QCE Biology Unit 3 Biodiversity and the Interconnectedness of Life: deep-dive 2026 guide
QCE Biology Unit 3 covers ecosystems as abiotic plus biotic systems through which energy flows one way (with only about 10 per cent passing between trophic levels) while matter cycles, with productivity measured as GPP and NPP, biodiversity measured across genetic, species and ecosystem levels using richness, evenness and indices such as Simpson's, organisms classified into a nested hierarchy and identified with dichotomous keys, and populations and communities regulated by density-dependent feedback toward carrying capacity and by succession toward a climax community.
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How Unit 3 fits into QCE Biology
Unit 3, Biodiversity and the interconnectedness of life, is the first Year 12 unit and the first half of the externally assessed content. Topic 1 covers describing biodiversity and ecosystem dynamics; Topic 2 covers the integration and regulation of ecosystems. The unit reuses the energy reactions from Unit 1 (photosynthesis fixes the energy that flows through every food web) and the negative feedback idea from Unit 2 (feedback also stabilises populations and ecosystems). Numerical questions on productivity, trophic efficiency and diversity indices appear most years, so this guide pairs the concepts with worked calculations.
Abiotic and biotic factors
An ecosystem is a community of organisms together with the non-living environment they interact with.
- Abiotic factors are the non-living physical and chemical components: temperature, light, water availability, pH, salinity, soil nutrients, dissolved oxygen and wind. They set the limits within which species can survive (the range of tolerance) and determine the distribution of organisms.
- Biotic factors are the living components and the interactions between organisms: predation, competition, mutualism, parasitism, disease and the supply of food.
Each species has a niche, the role it plays and the range of conditions and resources it uses. Where two species' niches overlap, competition results, and the more efficient competitor may exclude the other (competitive exclusion).
Energy flow through ecosystems
Energy flows one way through an ecosystem: from the sun, fixed by producers, and transferred with heavy losses through consumers, ultimately leaving as heat. Matter cycles, but energy does not.
- Producers (autotrophs) fix energy into organic molecules: photoautotrophs use light (plants, algae, cyanobacteria); chemoautotrophs use chemical energy (some bacteria). Trophic level 1.
- Consumers (heterotrophs) eat other organisms: primary consumers (herbivores, level 2), secondary consumers (carnivores, level 3), tertiary consumers (level 4), and omnivores that feed at more than one level.
- Decomposers (detritivores) break down dead matter and waste, returning nutrients to the abiotic pool.
A food chain is a single linear path of energy transfer (grass to kangaroo to dingo). A food web is the network of all feeding relationships in a community; it is more realistic because most species have multiple prey and predators, and it shows redundancy that makes the system more resilient. On a diagram, arrows point from the organism being eaten to the one that eats it, in the direction of energy flow.
Productivity and the 10 per cent rule
Productivity is the rate at which energy is fixed or biomass is produced per unit area per unit time, typically in kilojoules per square metre per year.
- Gross primary productivity (GPP): the total energy fixed by producers.
- Net primary productivity (NPP): the energy left after producers' respiration. NPP equals GPP minus plant respiration. NPP is the energy available to herbivores and accumulates as plant growth.
- Secondary productivity: the rate at which consumers convert ingested food into their own biomass.
The 10 per cent rule states that only about 10 per cent of the energy at one trophic level passes to the next. The remaining 90 per cent is lost as heat from respiration, used in movement, and lost in indigestible material and faeces. Because losses compound at each step, food chains rarely exceed four or five trophic levels, and energy pyramids always taper upward.
A grassland has a gross primary productivity (GPP) of 20 000 kJ per square metre per year. Plant respiration is 8 000 kJ per square metre per year. Assuming 10 per cent trophic efficiency, calculate the net primary productivity, the energy available to primary consumers, and the energy available to secondary consumers.
- Step 1. Net primary productivity
- NPP equals GPP minus plant respiration equals 20 000 minus 8 000 equals 12 000 kJ per square metre per year.
- Step 2. Primary consumers
- 10 per cent of NPP equals 0.10 times 12 000 equals 1 200 kJ per square metre per year.
- Step 3. Secondary consumers
- 10 per cent of 1 200 equals 120 kJ per square metre per year.
