Topic 10: Ecosystems — Edexcel A-Level Biology B
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Topic 10: Ecosystems explained
Topic 10: Ecosystems covers 6 specification points, including The nature of ecosystems i Understand what is… and Energy transfer through ecosystems i….
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10.1 The nature of ecosystems i Understand what is meant by the term ecosystem and that they range in size. ii Understand what is meant by trophic levels. iii Understand the advantages and disadvantages of pyramids of numbers, biomass (dry) and energy as useful representations of ecosystem structure and how biomass and energy are transferred within them. iv Know the ecological techniques used to assess abundance and distribution of organisms in a natural habitat, including types of quadrat, transects, ACFOR scales, percentage cover and individual counts. v Be able to select appropriate ecological techniques according to the ecosystem and organisms to be studied.
An ecosystem comprises interacting biotic and abiotic components, ranging in size from a small rock pool to a vast forest or biome. Trophic levels describe feeding tiers. Ecological pyramids model ecosystem structure: numbers pyramids count organisms (can invert when large producers host many consumers); dry biomass pyramids measure dry organic mass (reflects mass, but destructive and aquatic pyramids can invert); energy pyramids are always upright and measure energy flow over time (kJ m^-2 yr^-1). Ecological sampling methods assess abundance and distribution: point quadrats record pin hits; frame quadrats estimate percentage cover, frequency, or density. Random sampling via coordinates prevents bias in uniform habitats; systematic sampling along line or belt transects quantifies distribution along environmental gradients. ACFOR scales provide qualitative abundance scores.
10.2 Energy transfer through ecosystems i Understand how energy is transferred between trophic levels using the terms ‘net primary productivity’ and ‘gross primary productivity’. ii Be able to calculate the efficiency of energy transfer between different trophic levels and account for the loss of energy at each level. iii Understand the significance of microorganisms in the recycling of nutrients within an ecosystem.
Energy enters ecosystems as light and is converted by producers into chemical energy. Gross primary productivity (GPP) is the total energy fixed by producers; net primary productivity (NPP) is GPP minus energy lost as respiration (NPP = GPP − R). Energy transfer between trophic levels is inefficient: energy is lost in respiration, movement, uneaten parts, and egestion. Efficiency of transfer is calculated as (energy transferred to the next trophic level ÷ energy available at the previous trophic level) × 100. Microorganisms such as bacteria and fungi are decomposers; they break down dead organic matter and waste, releasing ammonium ions. Nitrifying bacteria then convert ammonium to nitrate, which producers absorb, ensuring nutrients are recycled within the ecosystem.
10.3 Changes in ecosystems i Understand how ecosystems can develop over time, including use of the terms colonisation and succession and types of climax communities. ii Understand the effects of biotic and abiotic factors on population size.
Ecosystems are dynamic: bare ground or open water is first occupied by pioneer species in a process called colonisation, where organisms arrive, survive and reproduce in a new habitat. Over time, succession changes the community as species alter conditions such as soil depth, nutrient content and light availability, allowing later species to outcompete earlier ones. Primary succession starts on newly exposed surfaces with no soil, while secondary succession follows disturbance where soil already exists. Succession may end in a relatively stable climax community, whose type depends on climate and soil, for example temperate deciduous woodland or, in drier regions, grassland. Population size is then limited by biotic factors such as predation, competition, disease and food supply, and by abiotic factors such as temperature, water, light, pH and mineral ions.
10.4 Human effects on ecosystems i Understand data relating to human influences on ecosystems, including climate change and depletion of biological resources, including overfishing. ii Understand the effect that treaties such as CITES have had on global biodiversity. iii Understand the idea that sustainability of resources depends on effective management of the conflict between human needs and conservation, as illustrated by attempts to conserve fish stocks and reduce possible causes of climate change. iv Understand the role of the scientific journals, the peer review process and scientific conferences in validating evidence related to the debate about climate change.
Human activity alters ecosystems in measurable ways. You interpret data on climate change (rising mean global temperature, shifting species ranges) and on depletion of biological resources, such as falling catch per unit effort in overfished stocks. Treaties like CITES regulate international trade in endangered species, so you judge their effect on global biodiversity from evidence of reduced trade or recovery. Sustainability means meeting human needs without exhausting the resource, so it depends on managing the conflict between demand and conservation: quotas, marine protected areas and emissions reduction all illustrate this. Finally, scientific journals, peer review and conferences validate climate evidence by checking methods and findings before wider acceptance.
