Q: What does H2 Biology: Ecology & Conservation cover? A: Energy flow through ecosystems, nutrient cycling, population ecology, biodiversity measurement, and conservation strategies - the key topics in Core Idea 4 of the 9477 syllabus that link living organisms to their environment.
TL;DR Ecology ties every other H2 Biology topic together at the ecosystem scale. Master energy budgets (GPP, NPP, R), carbon and nitrogen cycles, population growth models, Simpson's Diversity Index, and in-situ vs ex-situ conservation - these concepts dominate data-response and essay questions in Papers 2 and 3.
Concrete example: If GPP is 20,000 kJ m-2 yr-1 and respiration is 12,000 kJ m-2 yr-1, then NPP is 8,000 kJ m-2 yr-1. That is the energy stored as plant biomass and available to the next trophic level.
Ecology calculation chooser
Before calculating, decide what the question is measuring. The same data table can test energy storage, energy transfer, population size, or diversity.
Prompt cue
Use this idea
What to check before answering
Photosynthesis, respiration, biomass available to consumers
NPP=GPP−R
NPP is stored plant biomass, not total energy fixed by photosynthesis.
Reviewed by
Ezekiel Tan·Academic Advisor (Biology)
Energy moving from one trophic level to the next
Transfer efficiency
Divide energy at the next trophic level by energy at the previous trophic level, then multiply by 100.
Marked animals recaptured in a second sample
Lincoln index
Assumptions matter: marks must not be lost, and marked organisms must mix back into the population.
Species counts from two habitats
Simpson's Diversity Index
Compare both richness and evenness; a habitat with one dominant species can have lower diversity.
Common trap: do not choose an equation from the topic name alone. Choose it from what the data represents: energy, population size, or species abundance.
Status: SEAB's current H2 Biology (9477) syllabus PDF is labelled for 2026 and identifies 2026 as the first year of examination. Ecology content falls under Core Idea 4, covering ecosystems, energy flow, nutrient cycling, populations, biodiversity, and conservation. [1]
Quick revision box
What this topic tests: Energy flow and productivity calculations, nutrient cycling pathways and named organisms, population growth models, biodiversity indices, and conservation rationale with examples.
Top mistakes to avoid: Confusing GPP with NPP; forgetting named bacteria in nitrogen cycle steps; using species richness alone as a measure of diversity; failing to distinguish density-dependent from density-independent factors.
20-minute sprint plan: 5 min energy flow diagram with GPP/NPP/R; 5 min nitrogen cycle with bacteria names; 5 min Lincoln index and Simpson's D calculations; 5 min in-situ vs ex-situ comparison table.
1 Syllabus context - where ecology fits in 9477
Ecology appears in Core Idea 4 of the H2 Biology (9477) syllabus. It is the topic that brings together molecular biology, genetics, and evolution at the ecosystem level.
Paper 1 (1 h, 30 MCQs): Energy transfer calculations, population growth curve interpretation, and diversity index MCQs.
Paper 2 (2 h, structured questions): Data-response questions on productivity, population sampling data, and nutrient cycling diagrams.
Paper 3 (2 h, free-response): Essay prompts linking ecology to evolution, human impact, and conservation strategies.
Paper 4 (practical): Fieldwork-based questions on sampling techniques, quadrat data analysis, and biodiversity measurement.
Ecology questions frequently require quantitative skills - expect calculations on energy transfer efficiency, the Lincoln index, and Simpson's Diversity Index. [1]
2 Key definitions
Before diving into specific concepts, make sure you can define these terms precisely in exam answers:
Ecosystem: A community of organisms interacting with each other and their abiotic environment in a defined area.
Community: All the populations of different species living and interacting in a particular habitat at a given time.
Population: A group of organisms of the same species living in the same area at the same time, capable of interbreeding.
Habitat: The place where an organism lives, characterised by its physical and biological features.
Niche: The role an organism plays in its ecosystem, including its biotic interactions and its use of abiotic resources. No two species can occupy the same niche indefinitely (competitive exclusion principle).
Abiotic factors: Non-living components - temperature, light intensity, water availability, pH, mineral ion concentration.
