4 Ecology and the environment
- Syllabus
- 2024
- Section
- 4
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(a) The organism in the environment
- Syllabus
- 2024
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Build the ecological hierarchy
Ecologists describe organisms at nested scales, from one species in an area to all organisms interacting with the non-living environment.
| Term | Meaning | Example in one woodland |
|---|---|---|
| habitat | the place where an organism lives | the woodland floor |
| population | all organisms of one species in an area at a given time | all bluebells in the woodland |
| community | all the populations of different species in the area | bluebells, trees, fungi and animals |
| ecosystem | the community together with its abiotic environment and their interactions | the woodland community plus soil, water, light and temperature |
A community includes living populations only; an ecosystem adds the non-living environment. A habitat is a place, while a population is a group of one species living there.
Compare populations in two areas with quadrats
A fair quadrat investigation estimates the abundance of a plant or slow-moving organism in two areas using the same unbiased sampling method.
| Stage | Method and reason |
|---|---|
| 1 define areas | map each area and measure its total size |
| 2 sample randomly | use grid coordinates and a random-number generator to place equal-sized quadrats without choosing favourable patches |
| 3 measure | apply one counting rule to record individuals, frequency or percentage cover |
| 4 repeat | use many quadrats in each area to reduce the effect of patchiness |
| 5 compare | calculate abundance per quadrat or per m² and a mean for each area |
| 6 estimate population | mean density × total area, when a whole-area estimate is required |
Keep quadrat size, number of samples, species definition, counting rule, season and relevant time or weather conditions consistent between areas. Report variation as well as the mean when possible.
A quadrat samples a population; one quadrat is not the whole population. Convenience placement near paths or conspicuous organisms creates bias, and unequal effort makes the two areas incomparable.
Measure biodiversity as richness and evenness
Biodiversity is the variety of living organisms in an area. At species level it depends on both how many species are present and how evenly individuals are distributed among them.
| Component | Question it answers |
|---|---|
| species richness | How many different species are present? |
| relative abundance | What proportion of individuals belongs to each species? |
| evenness | Are individuals spread fairly evenly, or does one species dominate? |
| comparison | With equal richness, the community with greater evenness has greater biodiversity |
Two fields may each contain three plant species. A field with counts 15, 18 and 14 is more even—and therefore more diverse—than one with counts 19, 4 and 2, despite identical richness.
Counting species alone can be incomplete: two sites with the same richness may differ greatly in abundance and evenness. A large number of individuals of one species does not by itself mean high biodiversity.
Sample distribution and biodiversity with quadrats
Quadrats can reveal where organisms occur and allow biodiversity comparisons when placement is unbiased and sampling effort is sufficient.
| Investigation decision | Strong method |
|---|---|
| random distribution survey | overlay a coordinate grid; generate random x and y coordinates |
| change along a gradient | place quadrats at fixed intervals on a transect from the suspected factor |
| abundance measure | count individuals; for mats or lichens, estimate percentage cover with a gridded quadrat |
| biodiversity record | identify every species and record its abundance in every quadrat |
| reliability | repeat enough quadrats and compare means, spread, richness and evenness |
| valid comparison | keep quadrat area, sampling effort, identification rules, season and conditions consistent |
Density = total individuals counted ÷ total sampled area. Percentage cover = occupied grid area ÷ total quadrat area × 100. Distribution is supported by changes among locations, not just an overall total.
Random sampling tests a general area without observer choice; systematic transects test a spatial gradient. Neither means placing quadrats wherever organisms are easiest to see.
Explain population size and distribution with limiting factors
Population size and distribution reflect whether physical conditions and interactions with other organisms allow survival and reproduction at each location.
| Factor type | Examples | Route to population change |
|---|---|---|
| abiotic: non-living | light, temperature, water, oxygen, pH, salinity, mineral ions, wind | changes photosynthesis, respiration, enzyme activity, water balance or nutrient supply |
| biotic: living | food, competition, predation, disease, pathogens and mates | changes death rate, birth rate, access to resources or successful reproduction |
| distribution | a factor varies from place to place | organisms occur most where conditions fall within their tolerance and resources are accessible |
| population size | births and immigration versus deaths and emigration | size rises when gains exceed losses and falls when losses exceed gains |
Use data before proposing a mechanism: identify the population change, identify a factor that also changed, then explain the biological link. For example, more predators can increase prey deaths; fewer food plants can intensify competition and reduce reproduction.
