Chapter 19
Relationships of Organisms with One Another and with the Environment
Cambridge O Level Biology 5090 Topic 19 revision chapter covering the relationships of organisms with one another and with their environment. The chapter opens with the single organising boundary of the whole topic: energy flows through an ecosystem and is not recycled, while matter cycles between organisms and the environment and is used again. It establishes the Sun as the principal energy input to most ecosystems, explains how photosynthetic producers transfer light energy into chemical energy in organic molecules, and defines producer, consumer, herbivore, carnivore, decomposer and trophic level in syllabus wording. Food chains are taught through the meaning of the arrow, which shows transfer of energy, biomass and nutrients from the food organism to the feeding organism, never from predator to prey. Food webs are treated as networks of interconnected food chains, with a step-by-step method for tracing the direct and indirect consequences of removing or increasing one population. Decomposers are shown carrying out extracellular digestion, secreting enzymes onto dead material and waste and absorbing the soluble products, receiving material from several trophic levels rather than forming a single top level. The chapter explains why trophic transfer is inefficient through uneaten parts, egested undigested material, respiration and heat transfer to the environment, movement and other life processes, excretory losses and organisms dying before being eaten, and links this to short food chains and to the greater energy efficiency of eating crop plants directly rather than feeding crops to livestock. Percentage energy transfer efficiency is calculated with correct trophic ordering, consistent units and biological interpretation. All three ecological pyramids are constructed and compared: numbers, which can be inverted when one large producer supports many small consumers; biomass, preferably as dry mass for a stated area and time; and energy, which is always upright. The carbon cycle covers photosynthesis, feeding, respiration by plants, animals and decomposers, decomposition, fossil fuel formation and combustion. The nitrogen cycle covers nitrogen fixation by bacteria and lightning, decomposition to ammonium compounds, nitrification to nitrate ions, absorption of nitrate ions by roots for amino acid and protein synthesis, feeding, death and excretion, and denitrification. Population, community, ecosystem and biodiversity are defined precisely, and population growth rate is explained through food supply, competition, predation and disease, then scaled up to human population growth and its demand for food, water, energy, land and raw materials. Ecological sampling is taught as examinable method: random quadrat placement using random coordinates, counts and percentage cover, mean per quadrat, scaling to estimate a population, the assumptions and limitations of that estimate, and belt transects for investigating change along an environmental gradient. Human impacts are developed as complete causal chains for deforestation, over-harvesting, non-native species, eutrophication, greenhouse gases and climate change, insecticides and herbicides, and non-biodegradable plastics. The chapter closes with conservation of forests and fish stocks, the definition of a sustainable resource as one produced or replaced as rapidly as it is removed, and a framework for making conditional, evidence-based judgements about conservation measures.Show moreShow less
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What is Relationships of Organisms with One Another and with the Environment about?
Ecosystems run on one input and two very different accounting systems. Light energy from the Sun enters through photosynthesis, passes along food chains by feeding, and is transferred to the environment as thermal energy at every step — so energy flows through an ecosystem and is never recycled. The atoms carried with it are different: carbon and nitrogen move from air to producer to consumer to decomposer and back to the air, so matter cycles and is used again. Everything else in this topic — short food chains, ecological pyramids, the carbon and nitrogen cycles, eutrophication, conservation — is a consequence of those two sentences.
The most-asked questions about Topic 19, answered in the wording that would earn the mark. Each answer stands alone, so you can use any one of them as a model sentence in the exam.
Key ideas to remember
- Matter goes round. Energy goes through. Every mark in this topic sits on one side of that line.
- If you can rebuild this diagram from memory — four levels, five respiration arrows, one photosynthesis arrow, decomposers fed from every level, ammonium to nitrate to roots, and a river going wrong — you can answer almost anything in Topic 19.
- Three visits, spaced. Recall on day 1, produce on day 7, apply on day 30.
What you need to be able to do
- I can state that the Sun is the principal source of energy input to biological systems, and explain how photosynthesis transfers light energy into chemical energy in organic molecules.
- I can define producer, consumer, herbivore, carnivore, decomposer and trophic level in syllabus wording.
- I can explain what a food-chain arrow means and state the direction of transfer.
- I can construct and interpret a food web, and trace the direct and indirect effects of removing or increasing one population.
- I can describe how decomposers feed by extracellular digestion and absorption, and explain why they are not a single top trophic level.
- I can give at least five reasons why energy transfer between trophic levels is inefficient, using the phrase less energy remains available for transfer to the next trophic level.
- I can explain why food chains usually have fewer than five trophic levels.
- I can explain why eating crop plants directly is generally more energy-efficient than eating livestock fed on those crops.
- I can calculate percentage energy-transfer efficiency between two named trophic levels and interpret the result.
