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3.1.4Unit 3 — Life on Earth

Practical: Competing for Resources

5 min read 6 parts Lesson 5 of 31

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Section 1 of 6Investigating Interspecific Competition

3.1.4 Practical: Competing for Resources
1
Fill identical pots with the same mass of compost and sow five pots with species A alone, five with species B alone and five with a mixture of both.
2
Use the same total number of seeds in each pot, for example ten, so that total density is the same.
3
Water every pot with the same volume of water on the same schedule and keep them all in a well-lit place.
4
Measure growth weekly for two to three weeks, for example height of seedlings and number of leaves.
5
Finally measure the fresh mass of the plants, then compare each species in the mixture with the same species grown alone.

Section 2 of 6Expected Results

If competition matters
  • Each species grows less in the mixture than alone, with lower mean height, leaf number and final mass.
Stronger competitor
  • One species may be affected less by the mixture, while the weaker competitor shows a bigger fall in growth.
No limiting resources
  • If resources are plentiful, growth may be similar in all treatments.
Baseline
  • Each species is compared with its own monoculture because species differ naturally in growth rate.

Section 3 of 6Investigating Intraspecific Competition

Investigating Intraspecific Competition
Design
  • Sow the same species at different densities, such as low, medium and high, in trays of identical size.
Fixed conditions
  • Light, temperature, compost and watering are kept the same, and tray positions are swapped to even out light.
Measurements
  • Count surviving seedlings, measure shoot length and leaf number of a random sample, and find fresh mass.
Expected results
  • Average seedling size falls as density rises because the plants share the same light, water and minerals.

Section 4 of 6Reliability and Limitations

Reliability and Limitations
Light gradient
  • Randomise pot positions or rotate the tray daily so no pot always has the most light.
Soil variation
  • Mix one batch of compost thoroughly and avoid pressing it down in the pots.
Duration
  • A longer investigation shows the effects of competition more clearly than a short one.
Random sampling
  • Select seedlings to measure by random coordinates, which avoids choosing good-looking ones.

Section 5 of 6Worked Example

Question

  • The mean final mass of species A was 4.0 g per pot when grown alone and 2.6 g per pot in the mixture.
  • Calculate the percentage decrease caused by growing it in the mixture.

  1. Decrease in mass = 4.0 − 2.6 = 1.4 g.
  2. Divide by the original mass1.4 ÷ 4.0 = 0.35.
  3. Multiply by 100 to give a percentage.

Answer Growing with species B caused a 35% decrease in the mean mass of species A.

Section 6 of 6Exam Checklist & Tips

3.1.4 – Key Points for the Exam

  • Monoculture A pot or area containing a single species grown alone.
  • Density The number of individuals per unit area.
  • Variable A factor that can change and affect the result.
  • Replicate A repeat of the same treatment to improve reliability.

Common Mistakes

  • Comparing a mixed pot with a different species grown alone instead of the same species.
  • Using different volumes of water in different pots, so that more than one variable changes.
  • Measuring only the tallest seedlings rather than a random sample.
  • Concluding that competition has no effect after a very short experiment.
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