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Ecology and the Environment

The organism in the environment — Edexcel Linear - OL Revision Notes & Topical Questions

5 min read 12 parts Lesson 6 of 10 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Lesson summary

Abiotic and Biotic Factors: Evidence and Data Interpretation — EDEXCEL LINEAR - OL notes: Explain how abiotic and biotic factors affect populations and interpret data from investigations such as wheat grown at different carbon dioxide concentrations.An abiotic factor is a non-liv…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Competition
  • Indicator species

Part 1 of 5Abiotic factors

An abiotic factor is a non-living factor in the environment, such as temperature, light intensity and wind speed. Abiotic factors affect population size and distribution.

FactorEffectExample
Light intensityNeeded for photosynthesis; low light slows growth and reduces food for herbivoresOnly shade-tolerant plants live on a dark forest floor
TemperatureAffects the rate of enzyme-controlled reactions; too hot or cold lowers survival and reproductionPolar bears are limited to cold Arctic regions
Water availabilityDetermines which plants and animals can surviveCacti store water in deserts; amphibians need moist places
Soil quality (minerals, pH)Poor or acidic soils limit plant growth and the whole food webHeather grows well on acidic moorland soils
Oxygen in waterSome aquatic animals need high oxygen levelsIn warm stagnant ponds oxygen falls and fish die or move
PollutionKills sensitive species and reduces biodiversityFew lichens grow where there is air pollution; their presence indicates clean air

Part 2 of 5Worked data example: carbon dioxide and wheat

Scientists studied the effect of carbon dioxide concentration on wheat.

1
Five glasshouses each contained 100 wheat plants with carbon dioxide at 0.03%, 0.06%, 0.10%, 0.15% and 0.20%.
2
Light intensity, temperature and water were kept the same.
3
After two months the dry biomass was measured and the mean calculated for each concentration.
4
The scientists concluded that increasing carbon dioxide increases the growth of wheat.

Evaluating the conclusion (a five-mark answer uses five of these points).

Worked data example: carbon dioxide and wheat
Support: mean biomass increases with carbon dioxide concentration; the greatest increase is between 0.03% and 0.10%; carbon dioxide is a reactant in photosynthesis,
  • so more of it allows a higher rate and more glucose for growth.
Limitations: the graph levels off at higher concentrations, suggesting another factor such as light, temperature or nutrients becomes limiting; the conclusion is only true up to an optimum; only one species and only five concentrations were tested;
  • and glasshouses are not natural field conditions.

Part 3 of 5Biotic factors

A biotic factor is a living factor such as competition, predation and disease.

FactorEffectExample
Food availabilityMore food lets populations grow; scarce food means only the best adapted surviveRainforests support many species; deserts few
New predatorsCan disrupt balance and make prey populations declineRed foxes introduced to Australia reduced native small mammals and birds
New pathogensPopulations with no resistance may fall sharplyAsh dieback fungus has infected millions of European ash trees
CompetitionThe better-adapted species outcompetes the otherGrey squirrels outcompete red squirrels and carry a virus they resist

Part 4 of 5Using evidence in exams

When a question gives a chart, graph or table, quote specific figures and describe the trend (both the description and the biological explanation earn marks).

Using evidence in exams
For the squirrel question: the red squirrel population rose at first and then fell after grey squirrels were introduced in year 6,
  • while the grey squirrel population increased; grey squirrels outcompete red squirrels for food and habitat, are better adapted,
    • and may spread disease,
    • (for example they eat a wider range of foods)
  • so by the end the red squirrel population is very small.
Explanations about "better adapted" must be linked to a specific advantage.

Part 5 of 5Worked examples

  1. Explain why few lichens grow near a busy road. [2]2 marks

  2. A student concludes that more carbon dioxide always means more growth in wheat. Evaluate this. [3]3 marks

  3. Explain why the red squirrel population fell after grey squirrels arrived. [3]3 marks

  4. Give two abiotic factors that affect plant growth. [2]2 marks

Key terms

TermMeaning
Abiotic factorNon-living part of the environment, such as temperature.
Biotic factorLiving factor such as predation or competition.
CompetitionStruggle between organisms for the same resource.
PathogenMicroorganism that causes disease.
Indicator speciesSpecies whose presence shows the condition of the environment, such as lichens.
BiomassMass of living material.
Limiting factorFactor in shortest supply that limits the rate of a process.

Avoid these mistakes

1
Describing a graph without numbers — always quote figures.
2
Saying the species "died out" — say the population decreased.
3
Confusing abiotic with biotic — abiotic is non-living, biotic is living.
4
Concluding a factor always increases growth — another factor can become limiting.

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