- I can construct, label and compare pyramids of numbers, biomass and energy, and explain when a pyramid of numbers can be inverted.
- I can describe the carbon cycle including photosynthesis, feeding, respiration by plants, animals and decomposers, decomposition, fossil-fuel formation and combustion.
- I can describe the nitrogen cycle including fixation, decomposition to ammonium compounds, nitrification, absorption of nitrate ions, protein synthesis, feeding, death, excretion and denitrification.
- I can explain why matter cycles but the energy associated with these processes does not.
- I can define population, community, ecosystem and biodiversity precisely.
- I can explain how food supply, competition, predation and disease affect population growth rate, using a change → mechanism → consequence structure.
- I can explain how human population growth increases demand for food, water, energy, land and raw materials.
- I can describe a valid random quadrat sampling method and identify what must be standardised.
- I can estimate a population from quadrat data and state the assumptions that estimate depends on.
- I can explain when percentage cover is preferable to a count.
- I can describe a belt transect and distinguish systematic sampling from random sampling.
- I can explain the causes of deforestation and give complete causal chains for habitat loss, soil erosion, flooding and rising atmospheric carbon dioxide.
- I can define over-harvesting and trace its effect on a population.
- I can explain how an introduced non-native species may disrupt a food web.
- I can write the full eutrophication sequence from nutrient enrichment to the death of aerobic organisms.
- I can explain the enhanced greenhouse effect and distinguish it from ozone-layer depletion.
- I can explain how insecticides and herbicides may affect non-target organisms and food webs.
- I can explain the effects of non-biodegradable plastics on aquatic and terrestrial ecosystems.
- I can give reasons why conservation may be needed.
- I can explain how education, protected areas, quotas, replanting and monitoring conserve forests, and why replanting one species does not restore original biodiversity.
- I can explain how closed seasons, protected areas, mesh-size control, quotas, monitoring and education conserve fish stocks.
- I can define a sustainable resource and explain why renewable does not automatically mean sustainably managed.
- I can evaluate a conservation measure using evidence, benefit, limitation, monitoring need and a conditional judgement.
Key terms in Relationships of Organisms with One Another and with the Environment
- Producer
- An organism that makes its own organic nutrients, usually using energy from light through photosynthesis. Producers such as green plants and algae form the first trophic level of a food chain and are the point at which energy enters most ecosystems, because the organic molecules they build supply both energy and matter to every consumer that feeds on them.
- Energy Flow
- The one-way transfer of energy through an ecosystem: light energy is absorbed by producers and transferred into chemical energy by photosynthesis, passes to consumers by feeding, and is transferred to the environment as thermal energy at every step, mainly through respiration. Because energy leaves the ecosystem at each transfer and is never returned to the producers, energy flows through an ecosystem rather than cycling within it.
- Trophic Level
- The feeding position an organism occupies in a food chain, food web or ecological pyramid. Producers form the first trophic level, the herbivores that eat them form the second as primary consumers, the carnivores that eat those herbivores form the third as secondary consumers, and so on. An organism's trophic level depends on what it is eating in the chain being considered, so an omnivore can occupy different trophic levels in different chains of the same food web.
- Food Chain
- A diagram showing the transfer of energy and materials from one organism to the next, beginning with a producer. Each arrow points from the organism being eaten to the organism eating it, because that is the direction in which chemical energy, biomass and nutrients are transferred. A food chain shows one feeding route only, so the same organism may appear in several different chains within one ecosystem.
- Decomposer
- An organism, usually a bacterium or a fungus, that obtains its energy and nutrients from dead organic material and waste. A decomposer secretes digestive enzymes out onto the material, so large insoluble molecules are digested outside its body, and then absorbs the small soluble products. Its respiration returns carbon dioxide to the air and its activity releases nitrogen-containing compounds into the soil, which is how the matter locked in dead organisms becomes available to producers again.
- Pyramid of Energy
- A diagram in which the width of each bar represents the energy transferred at that trophic level over a period of time, for a stated area, using units such as kilojoules per square metre per year. Because energy is transferred out of the food chain at every level and is never returned, the energy transferred always decreases going up the chain, so a correctly constructed pyramid of energy is always upright. This makes it the most reliable of the three ecological pyramids for comparing energy flow between trophic levels.
- Ecosystem
- A unit containing the community of organisms in an area and the non-living parts of their environment, all interacting with one another. The living part is the community, made up of every population of every species present; the non-living part includes factors such as light, temperature, water, dissolved oxygen and mineral ions in the soil. Because the organisms and their surroundings affect each other continuously, an ecosystem is defined by the interaction, not merely by the boundary drawn around it.