Correlation suggests a possible factor but does not prove causation; several abiotic and biotic factors can change together. Biotic means involving living organisms, not simply any variable measured in biology.
(b) Feeding relationships
- Syllabus
- 2024
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Assign organisms to trophic levels
A trophic level is an organism's feeding position in a food chain; the position depends on what the organism eats in that particular chain.
| Trophic position | Name and role |
|---|---|
| 1 | producer: makes organic substances, usually by photosynthesis |
| 2 | primary consumer: feeds on a producer |
| 3 | secondary consumer: feeds on a primary consumer |
| 4 | tertiary consumer: feeds on a secondary consumer |
| across levels | decomposers such as bacteria and fungi digest dead organisms and waste, recycling substances |
In a food web, one organism can occupy more than one trophic level because it may eat prey from different levels. Trace each route separately from its producer.
Arrows point from food to feeder—the direction in which substances and energy transfer. A top predator is not automatically tertiary if the chain has a different number of steps.
Read chains, webs and ecological pyramids
Food chains show one transfer route; food webs join many routes. Ecological pyramids compare trophic levels using number, biomass or energy.
| Representation | What width means | Shape and limitation |
|---|---|---|
| food chain | not a width model; each arrow is a feeding transfer | one linear route |
| food web | linked feeding routes | shows alternatives and indirect effects |
| pyramid of numbers | number of organisms | may be inverted: one tree can support many insects |
| pyramid of biomass | dry mass at one time, often per unit area | usually upright, but fast-turnover producers can give an inverted snapshot |
| pyramid of energy transfer | energy transferred per area per time | always upright because usable energy decreases between levels |
Label every trophic level and keep the producer at the base. Use the quantity named in the question: organism size affects biomass but not the count used in a pyramid of numbers.
Pyramids of number and biomass are not interchangeable. Biomass is a standing mass snapshot; energy transfer includes a time interval and cannot increase up a chain.
Track substances and energy through food chains
Producers store light energy as chemical energy in organic substances; feeding transfers some of those substances and their chemical energy to consumers and decomposers.
| Route | Substance or energy change |
|---|---|
| photosynthesis | light energy becomes chemical energy in producer biomass |
| feeding | carbon compounds, mineral-containing molecules and stored chemical energy enter the consumer |
| assimilation | digested, absorbed food can be built into new biomass |
| respiration | chemical energy is released for cellular work and ultimately dissipated as heat |
| egestion and excretion | substances and their remaining energy leave as faeces or metabolic waste |
| death and uneaten material | organic matter passes to decomposers, which respire and recycle mineral ions |
Transfer efficiency (%) = energy or biomass transferred to the next level ÷ energy or biomass available at the previous level × 100. Compare like units and the same area and time interval.
Substances cycle through organisms and the environment, but energy flows one way and is dissipated as heat. Decomposers recycle matter; they do not return used energy to producers.
Account for the roughly 10% energy transfer
On average, only about 10% of the energy at one trophic level becomes biomass available to the next; the exact percentage varies among organisms and ecosystems.
| Energy route not transferred as new consumer biomass | Mechanism |
|---|---|
| organisms or parts not eaten | roots, bones, fur or whole organisms remain for decomposers |
| food not digested or absorbed | energy remains in faeces and is egested |
| excretory products | some chemical energy leaves in substances such as urea |
| respiration and activity | energy supports movement, active transport and other cell processes |
| heat | energy released in respiration is ultimately dissipated to surroundings |
If producers store 20,000 kJ m⁻² yr⁻¹, an approximate 10% model predicts 2,000 at primary consumers and 200 at secondary consumers. Reapply the fraction at every transfer rather than subtracting one fixed amount.
Energy is not destroyed: much is transferred to decomposers or dissipated as heat and is therefore unavailable to the next consumer level. '10%' is a useful approximate pattern, not a universal constant.
(c) Cycles within ecosystems
- Syllabus
- 2024
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Route carbon through an ecosystem
The carbon cycle transfers carbon among atmospheric carbon dioxide, living biomass, dead organic matter and fuels; each arrow must be labelled by a process.
| From → to | Process and mechanism |
|---|---|
| atmospheric CO₂ → producer biomass | photosynthesis fixes carbon into carbohydrates and other organic molecules |
| producer → consumer | feeding transfers carbon compounds along food chains |
| living organism → atmospheric CO₂ | respiration oxidises organic molecules and releases carbon dioxide |
| dead material and waste → decomposers | feeding and extracellular digestion transfer carbon to bacteria and fungi |
| decomposers → atmospheric CO₂ | decomposer respiration releases carbon dioxide |
| biomass or fossil fuel → atmospheric CO₂ | combustion releases carbon dioxide |
A carbon sink absorbs more carbon than it releases. Increased photosynthesis can strengthen a sink, while respiration, decomposition, combustion and reduced photosynthesis after deforestation can weaken it.