The sharp drop from 12 000 to 1 200 to 120 shows why higher trophic levels support far less biomass, and why long food chains are rare.
Biogeochemical cycles
While energy flows through and is lost, matter cycles. The carbon cycle moves carbon between the atmosphere (carbon dioxide), living organisms (organic molecules), the oceans (dissolved carbon) and geological stores (fossil fuels, limestone) via photosynthesis, respiration, decomposition, combustion and ocean exchange. The nitrogen cycle moves nitrogen through nitrogen fixation, nitrification, assimilation, ammonification and denitrification, mostly driven by bacteria. The water cycle moves water through evaporation, transpiration, condensation, precipitation and runoff. Human activities (burning fossil fuels, fertiliser use, land clearing) disturb these cycles and underpin climate change.
Measuring biodiversity
Biodiversity exists at three levels: genetic diversity within a species, species diversity within a community, and ecosystem diversity across a landscape.
Species diversity combines two ideas:
- Species richness: the number of different species present.
- Evenness: how equally individuals are distributed among those species.
A diversity index combines both into one number. Simpson's diversity index is commonly used at QCE level:
D equals 1 minus the sum, over all species, of (n divided by N) squared,
where n is the number of individuals of a species and N is the total number of individuals of all species. Higher D means greater diversity (more species and more evenness). Field sampling uses quadrats (for sessile or slow organisms such as plants), transects (to sample across an environmental gradient) and mark-recapture (for mobile animals) to estimate abundance.
A quadrat sample of a meadow contains: species A, 10 individuals; species B, 8 individuals; species C, 2 individuals. Calculate Simpson's diversity index, D equals 1 minus the sum of (n over N) squared.
- Step 1. Total individuals
- N equals 10 plus 8 plus 2 equals 20.
- Step 2. Proportions squared
- (10 over 20) squared equals 0.25; (8 over 20) squared equals 0.16; (2 over 20) squared equals 0.01.
- Step 3. Sum and subtract
- Sum equals 0.25 plus 0.16 plus 0.01 equals 0.42. D equals 1 minus 0.42 equals 0.58.
Simpson's diversity index for this sample is 0.58. A value closer to 1 would indicate higher diversity (more species and greater evenness).
Classification and identification keys
Classification organises life into a nested hierarchy: domain, kingdom, phylum, class, order, family, genus and species. The binomial name uses genus and species (for example, the red kangaroo, Osphranter rufus). Classification today is based on shared derived features and, increasingly, on molecular (DNA and protein) evidence, which is shown on phylogenetic trees (cladograms) that represent evolutionary relationships.
A dichotomous key identifies an organism through a series of paired either-or choices, each based on an observable feature, that progressively narrow the options until a single species remains. A good key uses clear, mutually exclusive statements and stable characteristics rather than features that vary with age or season.
Population ecology
A population is all the individuals of one species in an area. Population size changes with births, deaths, immigration and emigration. Two growth models are studied.
- Exponential growth produces a J-shaped curve when resources are unlimited; growth rate accelerates as numbers rise.
- Logistic growth produces an S-shaped curve when limited resources slow growth as the population approaches the carrying capacity (K), the maximum population the environment can sustain.
Density-dependent factors (competition, predation, disease) intensify as the population grows and tend to stabilise it around K through negative feedback. Density-independent factors (fire, flood, drought, extreme temperature) affect the population regardless of its size.
Ecological succession
Succession is the gradual, directional change in community composition over time.
- Primary succession begins on bare substrate with no soil (a new lava flow, bare rock). Pioneer species (lichens, mosses) colonise first, build soil as they die and decompose, and are gradually replaced by grasses, shrubs and eventually trees.
- Secondary succession follows a disturbance that leaves soil intact (a fire, clearing, abandoned farmland). Because soil and seeds remain, it proceeds faster.
Both tend toward a relatively stable climax community in equilibrium with the local climate and soil, though ongoing disturbance often keeps real ecosystems in earlier successional stages.
- Drawing food-web arrows the wrong way
- Arrows point from prey to predator, the direction energy flows.
- Confusing GPP and NPP
- NPP equals GPP minus plant respiration; NPP is what reaches consumers.
- Saying energy cycles
- Energy flows one way and is lost as heat; matter cycles.
