and 12; Cambridge IGCSE/A-Level ecology.

Model scope and limitations

Lotka–Volterra assumes a homogeneous environment, no migration and few limits; natural data are usually more complex.

Learning objectives

  • Distinguish food chains, webs and trophic levels.
  • Explain energy transfer losses.
  • Interpret and critique population models.

Prerequisite knowledge

  • Photosynthesis, respiration and graph reading.

Core knowledge

Energy flow

Energy flows one way and much is transferred through respiration, heat, waste or uneaten material.

Nutrient cycling

Elements are recycled through organisms, waste, decomposition and the abiotic environment.

Population lag

Predator peaks often lag prey peaks because reproduction and responses take time.

Worked example

Producers contain 20,000 kJ and primary consumers receive 2,400 kJ: transfer efficiency = 2,400/20,000 × 100 = 12%.

Misconceptions to avoid

  • Food-chain arrows show energy transfer from food to consumer.
  • Two changing population curves alone do not prove one species caused the other to change.

Virtual experiment procedure: Predator–prey dynamics

  1. Choose initial populations and run a baseline long enough.
  2. Change one parameter such as predation rate; record amplitude, period and lag.
  3. Repeat and compare results with a pre-run prediction.

Safety and cautions

  • The model does not justify releasing or removing organisms in nature; ecological interventions require expert assessment.

Evidence to collect

  • A two-population graph, lag estimate and explanation naming at least two model limitations.
Open the virtual experiment

Diagnostic check

1. The first trophic level is usually?

  1. Producers
  2. Apex predators
  3. Secondary consumers
Show answer and explanation

Correct answer: Producers

Producers introduce energy into the food web.

2. When does the predator peak usually occur relative to prey?

  1. After a lag
  2. Always much earlier
  3. Always exactly together
Show answer and explanation

Correct answer: After a lag

Food increases first, then predator population responds.

Application practice

1. 5,000 kJ to 500 kJ gives what efficiency?

  1. 10%
  2. 100%
  3. 1%
Show answer and explanation

Correct answer: 10%

500/5000 × 100 = 10%.

2. Which factor is density-dependent?

  1. Competition for food
  2. A major storm
  3. Volcanic eruption
Show answer and explanation

Correct answer: Competition for food

Competition often intensifies as population density rises.

3. How can causation be tested better than viewing two curves?

  1. Replicates, controls or a suitable experiment
  2. Choose the prettiest segment
  3. Ignore environmental factors
Show answer and explanation

Correct answer: Replicates, controls or a suitable experiment

Controlled comparison helps exclude alternative explanations.

Academic references

  1. OpenStax Biology 2e — peer-reviewed foundational biology

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer