3.1Unit 2 · Ecology & the Environment
The Organism in the Environment — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 7 parts Lesson 2 of 5 1 quiz
Lesson summary
Define population, community, habitat and ecosystem.Random sampling avoids bias; placing quadrats where you choose is biased.More quadrats give a more representative (reliable) estimate.A 50 cm × 50 cm quadrat has an area of 0.25 m2.Biodiversity is higher in stable habitats such …
- Investigating population size
- Reliability
- Investigating distribution
- Abiotic factors
- How temperature affects organisms
- Introduced species
Section 1 of 7The Organism in the Environment
Essential knowledge
Key terms
Key terms a population is one species in an area; a community is all the populations in that area; a habitat is where an organism lives; an ecosystem is the community plus its physical environment, interacting together.
Investigating population size
Investigating population size it is usually impossible to count every organism, so we sample. Random quadrats are placed using coordinates on a grid. The mean number per quadrat is scaled up to the whole area. For plants that are hard to count, like grass, percentage cover can be used.
Reliability
Reliability sample many quadrats, use the same quadrat size, and sample at the same time of year. Random placement prevents bias.
Biodiversity
Biodiversity the variety of species, and of individuals within species, in an area. It can be compared by counting the number of species in quadrats in two habitats. High biodiversity makes ecosystems more stable, as there are more food sources and interactions.
Investigating distribution
Investigating distribution lay a tape across a habitat with a change in conditions (e.g. from shade under a tree into open field, or from low to high tide on a shore). Place quadrats at regular intervals (a belt transect) and record species present and abiotic factors such as light intensity, soil moisture or pH.
Abiotic factors
Abiotic factors light intensity, temperature, water availability, carbon dioxide and oxygen levels, soil pH and mineral content, wind. Each affects which organisms can survive and grow in an area.
Biotic factors
Biotic factors availability of food, predators, pathogens (disease), competition between species and within a species, and new species arriving. For example, an increase in predators reduces prey numbers, which then lowers predator numbers.
How temperature affects organisms
Temperature affects the rate of enzyme-controlled reactions. Too cold or too hot and enzymes work poorly, so survival and reproduction fall. This limits where species can live, e.g. animals adapted to polar cold cannot live in warm regions.
Oxygen and pollution as abiotic factors
In water, low dissolved oxygen (e.g. in warm, still ponds) kills fish or makes them move away. Pollution kills sensitive species; lichens are sensitive to sulfur dioxide, so few lichens indicates polluted air.
Introduced species
A species brought into a new area can upset the community. A new predator can reduce native prey; a better-adapted competitor can outcompete a native species, e.g. introduced grey squirrels have replaced red squirrels in much of the UK; a new pathogen can kill organisms with no resistance.
Quadrat limitations
Organisms hidden under other plants can be missed, so move leaves aside carefully. Species may be misidentified, so use an identification key. Take at least 10 randomly placed quadrats so the sample is representative.
Section 2 of 7Process & sequence
1
Lay out two tape measures at right angles to form a grid over the area.
2
Use a random number generator to produce coordinates.
3
Place a quadrat (e.g. 0.25 m2) at each coordinate.
4
Count the number of the chosen species in each quadrat.
5
Repeat for many quadrats (e.g. 10–20) and calculate the mean per quadrat.
6
Estimate population = mean per quadrat × (total area ÷ quadrat area).
Estimating population sizewrite these steps in this order; each step is a marking point.
1
Mark out grid with tapes
2
Generate random coordinates
3
Place quadrats
4
Count organisms per quadrat
5
Calculate mean
6
Scale up to total area
Section 3 of 7Compare
| Type | Factor | Example effect |
|---|---|---|
| Abiotic | Light intensity | Fewer plants grow under dense tree canopy |
| Abiotic | Temperature / water | Determines which species survive in deserts |
| Abiotic | Soil pH and minerals | Some plants only grow in acidic soils |
| Biotic | Predation | More predators reduce prey population |
| Biotic | Competition / disease | Species compete for light, food, mates; disease reduces numbers |
| Method | Used to study | How |
|---|---|---|
| Random quadrats | Population size / abundance | Coordinates from random number generator on a grid |
| Belt/line transect | Distribution along a gradient | Quadrats at regular intervals along a tape |
| Percentage cover | Plants hard to count | Estimate % of quadrat squares covered |
Section 4 of 7Worked examples
12400 daisies
A student placed 10 random 0.25 m2 quadrats in a 200 m2 field and counted a total of 30 daisies. Estimate the daisy population.
- Mean per quadrat = 30 ÷ 10 = 3 daisies.
- Number of quadrats that fit in the field = 200 ÷ 0.25 = 800.
- Population ≈ 3 × 800 = 2400 daisies.
2Explain why fewer plants grow close to the trunk of a large tree.
- The tree canopy reduces light intensity, so plants beneath photosynthesise less.
- The tree roots also compete for water and mineral ions.
- These abiotic and biotic factors reduce growth and survival close to the trunk.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Population · Community · Habitat · Ecosystem · Quadrat · Biodiversity · Abiotic factor · Biotic factor · Transect
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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