- Treating richness as the whole story
- Diversity combines richness and evenness; two communities with the same number of species can differ in diversity.
- Mixing up primary and secondary succession
- Primary starts with no soil; secondary follows a disturbance that leaves soil intact.
Check your knowledge
A mix of recall, calculation and exam-style application questions covering Unit 3 subject matter. Answer all under timed conditions (about 1 minute per mark), retaining units throughout, then check against the solutions block.
- Distinguish between abiotic and biotic factors, giving two examples of each. (3 marks)
- Distinguish between a food chain and a food web, and explain why a food web is a more accurate representation of an ecosystem. (3 marks)
- A forest has a GPP of 30 000 kJ per square metre per year and plant respiration of 11 000 kJ per square metre per year. (a) Calculate the NPP. (b) Using the 10 per cent rule, calculate the energy available to primary and to secondary consumers. (c) Explain why food chains rarely exceed five trophic levels. (5 marks)
- A quadrat contains species A (12 individuals), species B (6 individuals) and species C (2 individuals). Calculate Simpson's diversity index, D equals 1 minus the sum of (n over N) squared. (3 marks)
- Define species richness and evenness, and explain how two communities with the same number of species could have different diversity. (3 marks)
- Explain the difference between exponential and logistic population growth, and define carrying capacity. (4 marks)
- Distinguish between density-dependent and density-independent factors, giving one example of each and explaining how density-dependent factors stabilise a population. (4 marks)
- Compare primary and secondary succession. (a) State the starting conditions of each. (b) Explain why secondary succession is usually faster. (c) Describe what a climax community is and why real ecosystems may not reach it. (5 marks)
- Q1
- Abiotic factors are non-living physical and chemical components, for example temperature and water availability (also light, pH, salinity, soil nutrients). Biotic factors are living components and their interactions, for example predation and competition (also mutualism, parasitism, disease).
- Q2
- A food chain is a single linear path of energy transfer from producer through successive consumers. A food web is the network of all feeding relationships in a community. A web is more accurate because most species have multiple prey and predators and feed at more than one trophic level; it also shows redundancy, so losing one species rarely collapses the whole system.
- Q3
- (a) NPP equals 30 000 minus 11 000 equals 19 000 kJ per square metre per year. (b) Primary consumers: 0.10 times 19 000 equals 1 900 kJ per square metre per year; secondary consumers: 0.10 times 1 900 equals 190 kJ per square metre per year. (c) About 90 per cent of energy is lost at each transfer (respiration, heat, movement, indigestible matter), so after several transfers too little energy remains to support a viable population at the next level.
- Q4
- N equals 12 plus 6 plus 2 equals 20. (12 over 20) squared equals 0.36; (6 over 20) squared equals 0.09; (2 over 20) squared equals 0.01. Sum equals 0.46. D equals 1 minus 0.46 equals 0.54.
- Q5
- Species richness is the number of different species present. Evenness is how equally individuals are distributed among species. Two communities with the same richness can differ in diversity if one is dominated by a single species (low evenness, lower diversity) while the other has individuals spread evenly across species (high evenness, higher diversity).
- Q6
- Exponential growth gives a J-shaped curve when resources are unlimited, with the growth rate accelerating as numbers rise. Logistic growth gives an S-shaped curve when limited resources slow growth as the population nears the carrying capacity. Carrying capacity (K) is the maximum population size the environment can sustain over time.
- Q7
- Density-dependent factors intensify as population density rises, for example competition for food (also predation, disease); they stabilise a population by reducing birth rates and raising death rates as numbers approach K, a negative feedback that returns the population toward carrying capacity. Density-independent factors affect a population regardless of its density, for example a flood (also fire, drought, extreme temperature).
- Q8
- (a) Primary succession starts on bare substrate with no soil (lava flow, bare rock); secondary succession starts after a disturbance that leaves soil intact (fire, clearing). (b) Secondary succession is faster because soil, nutrients and seeds or roots already remain, so colonisation does not have to wait for soil to form. (c) A climax community is a relatively stable community in equilibrium with the local climate and soil; real ecosystems may not reach it because ongoing disturbances (fire, grazing, storms) reset succession to earlier stages.