Pearson Edexcel International A Level Biology: Productivity, Populations, Niche and Succession (YBI11) – Revision Notes

Short-form revision for statements 5.9 to 5.15 of Topic 5 in the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2, with Core Practical 11. Topic 5 belongs to IA2, and no tiers apply. The study guide gives the full explanations and the practice questions test recall. Related pages: course hub, checklist and free diagnostics.

5.9 Productivity

Term	Meaning	Unit
GPP	Total rate of energy capture by producers in organic molecules	kJ m⁻² year⁻¹
R	Energy released by plant respiration	kJ m⁻² year⁻¹
NPP	Energy stored in new plant biomass: GPP − R	kJ m⁻² year⁻¹

• NPP = GPP − R. GPP = NPP + R. R = GPP − NPP.
• NPP is what is available to primary consumers and decomposers.
• Dry mass can replace energy: g m⁻² year⁻¹.

Method in steps

1. Write the equation you need.
2. Put in both values with the same unit and time period.
3. Calculate and give the unit.
4. If asked, express R or NPP as a percentage of GPP.

Reminder. GPP is 9 600 kJ m⁻² year⁻¹ and R is 3 360. NPP = 9 600 − 3 360 = 6 240 kJ m⁻² year⁻¹. R is 35.0% of GPP and NPP is 65.0%.

5.10 Efficiency of transfer

efficiency (%) = (energy or biomass passed on ÷ energy or biomass available) × 100

Reminder. NPP of 6 240 kJ m⁻² year⁻¹ gives herbivores 780. Herbivores give carnivores 78.

• Producers to herbivores: 780 ÷ 6 240 × 100 = 12.5%.
• Herbivores to carnivores: 78 ÷ 780 × 100 = 10.0%.
• Producers to carnivores: 78 ÷ 6 240 × 100 = 1.25%.

Why transfers are inefficient

• Not all of the organism is eaten (roots, bones, shells).
• Not all that is eaten is digested: egestion in faeces.
• Respiration releases energy as heat.
• Excretory products carry away energy.
• Only the remainder forms new tissue.

Long food chains are rare because so little reaches the top.

5.11 Key terms

Term	Definition
Population	Organisms of one species in one place at one time
Community	Every population of every species that shares a place
Habitat	The place an organism lives, with its surroundings
Ecosystem	A community plus the abiotic factors it interacts with

5.12 Biotic and abiotic factors

Abiotic	Biotic
Temperature, light, water, pH, salinity, oxygen, wind	Competition, predation, grazing, parasitism, disease, food supply

• Intraspecific competition: between members of the same species.
• Interspecific competition: between different species.
• A good explanation names the factor, says how it acts and links it to survival or reproduction.

5.13 Niche

• Niche = the role of a species in its habitat: its abiotic tolerances, resources used and interactions with other species.
• Distribution: a species occurs where its requirements are met.
• Abundance: high where resources are plentiful and competition is low.
• Two species with the same niche compete for a limiting resource. One tends to outcompete the other.
• Coexistence is possible when the species use different resources or different parts of the habitat.

5.14 Core Practical 11: ecology of a habitat

Method	Use
Random quadrats	Estimate abundance across an even habitat
Line or belt transect	Show change along a gradient, such as distance from a path or shore
Percentage cover	Plants that cannot be counted individually, such as grass
Frequency	Percentage of quadrats in which the species occurs

Method in steps (random sampling)

1. Lay two tapes at right angles to mark the area.
2. Use random numbers as coordinates.
3. Place the quadrat at each coordinate and record the abundance.
4. Take enough quadrats to give a reliable mean.
5. Measure abiotic factors at each point, in the same way each time.

Abiotic measurements

• Light: light meter.
• Soil moisture: mass lost on drying a sample.
• pH: soil probe or indicator.
• Temperature: thermometer.

Reminder. 20 quadrats of 1 m² are counted. The total is 140 individuals. Mean = 140 ÷ 20 = 7 per m². The area is 350 m², so the estimate is 7 × 350 = 2 450 individuals.

Another reminder: if the quadrat is 0.25 m² and six quadrats have 18 individuals in total, the mean is 3 per quadrat. Density = 3 ÷ 0.25 = 12 per m².