- Carbon Cycle
- The circulation of carbon between the atmosphere and water, living organisms and long-term stores. Carbon dioxide is removed from air or water by photosynthesis and built into the organic molecules of producers; feeding transfers that carbon to consumers; respiration by plants, animals and decomposers returns carbon dioxide; death, waste and excretion pass organic carbon to decomposers, whose respiration also returns carbon dioxide; and some carbon is stored over geological time as fossil fuels, which release carbon dioxide when they are burned.
- Quadrat
- A frame of known area, often a square of side 0.5 metres, placed on the ground so that the organisms inside it can be counted or their percentage cover estimated. Quadrats are suitable for plants and for animals that do not move far, and they are placed using pairs of random numbers as coordinates so that the sample is not biased by the person choosing where to put them. Results from several quadrats are averaged to give a mean per quadrat, which can then be scaled up to estimate the population of the whole area.
- Trophic Transfer Efficiency
- The proportion of the energy available at one trophic level that becomes new biomass in the next trophic level, usually expressed as a percentage. It is low, often around ten per cent, because not all of the food organism is eaten, some eaten material is egested undigested, energy is released in respiration and transferred to the environment as thermal energy, energy is used for movement and other life processes, some is lost in excretory products, and some organisms die without being eaten. The result is that less energy remains available for transfer to the next trophic level.
- Population Growth Rate
- The speed at which the number of organisms in a population changes, determined by the balance between births and deaths. It is influenced by food supply, competition, predation and disease, and each of these acts through survival or reproduction: more food allows more individuals to survive and reproduce; greater competition, more predators or more disease reduce survival or reproductive success. The factors are not independent, because a population that grows for one reason usually experiences greater competition and often greater predation and disease as a result.
- Percentage Energy Transfer
- A calculation expressing how much of the energy available at one trophic level becomes available at the next, as a percentage. The energy available at the higher trophic level is divided by the energy available at the lower trophic level and multiplied by one hundred. Both quantities must be in the same units and must refer to the same area and the same period of time, and the division must run from the higher level over the lower level, never the other way round.
- Food Web
- A network of interconnected food chains showing all the feeding relationships in an ecosystem. Because most organisms eat more than one kind of food and are eaten by more than one predator, a food web represents the real pattern of energy and material transfer far better than a single chain does. Tracing the arrows into and out of one organism reveals its food sources, its predators and its competitors, which is what makes it possible to predict the effect of a change in one population.
- Deforestation
- The large-scale removal of trees from an area of forest, usually to clear land for agriculture or settlement or to extract timber, fuel or minerals. Its consequences follow from what the trees were doing: habitats and food sources are lost so populations decline and biodiversity falls; fewer roots bind the soil and less canopy intercepts rain, so topsoil is washed away and fertility falls; less water is intercepted and infiltrates so surface runoff increases and flood risk rises; and less photosynthesis together with burning or decomposition of the trees increases the concentration of carbon dioxide in the atmosphere.
- Food Chain Length
- The number of trophic levels a food chain contains, which is usually fewer than five. Each transfer between levels is inefficient, so the energy available falls sharply from one level to the next. After a small number of transfers there is too little energy left to support a further population of predators large enough to sustain itself, which sets a practical limit on how many levels a chain can have.
- Conservation Evaluation
- A structured judgement about whether a conservation measure is likely to work. A complete evaluation identifies the biological problem, states the mechanism by which the proposed measure would address it, gives the expected benefit, names a genuine limitation, says what monitoring evidence would show whether it is working, and reaches a conclusion that is conditional on that evidence. Unsupported conclusions such as claiming a single measure always works, or that education alone is sufficient, do not meet the standard because they state an outcome without the mechanism or the evidence that would justify it.
- Nutrient Cycle
- The repeated movement of a chemical element between the non-living environment and living organisms. Producers take the element up in an inorganic form, feeding transfers it along the food chain, and respiration, excretion, death and decomposition return it to the environment where producers can take it up again. Because the same atoms are used repeatedly, an ecosystem does not need a continuous supply of new matter in the way it needs a continuous supply of energy.
- Conservation
- The protection and careful management of species, habitats and ecosystems so that they are maintained for the future. Conservation aims to keep biodiversity high, to reduce the risk of extinction, to protect vulnerable habitats, and to preserve the genetic variation within populations that allows them to adapt to disease and to changing conditions. It also maintains the processes ecosystems provide, such as nutrient cycling, pollination and soil stabilisation. Conservation manages human use of a resource rather than preventing all use of it.
- Nitrogen Cycle
- The circulation of nitrogen between the atmosphere, the soil and living organisms. Nitrogen gas is made available in compounds by nitrogen-fixing bacteria and by lightning; decomposition of nitrogen-containing material forms ammonium compounds; nitrifying bacteria convert these toward nitrate ions; plant roots absorb nitrate ions and use the nitrogen to make amino acids and proteins; feeding transfers protein to animals; death and excretion return nitrogen-containing material for decomposition; and denitrifying bacteria convert nitrate compounds back to nitrogen gas.
