Individuals, populations, communities, ecosystems, energy and matter. 5 subtopics, 26 key terms.
Learn this topic properly. Interactive diagrams, worked examples, check questions and exam-style practice with mark schemes. Open “2 · Ecology” in FortyFive →
2.1Individuals, populations, communities and ecosystems
Key terms
Species
Organisms that can interbreed to produce fertile offspring.
Population
Individuals of one species in the same area at the same time.
Niche
A species' role and the conditions and resources it uses: fundamental (possible) vs. realised (actual).
Carrying capacity (K)
The maximum population an environment can sustain indefinitely.
Biotic / abiotic factors
Living / non-living factors that affect organisms.
Limiting factor
The factor in shortest supply that restricts growth.
Interactions: competition (intra- and interspecific), predation, herbivory, parasitism, mutualism, disease.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: State the assumptions of any sampling method you use, and say how a broken assumption would affect the estimate.
2.2Energy and biomass in ecosystems
Key terms
Producer (autotroph)
Makes its own food by photosynthesis (or chemosynthesis).
Consumer (heterotroph)
Obtains energy by feeding on other organisms.
Trophic level
A feeding position in a food chain.
Gross primary productivity (GPP)
Total energy fixed by producers by photosynthesis per unit area per unit time.
Net primary productivity (NPP)
GPP minus energy lost in respiration: energy available to consumers.
Ecological efficiency
% of energy passed from one trophic level to the next.
Energy enters as sunlight, is transformed into chemical energy, and leaves as heat: ecosystems follow the laws of thermodynamics. Energy flows; it is not recycled.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Always include units in productivity answers, including the time period (per year).
2.3Biogeochemical cycles
Key terms
Biogeochemical cycle
The movement of an element between living and non-living storages.
Carbon sink / source
Absorbs more carbon than it releases / releases more than it absorbs.
Sequestration
Long-term storage of carbon (forests, soils, oceans, fossil fuels).
Nitrogen fixation
Conversion of N₂ into usable compounds (by bacteria, lightning, the Haber process).
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Name the storage, the flow and the process in each answer (e.g. “carbon flows from biomass to the atmosphere by combustion”).
2.4Climate and biomes
Key terms
Biome
A large area with a similar climate and characteristic vegetation (e.g. tropical rainforest, desert, tundra).
Climate
The long-term average of weather conditions (temperature, precipitation).
Tricellular model
Hadley, Ferrel and polar cells that redistribute heat in the atmosphere.
Climograph
A graph of monthly temperature and precipitation for a place.
Biome distribution depends mainly on temperature, precipitation and insolation, all controlled by latitude, altitude, distance from the sea and ocean currents.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Use a climograph from the question: quote actual temperature and rainfall values to justify the biome.
2.5Zonation, succession and change in ecosystems
Key terms
Zonation
Change in community along an environmental gradient (e.g. altitude, distance from the sea).
Succession
Change in a community over time.
Primary succession
On bare ground with no soil (e.g. new lava, retreating glacier).
Secondary succession
Where soil already exists (e.g. after a fire or abandoned farmland).
Climax community
A stable community at the end of succession.
Pioneer species
The first colonisers, e.g. lichens and mosses.
Succession stages: pioneer → early → mid → late → climax. Over time: soil depth, organic matter, biomass, biodiversity, niches and complexity increase.
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Distinguish zonation (space) from succession (time): mixing them up is a common error.