Biotic factors: Living components - predation, competition, parasitism, mutualism, disease.
3 Energy flow
3.1 Trophic levels, food chains, and food webs
Organisms are assigned to trophic levels based on how they obtain energy:
Producers (T1): Autotrophs (mainly photosynthetic organisms) that convert light energy into chemical energy.
Primary consumers (T2): Herbivores that feed on producers.
Secondary consumers (T3): Carnivores that feed on primary consumers.
Tertiary consumers (T4): Top predators.
Decomposers: Bacteria and fungi that break down dead organic matter at all trophic levels, returning nutrients to the soil.
A food chain is a linear sequence showing energy transfer from one trophic level to the next. A food web is a network of interconnected food chains within an ecosystem, providing a more realistic picture of feeding relationships.
3.2 GPP, NPP, and R - the productivity equations
These three quantities describe how energy moves through producers:
Gross Primary Productivity (GPP): The total rate of energy fixed by photosynthesis per unit area per unit time.
Net Primary Productivity (NPP): The rate of energy storage in plant biomass, available to the next trophic level.
Respiration (R): The energy used by producers for their own metabolic processes.
The fundamental relationship:
NPP = GPP − R
Units are typically kJ m⁻² yr⁻¹. In exam calculations, you may be given two of the three values and asked to calculate the third.
Worked example: If GPP = 20,000 kJ m⁻² yr⁻¹ and R = 12,000 kJ m⁻² yr⁻¹, then:
NPP = 20,000 − 12,000 = 8,000 kJ m⁻² yr⁻¹
3.3 Ecological pyramids
Pyramid of numbers: Shows the number of individual organisms at each trophic level. Can be inverted (e.g. one tree supporting many insects).
Pyramid of biomass: Shows the total dry mass of organisms at each trophic level. Usually upright; can be inverted in aquatic ecosystems where phytoplankton reproduce rapidly.
Pyramid of energy: Shows the rate of energy flow through each trophic level. Always upright because energy is lost at every transfer.
3.4 Energy transfer efficiency
Only about 10% of the energy at one trophic level is transferred to the next. Energy is lost between levels through:
Respiration (the largest loss - heat energy released).
Excretion (urine, urea).
Egestion (faeces - undigested material).
Not all organisms are consumed (parts not eaten, organisms that die without being eaten).
The efficiency of energy transfer is calculated as:
Efficiency (%) = (Energy at trophic level n+1 / Energy at trophic level n) × 100
This low efficiency explains why food chains rarely exceed four or five trophic levels - there is insufficient energy to support additional levels.
Energy transfer efficiency checkpoint
When an energy-flow table is given, label the donor trophic level and the receiving trophic level before calculating. Then explain the missing energy using named loss routes instead of just saying "energy is lost".
Question cue
Numerator
Denominator
Answer move
Common trap
Energy transfer from producers to primary consumers
Energy stored in primary consumers
NPP or producer energy available to consumers
Calculate the percentage transferred to the next trophic level.
Dividing producer energy by consumer energy and getting a percentage above 100.
Transfer between two named consumer levels
Energy in the higher consumer level
Energy in the lower consumer level
Use the receiving level over the donor level, then multiply by 100.
Treating every transfer as exactly 10% even when data is given.
A biomass pyramid is compared with an energy pyramid
Energy pyramid uses rate of energy flow, not standing mass
Use energy-flow data if the question asks for efficiency
Explain why biomass can mislead if organisms reproduce quickly.
Calculating transfer efficiency from organism numbers alone.
The question asks why transfer is inefficient
Not a calculation first
Not a calculation first
Name respiration, heat loss, egestion, excretion, and uneaten biomass where relevant.
Saying energy is destroyed instead of transferred to surroundings or waste.
Worked check: if producers store 8000kJ⋅m−2⋅yr−1 as NPP and primary consumers store 960kJ⋅m−2⋅yr−1, then
transfer efficiency=8000960×100=12%.
The remaining energy is not available to the next trophic level because some producer biomass is not eaten, some is egested or excreted after feeding, and much is released as heat during respiration.