- Non-native Species
- A species brought by human activity into an area where it does not naturally occur. A non-native species may have no natural predators, parasites or diseases in its new habitat, so its population can grow rapidly; it may compete with native species for food, water, light or space, prey on native organisms, or introduce a disease to which they have no resistance. Where this happens, native populations fall, food webs are disrupted and biodiversity decreases. Many introduced species have little effect, so the outcome is described as possible rather than certain.
- Pyramid of Numbers
- A diagram in which the width of each bar represents the number of organisms at that trophic level, with the producer placed at the base and each successive level stacked above it. The bars have equal height and share a common central axis. Because bar width counts individuals and takes no account of their size, a pyramid of numbers can be inverted: a single large producer such as an oak tree may support thousands of small primary consumers, giving a narrow base and a much wider second level.
- Enhanced Greenhouse Effect
- The additional warming of the lower atmosphere caused by rising concentrations of greenhouse gases such as carbon dioxide and methane. These gases absorb thermal radiation given off by the Earth's surface and re-radiate some of it back towards the ground instead of allowing it to escape into space. The natural greenhouse effect keeps the planet warm enough to support life; increasing the concentration of these gases increases the proportion of thermal radiation retained, so the average global temperature may rise, climate patterns may change and the distribution of habitats and species may shift.
- Pyramid of Biomass
- A diagram in which the width of each bar represents the total mass of living material at that trophic level, for a stated area and at a stated time, with the producer at the base. Biomass is preferably measured as dry mass, because the water content of organisms varies and contributes no energy or nutrients. A pyramid of biomass gives a better picture of the living material present than a count of individuals does, but it is a snapshot and the values may change with the season.
- Resource Demand
- The quantity of food, water, energy, land and raw materials a population requires. As the human population grows, total demand for each of these rises, and meeting that demand places specific pressures on ecosystems: land is cleared for agriculture and settlement, water is abstracted from rivers and groundwater, fossil fuels are burned releasing carbon dioxide, and timber, fish and minerals are harvested more heavily. Population size is one driver of resource demand; how much each person consumes and the technology used also affect it.
- Non-biodegradable Plastic
- A plastic that decomposers cannot break down, so it persists in the environment for a very long time after it is discarded. Because it is not removed by decomposition, it accumulates in aquatic and terrestrial habitats, where animals may swallow fragments that block or damage the gut, may become entangled in larger pieces so that they cannot feed, move or breathe, and habitats may be physically contaminated. Plastic broken into smaller particles may be taken in by small organisms and so enter food webs.
- Eutrophication
- The process in which nutrient enrichment of water leads to a fall in dissolved oxygen and the death of aerobic organisms. Nitrogen-containing fertiliser or untreated sewage entering a river or lake raises the concentration of nitrate and other nutrient ions, so algae and other producers grow rapidly. The dense growth reduces the light reaching plants below and producers die, decomposer populations increase as they feed on the dead material, and their aerobic respiration uses up the dissolved oxygen. Fish and other organisms that need dissolved oxygen then die. The fertiliser does not remove oxygen itself; the decomposers do.
- Population Estimate
- A calculated approximation of the number of organisms in a study area, obtained by finding the mean number counted per quadrat and scaling that mean up by the number of quadrat areas that fit into the whole study area. It is an estimate rather than a count, because only a small fraction of the area is examined and the calculation assumes the sampled quadrats are representative of the whole. Increasing the number of quadrats sampled generally improves reliability but never turns the estimate into an exact figure.
- Percentage Cover
- The proportion of the ground inside a quadrat that is covered by a particular species, expressed as a percentage. It is estimated using a quadrat divided into a hundred small squares, so that each square represents one per cent, and applying a single consistent rule about which squares to count. Percentage cover is used instead of counting individuals when plants overlap, when the boundaries between individuals cannot be seen, or when a species grows as a mat or a colony rather than as separate plants.
- Biodiversity
- The number of different species living in an area. Biodiversity counts species, not individuals, so an area containing an enormous number of organisms belonging to only two or three species has low biodiversity, while an area with the same total number of organisms spread across many species has high biodiversity. Because each species occupies a different position in the food web, falling biodiversity means fewer feeding routes and an ecosystem that is less able to absorb change.
- Sustainable Resource
- A resource that is produced or replaced as rapidly as it is removed from the environment, so that it does not run out. Sustainability is a relationship between two rates rather than a property of the material itself: while the rate of replacement matches or exceeds the rate of removal the resource is maintained, and when removal exceeds replacement it declines however renewable the resource is in principle. Judging sustainability therefore requires monitoring of both rates over an appropriate timescale, and consideration of any environmental damage caused by the harvesting itself.
