Ecology
Ecosystem, population, community, habitat, abiotic and biotic factors; food chains and webs; three types of ecological pyramids; why energy decreases between trophic levels; Extended carbon, water, and nitrogen cycles with processes and organisms; limiting factors; quadrat, transect, and mark-recapture sampling; Extended Lincoln-Petersen formula with assumptions.
Ecology
CORE EXTENDEDEcosystem Key Terms
| Term | Definition |
|---|---|
| Ecosystem | All the living organisms in an area together with the non-living (abiotic) components of the environment |
| Population | All the organisms of the same species living in the same area at the same time |
| Community | All the populations of different species living in the same area at the same time (biotic component only) |
| Habitat | The place where an organism lives, characterised by its physical features and the species present |
| Abiotic factors | Non-living factors of the environment (temperature, light intensity, water availability, soil pH, wind speed, salinity) |
| Biotic factors | Living components of the environment (predation, competition, disease, food availability, parasitism) |
Food Chains and Food Webs
Food chain: shows the linear transfer of energy from one organism to the next. Arrows point in the direction of energy flow (from prey to predator, from eaten to eater).
Food web: a network of interconnected food chains in an ecosystem. More realistic; shows multiple feeding relationships.
Producers (plants/algae/phytoplankton) are always at the start — they convert light energy to chemical energy by photosynthesis.
Consumers: primary (herbivores) → secondary → tertiary (carnivores).
Decomposers (bacteria and fungi) break down dead organic matter, returning minerals to the soil.
When asked to predict the effect of removing a species: trace all arrows connected to that species. Removing a prey species → predators above may decrease (less food). Removing a predator → prey below may increase (less predation), which may cause species further below to decrease (more competition for food/over-grazing).
Energy Flow and Ecological Pyramids
Energy enters ecosystems via photosynthesis and is transferred along food chains. At each trophic level, most energy is lost and cannot be passed to the next level.
| Type of pyramid | What it shows | Always upright? |
|---|---|---|
| Pyramid of numbers | Number of organisms at each trophic level | Not always — can be inverted (e.g. one tree supporting thousands of insects) |
| Pyramid of biomass | Total dry mass of organisms at each level | Usually upright; occasionally inverted in aquatic systems (fast-reproducing phytoplankton) |
| Pyramid of energy | Energy available at each level | Always upright — energy always decreases at each transfer |
At each transfer, energy is lost as:
- Heat from cellular respiration (metabolic processes)
- Undigested material in faeces (passed to decomposers, not to next consumer)
- Urine and excretory products (containing chemical energy not absorbed)
Typically only 10–20% of energy at one level is transferred to the next. This limits food chain length to about 4–5 links.
Carbon Cycle — Extended
| Process | Direction of carbon transfer | Organisms involved |
|---|---|---|
| Photosynthesis | Atmosphere → biosphere | Plants, algae, cyanobacteria |
| Respiration | Biosphere → atmosphere | All living organisms |
| Combustion | Fossil fuels/biomass → atmosphere | Human activity (burning) |
| Decomposition | Biosphere → atmosphere/soil | Bacteria and fungi (decomposers) |
| Feeding | Producer → consumer | Herbivores, carnivores |
| Fossilisation | Biosphere → lithosphere | Geological processes (over millions of years) |
Burning fossil fuels: releases CO₂ stored for millions of years → rapidly increases atmospheric CO₂ → enhanced greenhouse effect → global warming.
Deforestation: (1) removes trees that absorb CO₂ by photosynthesis; (2) burning or decomposing felled trees releases stored carbon → double impact on atmospheric CO₂.
Water Cycle — Extended
| Process | Description |
|---|---|
| Evaporation | Water from oceans, lakes, and soil heated by sun → water vapour rises into atmosphere |
| Transpiration | Water vapour released from plant leaves through stomata → into atmosphere |
| Condensation | Water vapour cools as it rises → condenses into water droplets → forms clouds |
| Precipitation | Water falls as rain, snow, hail, or sleet back to Earth’s surface |
| Surface runoff | Water flows over land into rivers, lakes, and eventually seas |
| Infiltration | Water soaks into soil → replenishes groundwater (aquifers) |
Nitrogen Cycle — Extended
Nitrogen is essential for making amino acids, proteins, and nucleic acids. Atmospheric N₂ cannot be used directly by most organisms — it must be converted to usable forms.
