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
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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
Section 1 of 12Abiotic 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.
| Factor | Effect | Example |
|---|---|---|
| Light intensity | Needed for photosynthesis; low light slows growth and reduces food for herbivores | Only shade-tolerant plants live on a dark forest floor |
| Temperature | Affects the rate of enzyme-controlled reactions; too hot or cold lowers survival and reproduction | Polar bears are limited to cold Arctic regions |
| Water availability | Determines which plants and animals can survive | Cacti store water in deserts; amphibians need moist places |
| Soil quality (minerals, pH) | Poor or acidic soils limit plant growth and the whole food web | Heather grows well on acidic moorland soils |
| Oxygen in water | Some aquatic animals need high oxygen levels | In warm stagnant ponds oxygen falls and fish die or move |
| Pollution | Kills sensitive species and reduces biodiversity | Few lichens grow where there is air pollution; their presence indicates clean air |
Section 2 of 12Worked 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.
Section 3 of 12Biotic factors
A biotic factor is a living factor such as competition, predation and disease.
| Factor | Effect | Example |
|---|---|---|
| Food availability | More food lets populations grow; scarce food means only the best adapted survive | Rainforests support many species; deserts few |
| New predators | Can disrupt balance and make prey populations decline | Red foxes introduced to Australia reduced native small mammals and birds |
| New pathogens | Populations with no resistance may fall sharply | Ash dieback fungus has infected millions of European ash trees |
| Competition | The better-adapted species outcompetes the other | Grey squirrels outcompete red squirrels and carry a virus they resist |
Section 4 of 12Using 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.
Section 5 of 12Worked examples
Explain why few lichens grow near a busy road. [2]2 marks
Lichens are sensitive to air pollutants such as sulfur dioxide, so pollution kills them. Their absence indicates polluted air.
A student concludes that more carbon dioxide always means more growth in wheat. Evaluate this. [3]3 marks
Growth rose up to about 0.10% then levelled off, so another factor became limiting. The conclusion is only true up to an optimum, and only wheat was tested.
Explain why the red squirrel population fell after grey squirrels arrived. [3]3 marks
Grey squirrels competed for the same food and nest sites and were better adapted. They may also have carried a virus the red squirrels could not resist.
Give two abiotic factors that affect plant growth. [2]2 marks
Light intensity and water availability (also temperature, soil minerals, pH).
Section 6 of 12Abiotic and biotic factors
Abiotic factors
Abiotic factors are non-living environmental conditions, such as light intensity, temperature, water availability, soil pH and carbon dioxide concentration. They affect where organisms can survive and how quickly they grow or reproduce.
Biotic factors
Biotic factors arise from living organisms. Examples include food supply, predation, competition and pathogens. A change in any of these factors can alter the size or distribution of a population.
Section 7 of 12Carbon dioxide and plant growth
Carbon dioxide and plant growth
Carbon dioxide is a raw material for photosynthesis. If it is in short supply, increasing its concentration can raise the rate of photosynthesis, produce more glucose and increase plant biomass. This explanation links the environmental factor to the observed growth.
In one investigation, wheat plants were grown in glasshouses at several carbon dioxide concentrations while light, temperature, water and growth time were controlled.
- Mean dry biomass rose most at lower concentrations and then levelled off.
- The levelling off suggests another resource became limiting,
- so a claim that extra carbon dioxide will always increase growth would be too broad.
Section 8 of 12Living factors and population change
Living factors and population change
A larger food supply can improve survival and reproduction.
- A newly introduced predator can reduce prey numbers.
- A novel pathogen may spread through a population with little resistance.
- Where two species need the same food or nesting sites, the better competitor can reduce the other species' population.
Population change usually has more than one possible cause. When interpreting a graph, describe the trend first, then propose a mechanism supported by the context. A statement such as “the species is better adapted” needs a named advantage, such as a wider food range or greater disease resistance.
Section 9 of 12Worked example: squirrels
Worked example: squirrels
Suppose grey squirrels enter a woodland in year 6. After their introduction, their numbers rise while red squirrel numbers fall. That is a description of the data; a complete answer must also explain why the populations may change.
Grey squirrels can compete with red squirrels for food and nesting places.
- A wider range of usable food can give grey squirrels an advantage, and disease carried by grey squirrels may further harm red squirrels.
- These are plausible biological mechanisms;
- a graph showing opposite trends alone does not prove which mechanism acted.
Section 10 of 12Answering ecological data questions
For a “comment on” question, state when a change begins, describe the direction of change in both populations, and quote relevant values if the graph provides them. Then explain the trends using competition, predation or disease as appropriate.
Check:
Check Why might wheat biomass stop increasing even while carbon dioxide continues to rise? Answer: A different factor, such as light, water or mineral availability, may become limiting.
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Abiotic factor | Non-living part of the environment, such as temperature. |
| Biotic factor | Living factor such as predation or competition. |
| Competition | Struggle between organisms for the same resource. |
| Pathogen | Microorganism that causes disease. |
| Indicator species | Species whose presence shows the condition of the environment, such as lichens. |
| Biomass | Mass of living material. |
| Limiting factor | Factor in shortest supply that limits the rate of a process. |
Section 12 of 12Exam technique
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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