- Pesticide Effects on Food Webs
- The consequences for non-target organisms when insecticides or herbicides are applied. An insecticide reduces the population of a target pest but may also kill other insects, removing food for the animals that ate them and disrupting the food web. A herbicide kills target weeds but may also remove non-target producers, reducing food and habitat for other species and lowering biodiversity. Pesticides that are not broken down remain in organisms that absorb or eat them, so their concentration can build up within an organism and can become greater at each successive trophic level.
- Over-harvesting
- The removal of organisms from a population faster than reproduction can replace them. Because fewer individuals are left to breed, fewer offspring are produced, so the population declines and the same harvesting effort removes an ever larger share of what remains. A shrinking population may also lose genetic variation, which reduces its ability to adapt to disease or to changing conditions, and the risk of local extinction rises. Not every harvested population becomes extinct, but the decline continues for as long as removal exceeds replacement.
- Fish Stock Conservation
- The management of a fished population so that it is not removed faster than reproduction replaces it. Closed seasons stop fishing while the fish are breeding so that spawning is not interrupted; protected areas provide places where fish can grow and reproduce with little or no harvesting; controlled net types and mesh sizes allow young fish to escape and reduce the catch of unwanted species; quotas limit the mass or number that may be landed; monitoring measures the size of the stock and the catch so that limits can be adjusted; and education increases understanding and compliance.
- Forest Conservation
- The management of forests so that they are maintained while still being used. Education improves understanding and encourages responsible use; protected areas restrict damaging activities such as clearance and logging; harvesting quotas limit the number of trees felled so that removal does not exceed replacement; replanting replaces harvested trees and restores tree cover and future supply; and monitoring measures forest area and biodiversity over time so that management can be adjusted. Replanting a single species restores tree cover but does not necessarily restore the biodiversity of the original forest.
- Belt Transect
- A line laid across an area along which quadrats are placed at fixed intervals, or continuously, so that the abundance or distribution of species can be recorded against distance. A belt transect is used to investigate how a community changes along an environmental gradient, such as increasing distance from water, increasing height up a shore, or a change in light intensity from open ground into woodland. Because the quadrat positions are decided by a fixed rule rather than by random numbers, a belt transect is systematic sampling, not random sampling.
Common mistakes to avoid
- “Food-chain arrows point from the predator to the prey.” Why it is wrong: the arrow shows the direction energy and material travel, and they travel into the organism that eats, not out of it. Exam-safe: the arrow points from the organism being eaten to the organism eating it, showing the transfer of energy and materials.
- “Producers create energy.” Why it is wrong: energy cannot be created. Photosynthesis changes the form energy is in. Exam-safe: producers transfer light energy into chemical energy stored in organic molecules.
- “Energy is recycled by decomposers.” Why it is wrong: decomposers respire like any organism, so the energy they release is transferred to the environment as thermal energy. Nothing returns it to the producers. Exam-safe: decomposers recycle matter, releasing inorganic nutrients. Energy flows through the ecosystem and is not recycled.
- “Decomposers are just scavengers.” Why it is wrong: scavengers are animals that swallow pieces of dead material and digest it in a gut. Decomposers digest it outside their bodies. Exam-safe: decomposers are bacteria and fungi that secrete enzymes onto dead material, digest it externally and absorb the soluble products.
- “Decomposers are the highest trophic level.” Why it is wrong: they receive dead material and waste from producers and from every consumer level, so no single level can be assigned to them. Exam-safe: draw decomposers to the side of the chain with arrows coming into them from several levels.
- “All the organisms at one level are eaten by the next.” Why it is wrong: many die of disease, age or cold, and much of each organism is never eaten anyway. Exam-safe: not all organisms are eaten, and not all parts of those that are eaten are consumed — both are reasons transfer is inefficient.
- “Respiration destroys energy.” Why it is wrong: respiration releases energy from glucose. The energy is not destroyed; it is transferred, largely to the environment as thermal energy. Exam-safe: energy is released in respiration and transferred to the environment, so less remains available for the next trophic level.
- “A pyramid of numbers is always upright.” Why it is wrong: bar width counts individuals regardless of size, so one large producer can support many small consumers. Exam-safe: a pyramid of numbers may be inverted, for example when a single oak tree supports thousands of caterpillars.
- “Biomass means the mass of one organism.” Why it is wrong: a pyramid of biomass compares whole trophic levels, not individuals. Exam-safe: biomass is the total mass of living material at that trophic level, preferably as dry mass, for a stated area at a stated time.
- “Plants do not respire, so plant respiration is not part of the carbon cycle.” Why it is wrong: plants respire continuously, day and night, and release carbon dioxide. Exam-safe: respiration by producers, consumers and decomposers all return carbon dioxide to the air. Plant respiration is the arrow most often missed.