| Process | Conversion | Organisms/agents |
|---|---|---|
| Biological nitrogen fixation | N₂ → ammonia / ammonium compounds (NH₃) | Rhizobium in legume root nodules; Azotobacter and other free-living nitrogen-fixing bacteria in soil |
| Atmospheric nitrogen fixation | N₂ → nitrogen oxides → nitrates in soil water | Lightning — energy of lightning converts N₂ to nitrogen oxides; these dissolve in rain to form nitrates absorbed by plant roots |
| Nitrification | NH₃ → NO₂⁻ → NO₃⁻ (nitrate) | Nitrifying bacteria (Nitrosomonas, Nitrobacter) in aerobic soil |
| Assimilation | NO₃⁻ → organic N (proteins, DNA) | Plants absorb nitrate via roots; animals eat plants |
| Ammonification | Organic N → NH₃ / NH₄⁺ | Decomposer bacteria and fungi break down dead organic matter |
| Denitrification | NO₃⁻ → N₂ (returns to atmosphere) | Denitrifying bacteria in waterlogged/anaerobic soils |
Crops absorb nitrate from soil. Harvesting removes this nitrogen permanently from the field — it is not returned via decomposition as it would be in a natural ecosystem. Over time, soil nitrogen is depleted. Farmers replenish it by adding inorganic nitrogen fertilisers (e.g. ammonium nitrate) or organic fertilisers (manure, compost), or by growing leguminous cover crops that fix atmospheric N₂.
Population Size and Limiting Factors
Population size is determined by the balance between births + immigration and deaths + emigration. In natural ecosystems, populations do not grow indefinitely — they are limited by:
| Limiting factor | Type | Effect on population |
|---|---|---|
| Food availability | Biotic | Less food → starvation → increased death rate |
| Predation | Biotic | More predators → prey population decreases |
| Disease | Biotic | Pathogens spread more easily in dense populations |
| Competition | Biotic | Intraspecific (same species) or interspecific (different species) competition for resources |
| Temperature | Abiotic | Too hot/cold reduces survival and reproduction |
| Water/rainfall | Abiotic | Drought reduces food and habitat; flooding alters habitat |
Sampling Techniques
To estimate population size or study species distribution without counting every individual, scientists use sampling methods.
| Method | Used for | How it works |
|---|---|---|
| Quadrat | Plants and slow-moving/sessile animals | A square frame (e.g. 0.5×0.5 m or 1×1 m) placed randomly in the study area. Count the number/percentage cover of species inside. Repeat many times and calculate the mean. Multiply mean by total area to estimate total population. |
| Transect | Distribution along an environmental gradient (e.g. shore, edge of woodland) | A line is laid across the area. Record species at regular intervals (belt transect: quadrats placed at intervals along the line). Shows how species distribution changes with the gradient. |
| Mark-recapture | Mobile animals | Catch a sample, mark and release. Later, catch a second sample. Count the proportion of marked individuals in the second sample. Lincoln-Petersen formula: N = (n₁ × n₂) ÷ m₂ |
Mark-Recapture Calculation — Extended
N = (n₁ × n₂) ÷ m₂
Where: N = estimated total population; n₁ = number caught and marked in first sample; n₂ = total number caught in second sample; m₂ = number of marked individuals in second sample.
Assumptions: (1) marks do not affect survival or behaviour; (2) marks are not lost between captures; (3) the population is closed (no births, deaths, immigration, emigration between samples); (4) marked individuals mix randomly with the population before recapture.
Example: n₁ = 40 marked; n₂ = 25 total recaptured; m₂ = 8 marked in second sample. N = (40 × 25) ÷ 8 = 1000 ÷ 8 = 125.
In an ecosystem, a pyramid of energy is always upright. Why can a pyramid of numbers be inverted?
- A. Energy can increase as it passes through a food chain
- B. One large producer organism can support many smaller consumers, so consumer numbers can exceed producer numbers
- C. Decomposers add energy to the food chain, reversing the pyramid
- D. The number of organisms always equals the energy available at each level
A student catches 60 woodlice from a garden, marks them with white paint, and releases them. Two days later, she catches 45 woodlice from the same area and finds 9 are marked. Estimate the total woodlouse population. Show your working. [2 marks]
- N = (n₁ × n₂) ÷ m₂ = (60 × 45) ÷ 9 [1 mark for correct substitution]
- N = 2700 ÷ 9 = 300 woodlice [1 mark]
Comprehensive Practice Questions
Mixed questions across Topic 19.
Which of the following is the main reason why energy is lost between trophic levels in a food chain?