CORE PRACTICAL 15: Investigate the effect of different sampling methods on estimates of the size of a population taking into account the safe and ethical use of organisms. vi Be able to use statistical tests to analyse data, including t-test and Spearman rank correlation coefficient.
Core Practical 15 asks you to compare sampling methods and judge how each affects a population-size estimate, while sampling safely and ethically. For a sessile population, place quadrats randomly using a random-number generator and coordinates, then count individuals or estimate percentage cover; for mobile organisms, use mark-release-recapture, marking without harm and allowing mixing before recapture. Estimate population size as (number in first sample × number in second sample) ÷ number of marked recaptures. Compare methods by repeating sampling and using a t-test to compare mean estimates, or Spearman rank correlation coefficient to test whether two ranked variables are associated. Safety includes risk assessment, hygiene and minimal disturbance; ethics includes licensed handling, reducing stress and returning organisms unharmed.
CORE PRACTICAL 16: Investigate the effect of one abiotic factor on the distribution or morphology of one species taking into account the safe and ethical use of organisms.
Core Practical 16 requires you to plan and carry out an investigation into how one named abiotic factor, such as light intensity, soil moisture, soil pH or temperature, affects either the distribution of one species or its morphology. Distribution is studied by sampling, for example using quadrats along a transect to record presence, frequency or percentage cover; morphology is studied by measuring features such as leaf length, leaf width or plant height on individuals of the chosen species. You must identify and control other variables, use random or systematic sampling appropriately, repeat measurements and process data with a suitable summary or statistical test. Safe and ethical practice means risk assessment, correct handling of equipment and soil, minimal disturbance, no harm to protected species and leaving the habitat as found.
Your focus
- Define the terms ecosystem and trophic level.
- Compare the advantages and disadvantages of different ecological pyramids.
- Select and justify appropriate ecological sampling techniques for given scenarios.
Show all 18 objectives
- Distinguish between gross primary productivity and net primary productivity and use the equation NPP = GPP − R.
- Calculate the efficiency of energy transfer between trophic levels and account for energy losses at each level.
- Explain how microorganisms recycle nutrients within an ecosystem, including the release of ammonium ions.
- Define colonisation and succession and distinguish primary from secondary succession.
- Explain how species modify abiotic conditions and how this drives community change towards a climax community.
- Describe how named biotic and abiotic factors affect population size and interpret data showing such effects.
- Interpret data showing human influences such as climate change and overfishing.
- Explain how treaties such as CITES affect global biodiversity.
- Explain how managing the conflict between human needs and conservation supports sustainability, and how peer review, journals and conferences validate climate evidence.
- Compare at least two sampling methods and explain how each affects the accuracy of a population-size estimate.
- Carry out a t-test or Spearman rank correlation coefficient and interpret the result in the context of the sampling method.
- Apply safe and ethical handling of organisms throughout a field investigation.
- Plan a valid investigation into the effect of one named abiotic factor on the distribution or morphology of one named species.
- Carry out sampling safely and ethically, recording quantitative data with appropriate units and repeats.
- Process and interpret the data to evaluate whether the abiotic factor affects distribution or morphology, and justify conclusions.
Topic 10: Ecosystems exam tips
Marking Points
- An ecosystem comprises all interacting organisms (biotic) and physical environment (abiotic) in a defined area, and ecosystems range in size from small micro-habitats (such as a rock pool) to large biomes.
- Compare pyramids of numbers (counts individuals; can be inverted), dry biomass (measures dried organic mass; accounts for size but destructive), and energy (always upright; reflects energy flow over time).
- Explain the use of point quadrats (recording hits per pin) and frame quadrats (measuring percentage cover, frequency, or individual counts).
- Distinguish random sampling (using random coordinates in uniform habitats to avoid bias) from systematic sampling (using line or belt transects across environmental gradients).
- Evaluate ACFOR scales as rapid qualitative or semi-quantitative abundance estimates.
- Defines gross primary productivity as the total energy fixed by producers and net primary productivity as GPP minus respiratory losses.
- Uses the equation NPP = GPP − R to calculate or compare productivity values.
- Calculates efficiency of energy transfer as (energy at next trophic level ÷ energy at previous trophic level) × 100.