- “Plants absorb nitrogen gas from the air.” Why it is wrong: nitrogen gas is unreactive and plants have no mechanism for using it. Only nitrogen-fixing bacteria and lightning make it available. Exam-safe: plant roots absorb nitrate ions from the soil solution and use the nitrogen to make amino acids and proteins.
- “Nitrifying and denitrifying bacteria do much the same thing.” Why it is wrong: they drive nitrogen in opposite directions. One makes it available to plants; the other removes it. Exam-safe: nitrifying bacteria convert ammonium ions toward nitrate ions; denitrifying bacteria convert nitrate compounds back to nitrogen gas.
- “Biodiversity means the total number of organisms in an area.” Why it is wrong: a wheat field holds millions of organisms and has very low biodiversity. Exam-safe: biodiversity is the number of different species living in an area.
- “A population is all the organisms living in an area.” Why it is wrong: that is a community. A population is restricted to one species. Exam-safe: a population is all the organisms of one species living in the same area at the same time.
- “Random quadrats can be placed wherever is convenient.” Why it is wrong: choosing the position, however casually, introduces bias — and throwing the quadrat is biased by direction, force and terrain. Exam-safe: use two tapes at right angles and place each quadrat at coordinates from a pair of random numbers.
- “A population estimate is the number of organisms present.” Why it is wrong: only a small fraction of the area was examined, and the calculation assumes those samples represent the whole. Exam-safe: it is an estimate. State that the organisms may not be evenly distributed and that more quadrats would improve reliability.
- “A belt transect is a form of random sampling.” Why it is wrong: quadrat positions along a transect are set by a fixed interval, which is systematic, not random. Exam-safe: a belt transect is systematic sampling, used to investigate change along an environmental gradient rather than to estimate abundance without bias.
- “Fertiliser removes the oxygen from the water.” Why it is wrong: fertiliser adds nutrients; it consumes no oxygen at all. Exam-safe: nutrient enrichment causes rapid producer growth, then death, then a rise in decomposers whose aerobic respiration uses up the dissolved oxygen.
- “The algae block the light, and that is why the fish die.” Why it is wrong: shading kills submerged plants; it does not deprive fish of oxygen. The oxygen is used by decomposers. Exam-safe: shading contributes to producers dying, but the fish die because decomposer respiration lowers the dissolved oxygen concentration.
- “Every non-native species becomes invasive.” Why it is wrong: most introduced species fail to establish, and many that do have no measurable effect. Exam-safe: an introduced species may compete with, prey on or bring disease to native species, especially if it has no natural predators there.
- “The greenhouse effect is the hole in the ozone layer.” Why it is wrong: different gases, different radiation, different consequence. Exam-safe: greenhouse gases absorb outgoing thermal radiation and warm the lower atmosphere; ozone depletion allows more ultraviolet radiation to reach the surface.
- “All pesticides build up through food chains.” Why it is wrong: persistence varies enormously, and many pesticides break down within days. Exam-safe: pesticides that are not broken down may accumulate within organisms and become more concentrated at successive trophic levels.
- “Replanting restores the forest, and sustainable means never using it.” Why it is wrong: a single-species plantation restores tree cover but not the original biodiversity; and a sustainable resource is one that is used, at a rate it can be replaced. Exam-safe: replanting restores tree cover and some habitat, but monitoring of biodiversity is needed to judge recovery. A sustainable resource is produced or replaced as rapidly as it is removed, so it does not run out.
Examiner tips
- Examiner's habit worth copying. In this topic, mark schemes are built from linked statements. Write in arrows on your rough paper first — fertiliser → nitrate rise → algal growth → death → decomposers → aerobic respiration → oxygen falls → fish die — then turn each arrow into a sentence. You will not lose a link, and links are what is being counted.
- Wording that is marked wrong. “Plants make energy”, “plants produce energy from the Sun”, “energy is created in photosynthesis”. Energy cannot be made. Write transfers light energy into chemical energy in organic molecules, or at minimum converts light energy to chemical energy.
- Two ways to lose the mark on one arrow. Reversing it is the obvious one. The subtler one is answering “what does the arrow show?” with “is eaten by”. That phrase names the event but not the transfer, so it is usually not enough. Write: the transfer of energy and materials from the organism being eaten to the organism eating it.
- Distinct points, not restatements. “Energy is lost as heat” and “energy is used for movement” are two points only if you make the link explicit — respiration releases the energy, movement uses it, and it ends up transferred to the environment. Writing “heat loss” three different ways scores once. Aim for one line per route out of the chain.