Planning points

• Independent variable: the distance or the abiotic factor. Dependent variable: the abundance. Control the other factors you can.
• State a null hypothesis: no relationship between the two.
• A correlation does not show cause.
• Treat the habitat with care: replace stones and avoid trampling.

5.15 Succession

Term	Meaning
Colonisation	The arrival of the first species, called pioneers
Primary succession	Starts on bare ground with no soil
Secondary succession	Starts where soil is already present
Climax community	Relatively stable community, determined mainly by climate

Sequence (temperate, from bare rock)

1. Pioneers such as lichens and mosses.
2. Weathering and dead material form thin soil.
3. Grasses and small herbs.
4. Shrubs.
5. Trees: the climax community.

Trends along the sequence

• Soil gets deeper, with more humus, nutrients and water-holding capacity.
• The abiotic environment becomes less extreme.
• Species diversity and biomass increase.
• Food webs become more complex.
• Each stage changes conditions so that the next stage can replace it.

Answer frameworks

Describe a trend in data. Say what happens, quote values with units from the start and end, and note any peak or plateau. Add a calculation if it helps, such as a percentage change.

Explain a distribution. Identify the factor, say how it acts on the organism, then link it to survival, growth or reproduction. Link it to niche when two species are involved: say which resource they compete for.

Explain a change during succession. Name the organisms present, say how they alter the soil, light or water, and say why that favours the next group and disadvantages the previous one.

Evaluate a sampling method. Comment on bias in placement, sample size, whether the quadrat size suits the organisms, whether abiotic factors were measured at the same points, and whether the data can show cause or only correlation.

Suggest a hypothesis. Link one factor to one measured outcome, for example: grass cover decreases as light intensity decreases.

Must-know distinctions

• GPP is before respiration; NPP is after.
• Population is one species; community is all species.
• Habitat is a place; niche is a role.
• Density is per unit area; frequency is percentage of quadrats; cover is area covered.
• Primary succession has no soil at the start; secondary succession does.
• Abiotic is non-living; biotic is living.

Quick self-test

1. A forest has GPP of 21 000 kJ m⁻² year⁻¹ and R of 8 400. Calculate NPP.
2. Give the equation linking GPP, NPP and R.
3. Of 900 kJ of energy in an insect population, 45 kJ passes to the birds that feed on it. Calculate the efficiency.
4. Give two reasons why not all the energy in a herbivore passes to its predator.
5. Distinguish between a habitat and a niche.
6. State two abiotic factors and two biotic factors that affect the distribution of a plant.
7. Explain why a random method is used to place quadrats.
8. Six quadrats of 0.25 m² contain 18 individuals in total. Calculate the density per m².
9. State when percentage cover is used rather than counting.
10. What is a pioneer species?
11. Explain why soil depth increases during succession.
12. State what determines the type of climax community.

Answers

1. NPP = 21 000 − 8 400 = 12 600 kJ m⁻² year⁻¹.
2. NPP = GPP − R.
3. 45 ÷ 900 × 100 = 5.0%.
4. Any two: part not eaten; part egested; part lost in respiration as heat; part lost in excretion.
5. A habitat is the place where an organism lives; a niche is its role there, including resources used and interactions with other species.
6. For example, light and soil pH; competition and grazing.
7. It avoids bias in where quadrats are placed, so the sample represents the whole area.
8. 18 ÷ 6 = 3 per quadrat; 3 ÷ 0.25 = 12 per m².
9. For plants that are hard to count as separate individuals, such as grass.
10. The first species to colonise bare ground, adapted to harsh conditions.
11. Weathering of rock, and the death and decay of organisms, add mineral particles and humus.
12. The climate of the area.

Where marks are usually lost

• Using GPP + R instead of GPP − R.
• Missing units or time periods on productivity.
• Dividing by the wrong trophic level in an efficiency calculation.
• Describing a niche as only the place where an organism lives.
• Saying “population” where the answer needs “community”.
• Describing a pattern without quoting values from the table or graph.
• Writing “light affects grass” without saying how it affects growth.
• Claiming a correlation proves a cause.
• Describing succession without saying how each stage changes the environment.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification, Issue 2, February 2021, Pearson Education Limited. Statements 5.9 to 5.15, Topic 5.