What you'll learn
- How carbon moves through ecosystems and why human activity can disrupt the cycle.
- How natural selection changes allele frequencies in populations over generations.
- How isolation can lead to speciation.
- How sustainable management balances biodiversity, carbon storage and human needs.
The starting point: ecosystems, populations and carbon
An ecosystem is a community of organisms interacting with the non-living, or abiotic, environment. A population is all the organisms of one species in a habitat at a particular time.
Carbon matters because it is found in carbohydrates, lipids, proteins and nucleic acids. In ecosystems, carbon is constantly transferred between living organisms, dead organic matter, the atmosphere, oceans, soils and long-term stores such as peat and fossil fuels.
The carbon cycle
Carbon cycle
The carbon cycle is the movement of carbon between living organisms and the physical environment, mainly through photosynthesis, feeding, respiration, decomposition, fossilisation and combustion.
In photosynthesis, producers such as plants convert carbon dioxide into organic molecules:
6CO2+6H2O→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_26CO2+6H2O→C6H12O6+6O2
That carbon then moves through food chains by feeding. It returns to the atmosphere when organisms respire, and when dead material is broken down by saprotrophic decomposers: microorganisms such as bacteria and fungi that secrete enzymes onto dead organic matter, digest it externally, absorb soluble products and respire.
The diagram shows the main carbon transfers you need to be able to describe.
Carbon stores, sinks and sources
A carbon store is a place where carbon is held, such as wood, soil, peat, oceans or fossil fuels. A carbon sink removes more carbon from the atmosphere than it releases. A carbon source releases more carbon than it removes.
Combustion is burning in oxygen. Burning fossil fuels releases carbon dioxide that had been locked away for millions of years. Deforestation also increases atmospheric carbon dioxide because it reduces photosynthesis and often releases carbon from burned or decomposing biomass.
A useful productivity relationship is:
NPP=GPP−Rplant\text{NPP} = \text{GPP} - R_{\text{plant}}NPP=GPP−Rplant
where gross primary productivity, GPP, is the total chemical energy or carbon fixed by photosynthesis, plant respiration, RplantR_{\text{plant}}Rplant, is the energy or carbon used in respiration, and net primary productivity, NPP, is what remains for growth, storage and transfer to consumers.
Estimating net carbon storage
A woodland has a GPP of 1.60 kg C m⁻² year⁻¹. Ecosystem respiration and decomposition release 1.10 kg C m⁻² year⁻¹, and harvesting removes 0.20 kg C m⁻² year⁻¹.
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Choose the carbon budget rule: storage increases if carbon inputs exceed carbon outputs, so use net storage=GPP−respiration and decomposition−harvest removal\text{net storage} = \text{GPP} - \text{respiration and decomposition} - \text{harvest removal}net storage=GPP−respiration and decomposition−harvest removal.
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Substitute the values with units: net storage=1.60 kg C m−2 year−1−1.10 kg C m−2 year−1−0.20 kg C m−2 year−1\text{net storage} = 1.60\ \text{kg C}\ \text{m}^{-2}\ \text{year}^{-1} - 1.10\ \text{kg C}\ \text{m}^{-2}\ \text{year}^{-1} - 0.20\ \text{kg C}\ \text{m}^{-2}\ \text{year}^{-1}net storage=1.60 kg C m−2 year−1−1.10 kg C m−2 year−1−0.20 kg C m−2 year−1.
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Calculate and interpret: net storage=0.30 kg C m−2 year−1\text{net storage} = 0.30\ \text{kg C}\ \text{m}^{-2}\ \text{year}^{-1}net storage=0.30 kg C m−2 year−1, so the woodland is acting as a carbon sink.
Carbon is recycled, energy is not
Carbon atoms are recycled through ecosystems, but energy flows through and is eventually lost as heat. Do not write that “energy is recycled” in the carbon cycle.
Climate change as a selection pressure
A greenhouse gas absorbs outgoing infrared radiation from Earth’s surface. Carbon dioxide and methane are important examples. Increased greenhouse gas concentrations can cause climate change, meaning long-term changes in average weather patterns, such as temperature, rainfall and frequency of extreme events.
Climate change can alter selection pressures. A selection pressure is an environmental factor that affects survival and reproductive success. Examples include drought, temperature, new predators, new pathogens, changed flowering times or reduced food availability.
Evolution by natural selection
Evolution
Evolution is a change in allele frequencies in a population over generations. An allele is a version of a gene, and allele frequency is the proportion of all copies of a gene in a population that are a particular allele.
Natural selection needs genetic variation, which arises from mutation and is reshuffled by meiosis and fertilisation during sexual reproduction. If a heritable allele gives an advantage in a particular environment, individuals with that allele are more likely to survive, reproduce and pass it on. Over many generations, that allele becomes more common.
Natural selection sequence
For full marks, link the sequence clearly: genetic variation → selection pressure → differential survival and reproduction → inheritance of advantageous alleles → allele frequency changes over generations.
Explaining drought-driven evolution
A grass population contains some individuals with alleles for deeper roots. Over several decades, the habitat becomes drier.
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Identify the relevant variation: some grasses have deeper roots because they carry alleles affecting root development.
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Apply the selection pressure: in drier conditions, deeper-rooted plants can access more water, so they are more likely to survive long enough to reproduce.
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Link survival to inheritance: these plants pass the advantageous alleles for deeper roots to more offspring than shallow-rooted plants do.
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State the population-level outcome: over generations, the frequency of the deeper-root allele increases, so the population becomes better adapted to drought.