- Keep it energetic, and keep it conditional. This objective is about energy transfer, so answer with transfers and losses. It is generally more energy-efficient to eat crop plants directly — it does not follow that every field should grow crops, because land unsuitable for crops may still support grazing animals, and diets involve nutritional, economic and cultural questions this syllabus point does not address. State the energy conclusion, note that it depends on context, and stop there.
- Units carry marks in this topic. Energy at a trophic level is a rate per unit area: \(\mathrm{kJ/m^2/year}\). It has to be per area, because a bigger field holds more energy, and per time, because energy keeps arriving. If a question supplies a unit, use that exact unit in your answer; if it asks you to comment on a pyramid of energy, the time component is part of why the pyramid is meaningful.
- “Which pyramid would be most useful, and why?” is a standard question, and the answer is nearly always the pyramid of energy, for a reason you must state: it measures the energy transferred over time, so it shows how much energy is actually available to the next level and it cannot be distorted by organism size or by a seasonal snapshot. Naming it without that reason scores one mark out of two.
- You are not required to name bacterial species. “Nitrogen-fixing bacteria”, “nitrifying bacteria” and “denitrifying bacteria” are the names the syllabus wants, and they are named for what they do — which makes them almost impossible to mix up once you notice. What you must get right is the direction each one drives the nitrogen: fixation and nitrification make it available to plants; denitrification takes it away.
- A compare question needs linked statements. “Carbon dioxide is absorbed by plants” on one line and “nitrogen is fixed by bacteria” on another is two facts, not a comparison. Write them as one linked sentence: plants absorb carbon dioxide from the air directly, whereas they cannot absorb nitrogen gas and must take up nitrate ions from the soil instead. One linked sentence, one mark.
- Watch the trap in “same area at the same time”. Questions sometimes describe two groups of the same species in ponds a kilometre apart, or the same pond in two different years. Neither is one population. If a definition question offers you a data table, check the place and the date before you commit.
- Use “may”, and mean it. An increase in food supply does not guarantee population growth — predators, disease or a shortage of nesting sites may hold the population where it is. Examiners accept and expect conditional wording here, and a flat “the population will increase” can cost the mark in an evaluation question.
- Answer with resources, not opinions. This objective is about demand for named resources and the pressures that follow. Answers that turn into a general argument about whether population growth is good or bad, or that propose social policy, do not answer the biology being asked. Name the resource, name the mechanism, name the ecological consequence.
- Never call it a count. If a question asks you to “estimate the number of…”, the word estimate should appear in your answer too, and a question asking for a limitation is nearly always fishing for either “only a small part of the area was sampled” or “the organisms may not be evenly distributed”.
- A precaution must match its hazard. “Be careful” earns nothing. “Wear gloves” earns nothing unless you have named what the gloves are protecting against. Write the pair: hazard — sharp shells on the rocks; precaution — wear gloves and sturdy shoes.
- “Habitats are destroyed” is one mark at most. The examiner is counting linked steps. Adding “so the animals that lived in those trees lose their food and shelter, and their populations fall” turns one mark into three. If a question is worth four or more marks on a single consequence, it wants the whole chain, not four different consequences.
- Pair every method with a mechanism. “Quotas” scores nothing. “Quotas limit the number of trees felled each year, so trees are removed no faster than new ones grow to replace them” scores. If a question asks for three methods, that is three method-plus-mechanism pairs, not three nouns.
- Four conclusions that lose marks. “Ban all fishing” — ignores that conservation manages use rather than ending it. “Education will solve it” — education changes understanding, not the removal rate, on its own. “Quotas always work” — they work only when set correctly and enforced. “Replanting restores the ecosystem” — it restores tree cover; biodiversity is a separate measurement. Each of these states an outcome without the mechanism or the evidence.
How Relationships of Organisms with One Another and with the Environment is examined
- Topic 19 is unusual: it appears on every paper, and it appears differently on each. Knowing which version you are looking at changes what a full-mark answer contains.
- The single most common command here. Never stop at the cause. “Trees are removed” is a cause; “fewer roots bind the soil, so rain washes topsoil away” is the mechanism and outcome that carry the marks.
- Used for the cycles and for sampling methods. A described sequence needs every required step present and in the right order; it does not need a reason unless you are also asked to explain.
- Common in food-web and conservation questions about an organism you have never met. The biology is standard; only the names are new. Answer with the mechanism, using the organisms in the question.
- Almost always a conservation measure. You need a benefit, a limitation, what evidence would settle it, and a conditional conclusion. An unqualified “quotas work” scores nothing.
- Examiner's habit worth copying. In this topic, mark schemes are built from linked statements. Write in arrows on your rough paper first — fertiliser → nitrate rise → algal growth → death → decomposers → aerobic respiration → oxygen falls → fish die — then turn each arrow into a sentence. You will not lose a link, and links are what is being counted.
Frequently asked questions
Which way do food chain arrows point?
