Ecosystems — OCR A-Level Biology
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Ecosystems explained
Ecosystems covers 6 specification points, including (a) ecosystems, which range in size, are… and (b) biomass transfers through ecosystems.
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Open a point below for its explanation and revision guidance.
(a) ecosystems, which range in size, are dynamic and are influenced by both biotic and abiotic factors
An ecosystem is a community of organisms interacting with each other and with their non-living environment. Ecosystems range in size from a drop of pond water or a rotting log to a whole forest or ocean, so there is no fixed minimum or maximum scale. They are dynamic: populations, species composition and physical conditions change over time, for example through succession or seasonal change. Both biotic factors, such as predation, competition and disease, and abiotic factors, such as light intensity, temperature, water availability and soil pH, influence which organisms survive and how the community is structured. A change in one factor can shift the balance of the whole system.
(b) biomass transfers through ecosystems
Biomass transfer means following the mass of living material as it moves from producers into consumers along a food chain. Learners should treat biomass as the mass of biological material present, usually measured as dry mass per unit area or volume over a stated time, and should recognise that only a fraction of the energy and matter in one trophic level becomes biomass in the next. The rest is lost through respiration, egestion, excretion and material not eaten, and some is lost as heat. A useful method is to compare dry mass at successive trophic levels and calculate transfer efficiency as biomass at the higher level divided by biomass at the lower level, multiplied by 100. A pyramid of biomass represents standing crop at each level, which can differ from a pyramid of numbers.
(c) recycling within ecosystems
Recycling within ecosystems means the return of elements such as carbon, nitrogen and phosphorus from dead organisms and waste back into usable forms. Decomposers, including bacteria and fungi, break down dead material and release inorganic ions and gases. Saprobionts absorb soluble nutrients from decaying matter, and detritivores fragment material to increase surface area for microbial action. The carbon cycle returns carbon dioxide to the atmosphere through respiration and combustion, while photosynthesis removes it. The nitrogen cycle involves nitrogen fixation, ammonification, nitrification and denitrification. Recycling keeps nutrients available for producers, so ecosystems can continue to function without constant new input. Learners should be able to describe these pathways, identify the organisms involved and explain why recycling is essential for sustained productivity.
(d) the process of primary succession in the development of an ecosystem
Primary succession is the colonisation of bare, previously uncolonised ground where no soil or seed bank exists, for example after a volcanic eruption or glacial retreat. Pioneer species such as lichens and mosses arrive by wind or water dispersal, tolerate harsh conditions and begin weathering rock. Their death and decay, together with trapped particles, slowly build a thin soil layer and add organic matter and nutrients. As soil deepens and water retention improves, larger plants such as grasses and shrubs can establish, outcompeting pioneers. Each stage is a seral stage; the community changes until a stable climax community forms, determined by climate. Biodiversity and soil depth generally increase, while pioneer species decline. The process is directional and slow, and it differs from secondary succession, which starts on existing soil.
(e) (i) how the distribution and abundance of organisms in an ecosystem can be measured
Distribution describes where organisms are found across a habitat, while abundance describes how many there are. Random sampling avoids bias: a quadrat is placed using random coordinates, and the number of individuals or percentage cover of a species is recorded. Percentage cover suits sessile organisms such as plants; counting individuals suits discrete organisms. To estimate abundance across a habitat, many quadrats are sampled and the mean is scaled to the total area. For mobile organisms, mark-release-recapture is used: animals are captured, marked, released, then a second sample is taken, and the Lincoln index estimates population size as (number in first sample × number in second sample) ÷ number of marked individuals in the second sample. Distribution along a gradient is measured using a transect, recording organisms at intervals.
(ii) the use of sampling and recording methods to determine the distribution and abundance of organisms in a variety of ecosystems.
Sampling estimates organisms without counting everything. Distribution is where organisms occur; abundance is how many or how much. In a habitat, a quadrat placed randomly (for example using random coordinates) gives percentage frequency, density per m² or percentage cover. A transect records how distribution changes along a line, with quadrats at intervals giving a kite diagram or belt transect data. For mobile animals, mark-release-recapture estimates population size using the Lincoln index: estimated population = (number marked in first sample × total in second sample) ÷ number of marked recaptured. Methods must be appropriate to the ecosystem: quadrats suit plants and slow-moving organisms on rocky shores or grassland; sweep nets and pitfall traps suit invertebrates; kick-sampling suits streams. Random sampling avoids bias, and sufficient samples improve reliability.