Photosynthesis removes atmospheric CO₂; respiration, decomposition through decomposer respiration, and combustion return it. Carbon matter cycles, but the energy driving these processes does not cycle.
Route nitrogen through bacteria, plants and animals
Nitrogen cycles between atmospheric nitrogen gas, ammonium compounds, nitrates and nitrogen-containing molecules in organisms. Bacteria make the key inorganic conversions.
| Process | Conversion and role |
|---|---|
| nitrogen fixation | nitrogen-fixing bacteria convert atmospheric N₂ into ammonia or ammonium compounds |
| decomposition / ammonification | decomposers digest dead organisms and waste; nitrogen in proteins and nucleic acids becomes ammonium compounds |
| nitrification | nitrifying bacteria oxidise ammonium to nitrite and then nitrite to nitrate |
| plant uptake | roots absorb nitrate ions and plants use nitrogen to make amino acids, proteins and nucleic acids |
| feeding | nitrogen-containing organic molecules pass from plants to animals |
| denitrification | denitrifying bacteria convert nitrate into nitrogen gas, returning it to the atmosphere |
Animal excretion and death return organic nitrogen to decomposers. Leached nitrate can leave soil for water, while denitrification lowers soil nitrate availability. Specific bacterial species names are not required.
Plants cannot use atmospheric N₂ directly in this cycle: nitrogen fixation must first create usable compounds. Nitrifying bacteria make nitrate; denitrifying bacteria remove nitrate by returning nitrogen to the atmosphere.
(d) Human influences on the environment
- Syllabus
- 2024
- Topic
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- Level
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Explain harm from sulfur dioxide and carbon monoxide
Sulfur dioxide and carbon monoxide harm organisms by different mechanisms: sulfur dioxide forms acidic pollution, while carbon monoxide reduces oxygen transport in blood.
| Pollutant | Route and biological consequences |
|---|---|
| sulfur dioxide, SO₂ | dissolves in atmospheric water and contributes to acid rain; acidic water and soil damage leaves and roots, kill aquatic organisms, disrupt food webs and reduce biodiversity; the gas also irritates airways |
| carbon monoxide, CO | binds strongly to haemoglobin to form carboxyhaemoglobin, leaving less haemoglobin able to carry oxygen; cells receive less oxygen for aerobic respiration, causing tiredness, unconsciousness or death at high exposure |
Pollution-sensitive lichens may have lower coverage near a sulfur-dioxide source, but distance data show correlation; other conditions must be controlled before attributing the pattern solely to pollution.
Carbon monoxide is toxic but is not one of the greenhouse gases named in objective 4.13. Sulfur dioxide's acid-rain mechanism is distinct from carbon dioxide's greenhouse effect.
Identify the five greenhouse gases in scope
Greenhouse gases absorb outgoing infrared radiation and re-emit it, reducing the rate at which energy escapes from Earth to space.
| Greenhouse gas required in this syllabus | Formula or name cue |
|---|---|
| water vapour | H₂O in the atmosphere |
| carbon dioxide | CO₂ |
| nitrous oxide | N₂O; not nitrogen gas, N₂ |
| methane | CH₄ |
| chlorofluorocarbons | CFCs |
Oxygen, nitrogen and carbon monoxide are not members of this required list. A gas can be an air pollutant without being a greenhouse gas, and vice versa.
Link human activities to greenhouse-gas emissions
Human activities raise greenhouse-gas concentrations by releasing stored carbon and nitrogen compounds, creating methane, removing carbon sinks and emitting manufactured gases.
| Human activity | Main greenhouse-gas contribution |
|---|---|
| burning coal, oil and natural gas for electricity, heating, industry and transport | carbon dioxide from combustion |
| deforestation and burning cleared vegetation | carbon dioxide released, plus less CO₂ removed by photosynthesis |
| cattle and other ruminants, rice fields and anaerobic decay in landfill | methane |
| nitrogen fertilisers and disturbed agricultural soils | nitrous oxide |
| leakage from older refrigeration, air-conditioning and aerosol systems | CFCs |
A complete explanation names both the activity and the process. For example, more petrol cars means more fuel combustion, so more carbon dioxide enters the atmosphere.