From the organism being eaten to the organism eating it. The arrow shows the transfer of energy, biomass and nutrients, and that transfer goes into the feeder. Reading it as “is eaten by” is a useful check, but if you are asked what the arrow represents, name the transfer and its direction.
Why does energy flow but matter cycle?
Energy enters as light, is transferred along the chain by feeding, and is transferred to the environment as thermal energy at every level — and nothing returns it to the producers, so it makes a single one-way pass. Matter is different: the same carbon and nitrogen atoms are returned to the environment by respiration, excretion, death and decomposition, and producers take them up again. Matter is reused; energy is not.
How much energy is transferred between trophic levels?
Only a small fraction, often around ten per cent, though the exact figure varies. Use the value a question gives you rather than quoting a general figure, and be ready to explain why so little is transferred: uneaten parts, egested undigested material, respiration and thermal transfer to the environment, movement and other life processes, excretory losses, and organisms dying without being eaten.
Why are food chains usually shorter than five levels?
Because transfer is inefficient, so the energy available falls sharply at each level. After a few transfers there is too little energy arriving to support another population of predators large enough to survive and reproduce. Say usually: some marine chains are longer because their producers are highly productive.
Can a pyramid of numbers really be upside down?
Yes. Bar width counts individuals and takes no account of size, so a single large producer such as an oak tree can support thousands of caterpillars, giving a narrow base and a much wider second level. Parasites do the same thing higher up a chain. A pyramid of energy, by contrast, is always upright when correctly constructed.
Do I need to learn the names of the bacteria in the nitrogen cycle?
No. Nitrogen-fixing, nitrifying and denitrifying bacteria are named for what they do, and those descriptive names are what the syllabus asks for. What you must get right is the direction each drives the nitrogen: fixation and nitrification make it available to plants, and denitrification returns it to the atmosphere.
Why can't plants use the nitrogen in the air?
Nitrogen gas is very unreactive and plants have no way of breaking the two atoms apart. Nitrogen becomes available only when nitrogen-fixing bacteria, or lightning, convert it into compounds. Roots then absorb nitrate ions from the soil solution and use the nitrogen to make amino acids and proteins.
What exactly kills the fish in eutrophication?
A shortage of dissolved oxygen. Nutrient enrichment makes producers grow rapidly; the dense growth reduces light and producers die; decomposer populations increase as they feed on the dead material; and those decomposers respire aerobically, using up the dissolved oxygen. The fertiliser itself removes no oxygen, and shading alone does not explain the fish deaths.
What is the difference between random quadrats and a transect?
Random quadrats are placed at coordinates from pairs of random numbers, so every part of the area has an equal chance of being sampled — they estimate abundance without bias. A belt transect places quadrats at fixed intervals along a line, which is systematic sampling, and it measures how abundance changes along an environmental gradient. Different questions, different designs.
Is a population estimate the same as a count?
No. Only a small fraction of the study area is examined, and the calculation assumes those quadrats represent the whole. Always call it an estimate, and be ready to give a limitation: the small proportion sampled, or the possibility that the organisms are not evenly distributed.
Is biodiversity the same as the number of organisms?
No. Biodiversity is the number of different species living in an area. A wheat field contains an enormous number of organisms and has very low biodiversity, because nearly all of them are one species.
Is every introduced species harmful?
No. Most fail to establish, and many that do have no measurable effect — most crops and farm animals are non-native where they are grown or kept. An introduced species may cause harm if it competes with native species, preys on them, brings a disease, or has no natural predators to limit it. Use conditional wording.
How is the greenhouse effect different from the hole in the ozone layer?
They involve different gases, different radiation and different consequences. Greenhouse gases such as carbon dioxide and methane absorb thermal radiation leaving the Earth's surface and re-radiate some of it back, warming the lower atmosphere. Ozone depletion concerns ultraviolet radiation arriving from the Sun, more of which reaches the surface when the ozone layer is thinned.
What makes a resource sustainable?
A sustainable resource is one that is produced or replaced as rapidly as it is removed from the environment, so that it does not run out. Sustainability describes a relationship between two rates, not a property of the material — which is why a renewable resource such as timber or fish can still be used unsustainably.
Does replanting a forest undo deforestation?
Partly. Replanting restores tree cover, so soil is bound again, rain is intercepted, photosynthesis resumes and a future timber supply is secured. But a plantation of one fast-growing species provides far fewer habitats and food sources than a diverse natural forest, so much of the lost biodiversity does not return. Monitoring biodiversity, not just tree cover, is what settles whether the ecosystem has recovered.
Syllabus reference and sources
Written against: Cambridge O Level Biology (5090) 2026–2028 Syllabus (Subject Content, Topic 19: Relationships of organisms with one another and with the environment).
Written by: Academiq Edu Instructor Panel
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