Misconception check: a low transfer efficiency does not mean energy disappeared. It means less chemical energy is stored as biomass in the next trophic level.
4 Nutrient cycling
Unlike energy, nutrients are recycled within ecosystems. Two cycles are essential for H2 Biology.
4.1 The carbon cycle
Carbon moves between the atmosphere, biosphere, hydrosphere, and lithosphere:
Photosynthesis removes CO2 from the atmosphere, fixing it into organic molecules (glucose).
Respiration by all living organisms returns CO2 to the atmosphere.
Decomposition by saprotrophic bacteria and fungi releases CO2 from dead organic matter.
Combustion of fossil fuels and biomass releases stored carbon as CO2.
Fossilisation locks carbon in fossil fuels over geological timescales.
Human activities - burning fossil fuels, deforestation - have increased atmospheric CO2 concentration, driving climate change and threatening ecosystems globally.
4.2 The nitrogen cycle
Nitrogen must be converted into usable forms before organisms can incorporate it. Know the named bacteria for each step:
Process
Description
Key organisms
Nitrogen fixation
N2 converted to NH3/NH4+
Rhizobium (symbiotic, in root nodules of legumes); free-living Azotobacter
Nitrification
NH4+ oxidised to NO2−, then to NO3−
Assimilation
Plants absorb NO3− or NH4+ and incorporate nitrogen into amino acids and proteins
Plants
Ammonification
Decomposers break down organic nitrogen compounds in dead organisms and waste into NH4+
Saprotrophic bacteria and fungi
Denitrification
NO3− converted back to N2, returning it to the atmosphere
Pseudomonas (anaerobic conditions, e.g. waterlogged soil)
Lightning can also fix atmospheric nitrogen, but biological fixation is the dominant natural pathway.
5 Population ecology
5.1 Population growth curves
Two idealised models describe how populations change over time:
Exponential (J-curve) growth: Occurs when resources are unlimited. The population grows at an accelerating rate with no upper limit. Seen in populations colonising a new habitat with abundant resources and no predators.
Logistic (S-curve) growth: More realistic. The population grows exponentially at first, then growth rate declines as resources become limited, eventually stabilising at the carrying capacity (K) - the maximum population size the environment can sustain.
5.2 Factors affecting population size
Density-dependent factors: Their effect intensifies as population density increases - intraspecific competition for food, water, and space; predation; disease; accumulation of toxic waste. These factors drive logistic growth toward K.
Density-independent factors: Their effect does not change with population density - natural disasters, fires, drought, extreme weather events.
5.3 Interspecific interactions
Interspecific competition: Two species competing for the same limited resource. The competitive exclusion principle states that two species cannot coexist indefinitely on the same limiting resource - one will outcompete the other. In practice, species coexist through resource partitioning (occupying slightly different niches).
Predator-prey relationships: Predator and prey populations oscillate in linked cycles. An increase in prey leads to an increase in predators, which then reduces the prey population, causing the predator population to decline - and the cycle repeats.
5.4 Sampling techniques
Ecologists estimate population size using field sampling methods:
Quadrat sampling (sessile organisms, e.g. plants):
Place quadrats randomly in the study area.
Count the number of individuals of the target species in each quadrat.
Calculate the mean number per quadrat and scale up to estimate total population.
Mark-recapture method (mobile organisms, e.g. animals):
Capture a sample, mark the individuals, and release them.
After a suitable time period, capture a second sample.
Count the total caught and the number of marked individuals recaptured.
The Lincoln index estimates population size:
N = (n₁ × n₂) / m₂
Where:
N = estimated total population size
n1 = number caught and marked in the first sample
n2 = total number caught in the second sample
m2 = number of marked individuals recaptured in the second sample
Assumptions of mark-recapture:
Marked individuals mix randomly with the population.
No immigration, emigration, births, or deaths between sampling events.
Marks do not affect survival or behaviour.
Marks are not lost.