- Accounts for energy loss between trophic levels through respiration, movement, uneaten parts and egestion as faeces.
- Explains that microorganisms decompose dead organic matter and waste, releasing ammonium ions.
- Describes how nitrifying bacteria convert ammonium ions into nitrates for uptake by producers, recycling nutrients within an ecosystem.
- Colonisation is the arrival, establishment and reproduction of pioneer species in a previously unoccupied or newly available habitat.
- Succession is the directional change in community composition over time, driven by organisms modifying abiotic conditions and by competitive exclusion.
- Primary succession begins on surfaces without soil, such as bare rock; secondary succession begins where soil and a seed bank or surviving roots remain after a disturbance.
- A climax community is the relatively stable end stage whose dominant species reflect the prevailing climate and soil conditions.
- Biotic factors affecting population size include predation, competition, disease and availability of food.
- Abiotic factors affecting population size include temperature, light intensity, water availability, pH and mineral ion concentration.
- Population size reflects the balance between birth and immigration adding individuals and death and emigration removing them.
- Interpret data on human influences: identify trends such as rising mean global temperature or declining catch per unit effort and link them to human causes.
- Explain depletion of biological resources using overfishing as the example, for instance catches falling while fishing effort rises.
- Describe the effect of treaties such as CITES on global biodiversity, for example restricting trade in endangered species to reduce exploitation.
- Explain that sustainability of resources depends on managing the conflict between human needs and conservation, illustrated by fish-stock conservation and reducing causes of climate change.
- Describe how scientific journals, peer review and scientific conferences validate evidence in the climate change debate by checking methods, data and conclusions.
- Describes random quadrat placement using random numbers and coordinates, and explains that this reduces bias in the population estimate.
- Explains mark-release-recapture for mobile organisms, including marking without harm, allowing mixing, and using the Lincoln index to estimate population size.
- Compares at least two sampling methods and explains how each method's assumptions affect the accuracy of the estimate.
- Uses a t-test to compare mean population estimates from two sampling methods, stating a null hypothesis and interpreting the probability value.
- Uses Spearman rank correlation coefficient to test an association between two ranked variables, such as quadrat distance from a path and percentage cover.
- Applies safe and ethical practice: risk assessment, correct handling, minimal disturbance, and return of organisms to the habitat.
- State one clearly identified abiotic factor, such as light intensity, soil moisture, soil pH or temperature, and explain how it will be measured with a named instrument or method.
- State one clearly identified species and justify whether distribution or morphology is being investigated.
- Describe a sampling method: random quadrat placement for distribution across an area, or a belt transect with quadrats at intervals for a gradient in the abiotic factor.
- Explain how distribution is quantified, for example frequency, percentage cover or density, or how morphology is quantified, for example leaf length in mm or plant height in cm.
- Identify controlled variables such as time of day, season, slope, soil type or sampling area size, and explain why each is controlled.
- Describe repeats and replication, such as multiple quadrats per site or multiple individuals measured per quadrat, and how a mean is calculated.
- Present or process data appropriately, for example a scatter graph or bar chart with units, and use a suitable test such as correlation or a t-test where justified.
- Describe safe and ethical practice: risk assessment, safe use of equipment and chemicals, hand hygiene after soil contact, avoiding damage to habitats and organisms, and complying with legislation on protected species.
Examiner Tips
- 💡Always specify 'dry biomass' rather than just 'biomass' when evaluating ecological pyramids.
- 💡Justify your choice of ecological technique by referencing the specific organism's mobility and the habitat's characteristics.
- 💡Remember that energy transfer between trophic levels is inefficient due to respiration, excretion, and inedible parts.
- 💡Write the productivity equation as NPP = GPP − R and state the meaning of each term before substituting values.
- 💡For efficiency calculations, identify the energy value for the previous trophic level and the next trophic level, then divide the next by the previous and multiply by 100.
- 💡When explaining energy loss, name at least two specific processes such as respiration and egestion rather than saying 'energy is lost'.
- 💡For nutrient recycling, distinguish between the role of decomposers (releasing ammonium ions) and nitrifying bacteria (converting ammonium to nitrate).
- 💡Read the question stem to identify whether it asks about colonisation, succession, climax type or population limitation before choosing an option.
- 💡Eliminate options that describe a single factor as the only influence on population size, because populations are affected by interacting biotic and abiotic factors.