Your focus
- Define an ecosystem and explain that it includes biotic and abiotic components.
- Describe how ecosystems range in size and are dynamic.
- Explain how named biotic and abiotic factors influence an ecosystem.
Show all 19 objectives
- Define biomass and state how it is measured.
- Explain why biomass transfer between trophic levels is inefficient, naming at least three loss processes.
- Calculate percentage transfer efficiency between two trophic levels from given biomass values.
- Interpret a pyramid of biomass and distinguish it from a pyramid of numbers.
- Describe the roles of decomposers, saprobionts and detritivores in recycling nutrients.
- Outline the main stages of the carbon and nitrogen cycles.
- Explain why recycling of nutrients is essential for continued ecosystem productivity.
- Describe the sequence of stages in primary succession from bare ground to climax community.
- Explain how pioneer species change abiotic conditions to allow later species to establish.
- Distinguish primary succession from secondary succession.
- Describe methods for measuring the distribution and abundance of organisms.
- Select an appropriate sampling method for sessile or mobile organisms.
- Calculate population size using the Lincoln index.
- Describe how quadrats and transects are used to determine distribution and abundance.
- Apply the Lincoln index to estimate population size from mark-release-recapture data.
- Justify a sampling method appropriate to a named ecosystem.
Ecosystems exam tips
Marking Points
- Defines an ecosystem as a community of organisms interacting with their abiotic environment.
- States that ecosystems range in size, giving an example of a very small and a very large ecosystem.
- Explains that ecosystems are dynamic, with populations and conditions changing over time.
- Identifies biotic factors such as competition, predation or disease that influence the community.
- Identifies abiotic factors such as light, temperature, water or soil pH that influence the community.
- Explains how a change in a biotic or abiotic factor can affect the organisms present.
- Biomass is the mass of living material, commonly measured as dry mass per unit area or volume over a stated period.
- Transfer between trophic levels is inefficient because energy and matter are lost through respiration, egestion, excretion and parts not consumed.
- Respiration releases energy, much of which is lost as heat, so it is not available to form biomass at the next level.
- Transfer efficiency can be calculated as biomass at the higher trophic level divided by biomass at the lower trophic level, multiplied by 100.
- A pyramid of biomass shows standing crop at each trophic level and may differ from a pyramid of numbers.
- Not all material at one level is eaten, and some eaten material is indigestible and lost in faeces.
- Decomposers such as bacteria and fungi break down dead organisms and waste, releasing inorganic nutrients.
- Saprobionts absorb soluble nutrients from decaying material, while detritivores fragment it to increase surface area.
- The carbon cycle returns carbon dioxide to the atmosphere by respiration and combustion, and removes it by photosynthesis.
- The nitrogen cycle includes nitrogen fixation, ammonification, nitrification and denitrification.
- Recycling makes nutrients available to producers, sustaining ecosystem productivity without constant new input.
- Microorganisms are central to recycling because they convert organic matter into inorganic forms usable by plants.
- Primary succession begins on bare, previously uncolonised ground with no pre-existing soil or seed bank.
- Pioneer species such as lichens and mosses colonise, tolerate harsh conditions and begin to weather rock.
- Death and decay of pioneers, plus trapped particles, build a thin soil layer and add organic matter and nutrients.
- As soil deepens and retains more water, larger plants such as grasses and shrubs establish and outcompete pioneers.
- Each community is a seral stage, and succession continues until a stable climax community determined by climate forms.
- Biodiversity and soil depth generally increase through succession, while pioneer species decline.
- Primary succession differs from secondary succession, which begins on already-formed soil.
- Distribution is where organisms are found; abundance is how many there are.
- Random sampling with quadrats uses random coordinates to avoid bias, recording individuals or percentage cover.
- Percentage cover suits sessile organisms such as plants, while counting individuals suits discrete organisms.
- Mean abundance from many quadrats is scaled to the total area to estimate population size.
- Mark-release-recapture estimates mobile populations using the Lincoln index: (first sample × second sample) ÷ marked individuals in the second sample.
- A transect records distribution along a gradient by sampling at intervals.
- States that quadrats estimate abundance (frequency, density or percentage cover) and transects show distribution along a gradient.
- Explains that random placement of quadrats reduces bias, and that larger numbers of samples improve reliability.