Worked example: 50 butterflies are caught, marked, and released. A week later, 60 butterflies are caught, of which 10 are marked. Estimated population:
N = (50 × 60) / 10 = 300
Mark-recapture bias checkpoint
After calculating the Lincoln index, always ask whether the second sample caught a fair proportion of marked animals:
first sample marked
-> marked animals mix normally: estimate is more reliable
-> marked animals avoid traps or marks are lost: too few marked recaptured
-> marked animals become trap-happy: too many marked recaptured
Broken assumption
What happens to m2
Effect on N=m2n1n2
How to phrase the evaluation
Marks are lost before recapture
m2 is too small
Population is overestimated
Some marked individuals are counted as unmarked, so the recaptured marked proportion looks lower than it really is.
Marking makes animals avoid traps
m2 is too small
Population is overestimated
Fewer marked animals enter the second sample, so the formula assumes the total population is larger.
Marking makes animals easier to recapture
m2 is too large
Population is underestimated
Marked animals are over-represented in the second sample, so the formula assumes the total population is smaller.
Immigration occurs before recapture
Often lowers the marked proportion
Usually overestimates population size
New unmarked animals dilute the marked fraction, so the second sample no longer represents the original closed population.
Worked check: if n1=50, n2=60, and m2=10, then N=300. If trap avoidance means only 5 marked butterflies are recaptured, the estimate doubles to 600, even if the real population has not changed.
Misconception check: a larger calculated N is not automatically evidence that the population grew. It may be evidence that a sampling assumption failed.
6 Biodiversity
6.1 Three levels of biodiversity
Species diversity: The variety of species in an area and their relative abundance.
Genetic diversity: The range of alleles and genotypes within a population. Higher genetic diversity increases a population's ability to adapt to environmental change.
Ecosystem diversity: The variety of different ecosystems (habitats) in a region.
6.2 Simpson's Diversity Index
Species richness (the number of species present) alone does not capture diversity - a community where one species dominates is less diverse than one with even distribution. Simpson's Diversity Index accounts for both richness and evenness:
D = 1 − [sum of n(n−1)] / [N(N−1)]
Where:
D = diversity index (ranges from 0 to 1; higher values indicate greater diversity)
n = number of individuals of each species
N = total number of individuals of all species
Worked example: A pond contains 30 dragonflies, 15 damselflies, and 5 water beetles N=50.
A D value of 0.551 indicates moderate diversity. Compare this to another site with the same three species but equal numbers (e.g. 17, 17, 16) and you would get a higher D, illustrating the importance of evenness.
Simpson comparison checkpoint
When comparing two habitats, calculate D first, then explain the difference using richness and evenness. Do not stop at the species count.
Comparison cue
What to check
How to phrase the answer
Same number of species, but one species dominates
Evenness is lower in the dominated habitat
The habitat with more even abundances has the higher D, so it is more diverse even though richness is the same.
More species in one habitat, but most individuals belong to one species
Richness is higher, but evenness may be lower
Higher richness alone does not guarantee a higher D; compare the calculated indices.
One habitat has fewer species, but counts are balanced
Evenness may compensate partly for lower richness
A balanced habitat can have a relatively high D, but it may still be lower than a richer habitat with similar evenness.
D values are close together
Sampling reliability matters
State that the habitats have similar diversity and support the claim with repeated sampling or larger sample size.
Worked check: Site A has counts 40,5,5, while Site B has 20,15,15. Both have three species and N=50, but Site A is dominated by one species. Site B will have the higher Simpson's D because individuals are distributed more evenly.
Misconception check: Simpson's D is not just a "rare species score". It increases when a habitat has more species and when individuals are more evenly spread among those species.
6.3 Measuring biodiversity in the field
Use quadrats or transects to record species and abundance.
Identify specimens to species level where possible.
Calculate Simpson's D for each site to enable quantitative comparison.
Repeat sampling to increase reliability.