- 💡Check that any named climax community matches the climate and soil described in the question rather than assuming woodland everywhere.
- 💡Read the graph or table axes and units before choosing an option; eliminate options that contradict the stated trend.
- 💡For CITES questions, check whether the option describes trade regulation rather than habitat creation or direct protection.
- 💡For sustainability items, choose the option that balances human need with conservation rather than banning all use.
- 💡State the sampling method, the variable measured, and how repeats were used before describing the statistical test.
- 💡When interpreting a statistical test, refer to the probability value and the null hypothesis rather than saying the result is 'proved'.
- 💡Link each safety and ethical point to a specific action, such as using a risk assessment, wetting hands before handling invertebrates, or returning organisms to the same location.
- 💡For mark-release-recapture, explain why the marking must not harm the organism or make it more visible to predators.
- 💡Write a null hypothesis and an alternative hypothesis that name the species, the abiotic factor and the measured variable with units.
- 💡Justify your sampling strategy in terms of the question: random sampling avoids bias across an area, while a transect is appropriate when the abiotic factor changes along a gradient.
- 💡Include a risk assessment and an ethical statement in your plan, and explain how you will minimise disturbance to the habitat and organisms.
Common Mistakes
- Confusing pyramids of numbers with pyramids of biomass. Correction: Pyramids of numbers count individuals and can be inverted, whereas pyramids of biomass measure dry mass and are rarely inverted.
- Using random sampling for a habitat with a clear environmental gradient. Correction: Systematic sampling using a transect is required to assess distribution changes along a gradient.
- Assuming percentage cover can only total 100%. Correction: Percentage cover can exceed 100% if different plant species overlap in the canopy.
- Misunderstanding: NPP is always larger than GPP. Correction: NPP = GPP − R, so NPP is smaller because respiratory losses are subtracted.
- Misunderstanding: energy is lost mainly as heat from the Sun. Correction: energy is lost from the food chain mainly through respiration, movement, uneaten parts and egestion.
- Misunderstanding: decomposers release nitrates directly from dead organic matter. Correction: decomposers release ammonium ions, which are then converted to nitrates by nitrifying bacteria.
- Misunderstanding: efficiency of energy transfer is calculated using energy at the previous level divided by energy at the next level. Correction: divide energy at the next trophic level by energy at the previous trophic level, then multiply by 100.
- Thinking succession is random change: it is directional because each community alters conditions and makes the habitat less suitable for itself and more suitable for later species.
- Confusing colonisation with succession: colonisation is the initial establishment of pioneer species, whereas succession is the longer-term sequence of community change.
- Assuming every succession ends in forest: the climax type depends on climate and soil, so grassland or other communities can be the stable end point.
- Treating biotic and abiotic factors as alternatives: both act together, and a named factor such as food supply is biotic while temperature is abiotic.
- Treating climate change as a single cause rather than interpreting data trends: correct by reading axes and units and describing the trend before explaining it.
- Assuming CITES directly protects habitats: correct by stating CITES regulates international trade in listed species, which affects biodiversity indirectly.
- Confusing sustainability with zero use of a resource: correct by explaining sustainable use meets present human needs without preventing future supply.
- Believing peer review proves a claim true: correct by explaining it checks quality and validity before publication, not certainty.
- Misunderstanding: any quadrat placement is random. Correction: random placement requires a random-number generator and coordinates so every point in the area has an equal chance of selection.
- Misunderstanding: the Lincoln index gives an exact population size. Correction: it is an estimate based on assumptions such as no migration, no births or deaths, and equal catchability.
- Misunderstanding: a t-test and Spearman rank correlation coefficient are interchangeable. Correction: a t-test compares mean values between two groups, while Spearman rank tests for an association between two ranked variables.
- Misunderstanding: ethical handling only matters for protected species. Correction: all organisms should be handled safely and ethically, with minimal stress and disturbance, and returned to the habitat.
- Choosing more than one abiotic factor to vary: the investigation must isolate one factor so its effect can be attributed correctly.
- Recording distribution without a defined sampling area or quadrat size: frequency and percentage cover are meaningless unless the area sampled is stated.
- Ignoring ethical and legal requirements: disturbing protected species or removing organisms without permission is unacceptable even if the sampling method is otherwise sound.
- Failing to repeat measurements: a single quadrat or single plant cannot support a conclusion about the effect of the abiotic factor.