Ecology ties every other H2 Biology topic together at the ecosystem scale.
Key points
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.
Planning a revision session? Use our study places near me map to find libraries, community study rooms, and late-night spots.
Read in layers
1 second
Read the summary above.
10 seconds
Scan the first few sections below.
100 seconds
Jump into the section that matches your decision.
Quick ecology map
Quick revision box
1 Syllabus context - where ecology fits in 9477
2 Key definitions
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.
Quick ecology map
Read depth
What to take away
1 second
Ecology tracks energy, nutrients, populations, diversity, and conservation.
10 seconds
Energy flows one way through trophic levels; nutrients cycle through organisms and the environment.
100 seconds
In data questions, identify the variable, choose the right index or equation, then link the result back to ecosystem stability.
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.
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.
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
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.
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.