7 Conservation
7.1 In-situ vs ex-situ conservation
Feature
In-situ conservation
Ex-situ conservation
Where
Within the natural habitat
Outside the natural habitat
Examples
National parks, nature reserves, marine protected areas
Preserves the entire ecosystem and ecological interactions; protects many species simultaneously; maintains natural selection pressures
Protects species from immediate threats; allows controlled breeding; can maintain genetic diversity through studbooks
Limitations
May not protect against large-scale threats (climate change, pollution); requires enforcement and management
Expensive; limited space; animals may lose natural behaviours; small populations risk inbreeding
Ideal use
Primary strategy for conserving habitats and communities
Complementary strategy for critically endangered species facing imminent extinction
7.2 Reasons for conservation
Exam answers should address multiple justifications:
Ecological: Every species plays a role in its ecosystem. Loss of one species can trigger cascading effects through food webs. Keystone species removal can cause ecosystem collapse.
Economic: Many industries depend on biodiversity - agriculture (pollination, pest control), fisheries, forestry, pharmaceuticals (many drugs derived from natural compounds), and ecotourism.
Ethical: Species have an intrinsic right to exist, independent of their usefulness to humans. Current generations have a moral responsibility to preserve biodiversity for future generations.
Aesthetic: Natural landscapes and wildlife have cultural, recreational, and psychological value.
7.3 Singapore-specific examples
Singapore is a useful case study because it demonstrates conservation in a highly urbanised context:
Bukit Timah Nature Reserve: One of the largest remaining patches of primary tropical rainforest in Singapore. Protects native flora and fauna, including the critically endangered Sunda pangolin. Demonstrates in-situ conservation within an urban setting.
Sungei Buloh Wetland Reserve: Protects mangrove and mudflat habitats. Important stopover site for migratory shorebirds on the East Asian-Australasian Flyway. Illustrates the role of protected areas in conserving migratory species.
Coral reef restoration: Singapore's waters support coral reefs despite heavy shipping traffic. Active restoration programmes transplant coral fragments to degraded areas and use artificial reef structures to promote recolonisation.
Garden City to City in Nature: Singapore's broader conservation strategy aims to integrate nature into urban planning, creating ecological corridors (e.g. the Rail Corridor) that connect fragmented habitats and reduce edge effects.
8 How ecology appears in exams
Data-response questions (Papers 2 and 4)
Energy flow diagrams: You may be given a diagram showing energy inputs and outputs at each trophic level. Calculate GPP, NPP, or transfer efficiency. Show your working clearly and include units.
Population data: Interpret mark-recapture data, apply the Lincoln index, and evaluate whether assumptions are met.
Diversity index calculations: Calculate Simpson's D from species abundance data. Compare values between sites and suggest reasons for differences.
Essay topics (Paper 3)
Common essay themes include:
The importance of biodiversity and the consequences of its loss.
Compare and contrast in-situ and ex-situ conservation strategies.
Discuss how human activities disrupt nutrient cycling and energy flow.
Explain how interspecific interactions regulate population size.
Exam tip: Ecology essays earn higher marks when you include specific examples, named organisms (especially bacteria in the nitrogen cycle), and quantitative reasoning (e.g. only ~10% energy transfer between trophic levels explains food chain length).
Where can I find the full H2 Biology notes series? Start at the H2 Biology notes hub, then work through Core Ideas 1-4 and the Extension Topics.
Is ecology heavily tested in the A-Level exam? Yes. Ecology spans all four papers. Papers 2 and 3 frequently include data-response questions on energy flow, population sampling, and biodiversity indices. Paper 4 may include fieldwork-based ecology tasks. Ecology essays are among the most common free-response prompts. [1]
Do I need to memorise the bacteria names in the nitrogen cycle? You should know Rhizobium (nitrogen fixation), Nitrosomonas and Nitrobacter (nitrification), and Pseudomonas (denitrification). Examiners expect named organisms when you describe each step of the cycle.
How do I know when to use Simpson's Diversity Index vs species richness? Species richness counts only the number of species present. Simpson's D is more informative because it accounts for both richness and the relative abundance (evenness) of each species. In exam answers, use Simpson's D when comparing diversity between habitats quantitatively.
What is the difference between a food chain and a food web? A food chain shows a single linear pathway of energy transfer. A food web shows multiple interconnected food chains in an ecosystem, giving a more complete and realistic picture of feeding relationships. Exam answers should acknowledge that food webs better represent real ecosystems because most organisms feed at multiple trophic levels.