Electric vehicles have no tailpipe carbon dioxide, but their total contribution depends on how electricity and materials are produced. Do not claim an activity has zero emissions without defining the system boundary.
Trace the enhanced greenhouse effect to global warming
Increasing greenhouse-gas concentrations enhances the natural greenhouse effect, raising Earth's average temperature and potentially causing global warming and wider climate change.
| Stage | Energy or biological consequence |
|---|---|
| 1 | incoming short-wave solar radiation reaches Earth's surface and warms it |
| 2 | the warm surface emits long-wave infrared radiation |
| 3 | greenhouse gases absorb and re-emit some infrared radiation |
| 4 | higher greenhouse-gas concentrations slow net energy loss to space |
| 5 | the climate system warms until incoming and outgoing energy balance again |
| 6 | warming can melt land ice, raise sea level, alter rainfall, increase drought or flooding, shift habitats, disrupt food webs, bleach coral and change species distributions |
A biological explanation links the physical change to organisms: melting sea ice can reduce the surface available for microscopic producers, reducing food for krill and then whales.
The greenhouse effect is a natural warming mechanism; the enhanced greenhouse effect is its strengthening by increased gas concentrations. Greenhouse gases absorb infrared radiation rather than simply trapping incoming sunlight.
Explain the biological consequences of sewage pollution
Untreated sewage adds pathogens, organic matter and mineral nutrients to water, so it can cause disease and deplete dissolved oxygen.
| Sewage component | Biological consequence |
|---|---|
| pathogenic microorganisms | water-borne disease can spread among humans and other animals |
| organic matter and faeces | decomposer bacteria multiply and respire while breaking it down; biochemical oxygen demand rises and dissolved oxygen falls |
| nitrates and phosphates | stimulate excessive algal and plant growth, adding a eutrophication route |
| low dissolved oxygen | fish and many aquatic invertebrates cannot respire aerobically enough; populations fall or die |
| community change | tolerant decomposers may increase while sensitive species disappear, lowering biodiversity |
Bacteria do not remove oxygen merely by being present: their increased aerobic respiration consumes it while organic sewage is decomposed. Sewage pollution also has a direct pathogen hazard beyond eutrophication.
Trace fertiliser leaching through eutrophication
Eutrophication begins when soluble mineral ions from excess fertiliser leach or run off into water and ends with oxygen depletion and loss of aquatic life.
| Stage | Causal change |
|---|---|
| 1 | rain washes nitrate and phosphate ions from soil into a river or lake |
| 2 | extra mineral nutrients cause rapid algal growth—an algal bloom |
| 3 | the bloom shades submerged plants, reducing photosynthesis; plants and algae die |
| 4 | decomposer bacteria break down the dead organic matter and their populations increase |
| 5 | bacterial aerobic respiration consumes dissolved oxygen |
| 6 | oxygen-sensitive fish and invertebrates die or leave; food webs simplify and biodiversity falls |
Downstream of a fertilised field, repeated samples may show lower mean dissolved oxygen than upstream. The position and flow direction support the mechanism, while repeats strengthen reliability.
Fertiliser does not poison fish directly in the standard eutrophication chain. The critical link is extra nutrients → bloom and death → decomposer respiration → oxygen depletion.
Connect deforestation to soil, water and atmospheric change
Removing a forest removes canopy, roots and photosynthetic biomass, so deforestation changes soil retention, water transfer, the carbon cycle and atmospheric gases together.
| Lost forest function | Consequence of deforestation |
|---|---|
| canopy intercepts rain; roots bind soil | more surface runoff and soil erosion; sediment can enter and block waterways |
| roots take up mineral ions; litter returns nutrients | soluble ions leach from exposed soil, reducing fertility |
| leaves transpire water | less evapotranspiration can reduce atmospheric water vapour and local rainfall, disturbing the water cycle |
| photosynthesis removes CO₂ and releases O₂ | less uptake of carbon dioxide and less oxygen production |
| biomass stores carbon | decay or combustion releases carbon dioxide, strengthening the enhanced greenhouse effect |
| trees provide habitat and food | populations and food-web links are lost, reducing biodiversity |
Replanting can reduce flooding because roots increase soil stability and water uptake, foliage intercepts rain, transpiration returns water to the atmosphere, and slower runoff reduces rapid river rise.
Deforestation changes stocks and flows: carbon already stored may be released, while future CO₂ removal falls. Soil erosion is physical loss of soil; leaching is dissolved mineral ions being washed through or out of it.