- Applies the Lincoln index correctly: estimated population = (marked in first sample × total in second sample) ÷ marked recaptured.
- Selects a method suited to the ecosystem, for example quadrats for plants on a rocky shore, pitfall traps for ground invertebrates, or kick-sampling for stream organisms.
- Recognises that consistent recording (same quadrat size, same cover scale) allows valid comparison between ecosystems.
Examiner Tips
- 💡Distinguish biotic from abiotic factors clearly when the question asks for both.
- 💡Give a named example of a small and a large ecosystem to show the range in size.
- 💡When explaining dynamism, refer to a specific change such as succession or seasonal variation.
- 💡Read the question carefully to see whether it asks about biomass, energy or numbers, and use the correct term in your answer.
- 💡When calculating efficiency, show the division and multiplication by 100 clearly, and check units are consistent.
- 💡Link each loss to a named process such as respiration, egestion or excretion rather than saying vaguely that energy is lost.
- 💡Name the specific organism group and process for each step, for example nitrifying bacteria for nitrification.
- 💡Use a flow diagram or cycle to show inputs, outputs and transformations clearly.
- 💡Explain the significance of recycling for producers, not just list the processes.
- 💡Link each stage to a change in an abiotic factor, such as soil depth or water retention, to show the mechanism.
- 💡Use the term seral stage and climax community precisely when describing the sequence.
- 💡Contrast primary with secondary succession explicitly when a question asks you to distinguish them.
- 💡State clearly whether you are measuring distribution or abundance, as the method differs.
- 💡Show the Lincoln index calculation with the correct numerator and denominator.
- 💡Explain why random sampling reduces bias when justifying a method.
- 💡Read the ecosystem named in the question and choose a sampling method that physically suits it before calculating anything.
- 💡When using the Lincoln index, keep the two samples distinct and substitute values carefully, checking the arithmetic.
- 💡Link each method to whether it measures distribution (transect) or abundance (quadrat counts, cover estimates).
Common Mistakes
- Treating an ecosystem as only the living organisms: correction — it includes the abiotic environment and the interactions between them.
- Assuming ecosystems must be large: correction — they range in size from tiny to vast.
- Describing an ecosystem as static: correction — it is dynamic, with populations and conditions changing over time.
- Assuming all biomass at one level is passed to the next; correction: only a fraction becomes biomass in the next level because of losses.
- Confusing biomass with energy; correction: biomass is a mass of material, while energy flow is a separate measure, though the two are linked.
- Treating a pyramid of numbers as identical to a pyramid of biomass; correction: a single large producer can support many consumers, so the shapes can differ.
- Ignoring egestion and excretion as losses; correction: material not absorbed or excreted is not converted into consumer biomass.
- Thinking decomposers are the same as detritivores; correction: detritivores are animals that ingest and fragment dead material, while decomposers are microorganisms that break it down chemically.
- Believing nutrients disappear when organisms die; correction: elements are recycled into inorganic forms and reused by producers.
- Confusing nitrification with nitrogen fixation; correction: fixation converts atmospheric nitrogen into ammonia or ammonium compounds, while nitrification converts ammonium into nitrite then nitrate.
- Assuming recycling only involves carbon; correction: nitrogen, phosphorus and other elements are also recycled.
- Thinking primary succession starts on bare rock that already has soil: correct this by stressing that no soil or seed bank exists at the start.
- Confusing primary with secondary succession: correct this by noting secondary succession begins on existing soil after a disturbance.
- Assuming the climax community is always the same everywhere: correct this by explaining it is determined by the prevailing climate.
- Believing pioneers are outcompeted immediately: correct this by describing the gradual build-up of soil before larger plants establish.
- Placing quadrats where organisms look abundant: correct this by using random coordinates to avoid bias.
- Using percentage cover for mobile animals: correct this by choosing mark-release-recapture for mobile organisms.
- Forgetting to scale the mean to the total area: correct this by multiplying the mean per quadrat by the number of quadrats that fit the area.
- Assuming every marked animal is recaptured: correct this by using the number of marked individuals actually found in the second sample.
- Thinking a single quadrat gives the abundance of the whole habitat; correction: many randomly placed quadrats are needed to estimate the population.
- Confusing distribution with abundance; correction: distribution is where organisms are found, abundance is how many or how much.
- Using mark-release-recapture for plants; correction: it suits mobile animals, while quadrats suit plants and sessile organisms.