Individuals do not evolve
An individual organism may grow, acclimatise or die, but it does not evolve. Evolution is measured as a change in allele frequencies in a population over generations.
Speciation: when populations become separate species
A species is usually defined as a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of a new species.
Speciation often begins when gene flow — movement of alleles between populations by interbreeding — is reduced or stopped. In allopatric speciation, populations are geographically isolated, for example by mountains, rivers, islands or habitat fragmentation. In sympatric speciation, reproductive isolation develops without a physical barrier, for example through differences in behaviour, breeding season or ecological niche.
Over time, different mutations occur in each isolated population, and different selection pressures act on them. Allele frequencies diverge. Eventually, the populations may become reproductively isolated, meaning they can no longer interbreed to produce fertile offspring.
Recognising allopatric speciation
A beetle population is split when rising sea level forms two islands. One island becomes wetter; the other becomes drier.
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Identify the isolating mechanism: the sea prevents beetles from moving between islands, so gene flow between the populations is reduced.
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Compare selection pressures: wetter conditions may favour alleles for fungal resistance, while drier conditions may favour alleles for water conservation.
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Track allele frequencies: mutation, natural selection and chance effects cause the two gene pools to become increasingly different.
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Decide whether speciation has occurred: if beetles from the two islands later meet but cannot produce fertile offspring, they are now separate species.
Sustainable management
Sustainable management
Sustainable management means using resources in a way that meets human needs now without reducing the ability of future generations to meet their needs, while maintaining ecosystem function and biodiversity.
Biodiversity is the variety of life. It includes species diversity, genetic diversity and ecosystem diversity. Genetic diversity is especially important for evolution because it provides the raw material for natural selection. A population with very low genetic diversity may be less able to adapt to new diseases or climate change.
Sustainable management is not the same as never using a resource. It means setting harvest levels, farming methods or conservation plans so that populations, carbon stores and ecosystem processes can recover.
Examples of sustainable approaches
In forests, sustainable management may include selective felling, replanting native species, coppicing, maintaining dead wood habitats, protecting soils, and using longer rotation times so carbon remains stored in biomass.
In fisheries, it may include catch quotas, minimum mesh sizes, closed seasons, protected breeding areas and monitoring population size.
In peatlands, the most sustainable action is often restoration: blocking drainage channels, keeping soils waterlogged and preventing peat extraction. Peat forms under anaerobic conditions, meaning conditions without oxygen, so decomposition is slow and carbon remains stored.
Not all biofuels are carbon neutral
A biofuel only has a low net carbon impact if regrowth reabsorbs the carbon released and if land clearance, fertiliser use, processing and transport do not add large extra emissions.
Judging a sustainable timber harvest
A managed woodland produces 1.20 kg dry biomass m⁻² year⁻¹ as NPP. Managers estimate that 0.50 kg dry biomass m⁻² year⁻¹ must remain for root growth, litter, decomposers and food webs. A company wants to harvest 0.80 kg dry biomass m⁻² year⁻¹.
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Calculate the maximum biomass available for harvest: available harvest=1.20 kg dry biomass m−2 year−1−0.50 kg dry biomass m−2 year−1\text{available harvest} = 1.20\ \text{kg dry biomass}\ \text{m}^{-2}\ \text{year}^{-1} - 0.50\ \text{kg dry biomass}\ \text{m}^{-2}\ \text{year}^{-1}available harvest=1.20 kg dry biomass m−2 year−1−0.50 kg dry biomass m−2 year−1.
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State the sustainable limit: available harvest=0.70 kg dry biomass m−2 year−1\text{available harvest} = 0.70\ \text{kg dry biomass}\ \text{m}^{-2}\ \text{year}^{-1}available harvest=0.70 kg dry biomass m−2 year−1.
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Compare the proposal with the limit: 0.80−0.70=0.10 kg dry biomass m−2 year−10.80 - 0.70 = 0.10\ \text{kg dry biomass}\ \text{m}^{-2}\ \text{year}^{-1}0.80−0.70=0.10 kg dry biomass m−2 year−1 above the estimated sustainable harvest.
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Make a judgement: the proposed harvest is not sustainable unless the estimate changes, because it removes more biomass than the woodland can spare each year.
Using evidence in management
Good management decisions should be based on repeated measurements, not guesses. Ecologists might use random quadrats to estimate plant abundance, transects to study changes across a habitat, dry mass to estimate biomass, or population surveys to monitor animals.
To evaluate a management plan, ask: has biodiversity increased, has carbon storage improved, are populations reproducing successfully, and are local people still able to use the resource responsibly?
Evaluating management plans
A strong answer usually considers ecological effects, economic costs, social impacts, time scale, monitoring and uncertainty. Sustainable management is about trade-offs, not one perfect solution.
In the exam
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For natural selection, always write the full chain: variation, selection pressure, differential survival and reproduction, inheritance, then allele frequency change.
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For carbon cycle questions, name the process and direction of carbon transfer, for example “photosynthesis removes carbon dioxide from the atmosphere into biomass”.
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For sustainability questions, use data where given, compare rates of removal with rates of replacement, and mention biodiversity, carbon storage and long-term monitoring.
Check yourself
- Why can burning fossil fuels increase atmospheric carbon dioxide even if plants are still photosynthesising?
- How can climate change cause allele frequencies to change in a population?
- A woodland has an NPP of 0.90 kg dry biomass m⁻² year⁻¹ and managers want to harvest 0.70 kg dry biomass m⁻² year⁻¹ while leaving 0.30 kg dry biomass m⁻² year⁻¹ for ecosystem processes. Is the plan sustainable?