- A. Organisms at higher trophic levels are smaller and need less energy
- B. Energy is released as heat during cellular respiration and some material is lost as undigested faeces
- C. Producers convert only a small fraction of sunlight into chemical energy
- D. Decomposers absorb most of the energy before it reaches consumers
(a) State the names of two types of bacteria involved in the nitrogen cycle and describe the role of each. [4 marks]
(b) A student uses quadrats to estimate the number of daisy plants in a field of area 200 m². In 10 randomly placed 0.5 m × 0.5 m quadrats, she counts: 3, 5, 2, 4, 3, 6, 2, 4, 5, 6 daisy plants. Estimate the total number of daisies in the field. [3 marks]
- (a) Any two of:
Nitrogen-fixing bacteria (e.g. Rhizobium): convert atmospheric N₂ into ammonia/ammonium ions that plants can use [2 marks]
Nitrifying bacteria (e.g. Nitrosomonas/Nitrobacter): convert ammonia to nitrite, then nitrate in the soil [2 marks]
Denitrifying bacteria: convert nitrate back to N₂ gas, returning nitrogen to the atmosphere [2 marks]
Decomposers (bacteria/fungi): convert organic nitrogen in dead matter to ammonia (ammonification) [2 marks]
[1 mark for name + 1 mark for correct role; any two pairs] - (b) Mean per quadrat: (3+5+2+4+3+6+2+4+5+6) ÷ 10 = 40 ÷ 10 = 4 daisies [1 mark]
Area of one quadrat = 0.5 × 0.5 = 0.25 m² [1 mark]
Estimated total = 4 ÷ 0.25 × 200 = 4 × 4 × 200 = 3200 daisies [1 mark]
The diagram below represents a simple food web in a pond:
phytoplankton → water fleas → small fish → large fish
phytoplankton → water fleas → large fish
(a) Identify the producer in this food web and explain its role in the carbon cycle. [3 marks]
(b) A disease kills most of the small fish. Predict the short-term effect on (i) large fish and (ii) water fleas. Explain your answers. [4 marks]
- The producer is phytoplankton [1 mark]
- Phytoplankton carry out photosynthesis, absorbing CO₂ from the water/atmosphere and converting it to organic carbon compounds (glucose, etc.) [1 mark]
- This transfers carbon from the atmosphere/water into the biosphere (living organisms), forming the basis of the food web [1 mark]
- (i) Large fish: population likely to decrease initially [1 mark]; small fish are a food source for large fish — less food available → increased competition among large fish / starvation [1 mark]
- (ii) Water fleas: population likely to increase initially [1 mark]; small fish eat water fleas — with fewer small fish, less predation pressure on water fleas → water flea population rises [1 mark]
High-Frequency Mistakes — Topic 19 Overall
- →Drawing food chain arrows the wrong wayArrows in food chains and webs show the direction of energy flow — they point FROM the organism being eaten TO the organism eating it (from prey to predator). Never draw arrows pointing from predator to prey (showing who eats whom in reverse).
- 📈Saying energy pyramids can be invertedPyramids of energy are always upright — energy always decreases at each trophic level (due to heat loss and faeces). Only pyramids of numbers and occasionally pyramids of biomass can be inverted. This is frequently tested as a spot-the-error question.
- 🌿Confusing community and ecosystemCommunity = all the different populations of species in an area (biotic only). Ecosystem = community + all abiotic factors. Adding "abiotic factors" converts a community into an ecosystem. This is the same key distinction from Topic 1 — it reappears in ecology questions.
- 🔄Ext: Confusing nitrification and nitrogen fixationBiological nitrogen fixation: N₂ → ammonia/ammonium compounds, by nitrogen-fixing bacteria (e.g. Rhizobium). Atmospheric nitrogen fixation: lightning converts N₂ to nitrogen oxides, which dissolve in rain to form nitrates. Nitrification: NH₃ → NO₂⁻ → NO₃⁻ by nitrifying bacteria in aerobic soil. They are different processes — do not merge biological and atmospheric fixation into one equation.
- 📈Ext: Applying mark-recapture formula incorrectlyN = (n₁ × n₂) ÷ m₂. A common error is dividing by n₂ instead of m₂, or swapping n₁ and m₂. Write out the formula before substituting. Remember: m₂ is always the smaller number (the recaptured marked individuals), not the total second sample.
- 🌎Saying decomposers are consumers in a food chainDecomposers are not shown as primary, secondary, or tertiary consumers in a simple grazing food chain — they form a separate decomposer pathway. They break down dead organic matter from all trophic levels and return mineral ions (including nitrogen as ammonia/nitrate) to the soil for plant uptake. They are heterotrophs but are always shown separately from the main consumer levels in food web diagrams.
- ☀Saying producers make all the energy in an ecosystemProducers convert light energy into chemical energy by photosynthesis — they do not "make" energy. Energy enters the ecosystem from the sun; producers trap it in organic molecules. Energy is never created or destroyed, only converted and transferred.
Highest-yield Core items: the six ecosystem key terms (especially community vs ecosystem); food chain/web arrows direction (prey to predator); three types of pyramid and which can/cannot be inverted (energy pyramid always upright); why energy decreases at each trophic level (heat from respiration + faeces); limiting factors (biotic and abiotic); quadrat sampling calculation (mean per quadrat → scale up to total area). For Extended: carbon cycle process table (photosynthesis/respiration/combustion/decomposition); nitrogen cycle five processes (fixation/nitrification/assimilation/ammonification/denitrification — match each to the correct bacterial species); mark-recapture formula N = (n₁ × n₂) ÷ m₂ and its four assumptions. Nitrogen cycle and mark-recapture are near-certain Paper 4 targets.