Population, community, habitat and ecosystem
Learn each term on its mark-earning element. Population = all the organisms of ONE species in a habitat. Community = all the populations of the DIFFERENT species living together in a habitat. Habitat = the PLACE (area) an organism lives. Ecosystem = a community of organisms together with the NON-LIVING (abiotic) parts of the environment, interacting. Vague answers ("all the animals", "where things live") miss the defining element.
Quadrat estimate: mean per quadrat scaled to total area
A quadrat is a square frame of known area, used for non-moving organisms like plants or limpets. Place it AT RANDOM several times, count the organisms (or record percentage cover), then take a MEAN. Estimate the population as the mean per square metre (mean per quadrat ÷ quadrat area) times the total habitat area. Percentage cover (small squares covered ) is used when individuals are hard to count. Compare two areas with the same-size quadrat and method.
Abiotic and biotic factors affect populations
Abiotic factors are NON-LIVING (physical/chemical): light, temperature, water, soil pH, minerals, carbon dioxide, wind. Biotic factors come from OTHER LIVING organisms: food, predation, competition (light, water, space, mates), disease. A change alters population size and distribution: more light lets plants photosynthesise, so they are commoner in the open; more predators reduce prey. A full answer names the factor, labels it abiotic or biotic, and gives a DIRECTION of change.
Drawn from real examiner reports.
Independent vs dependent variable
When comparing a species in two areas, the INDEPENDENT variable is what you change or compare (shaded vs open, or light intensity); the DEPENDENT variable is what you measure (plants per quadrat). Candidates often give the independent variable when asked for the dependent one. Keep other factors (soil, water, temperature) the same, or the conclusion is not valid.
Habitat vs ecosystem
A habitat is just the PLACE an organism lives. An ecosystem is much more — the community of living things PLUS the non-living surroundings, all interacting. A definition of ecosystem that mentions only living things, or that treats it as merely a location, is incomplete and loses the mark. Habitat = where; ecosystem = the living community and its abiotic environment together.
Label factors abiotic or biotic correctly
Abiotic = non-living (light, temperature, water, pH, minerals); biotic = caused by living organisms (food, predators, competition, disease). Naming the wrong type loses the mark even when the factor itself is sensible — e.g. calling predation "abiotic", or temperature "biotic". Decide first whether the cause is a living thing or a physical/chemical condition, then label it.
Scale up using the whole, correct area
Divide by the area actually SAMPLED — the number of quadrats times the quadrat area — not one quadrat's area, before multiplying by the total habitat area. For an irregular habitat, add EVERY part of the shape (e.g. a rectangle plus a triangle), not just the obvious rectangle. A wrong sampled or total area ruins the estimate even when the counting was fine.
Random placement improves validity
Placing quadrats AT RANDOM (random coordinates on a grid) removes bias, so the sample fairly represents the area — improving VALIDITY, not reliability. Reliability is improved separately, by taking MORE quadrat placements and a MEAN. Candidates routinely swap the words. Keep them apart: random → reduce bias / valid; many repeats + mean → reliable.
A population is one species, not all animals
A population is "all the organisms of ONE species in a habitat". Writing "all the animals in an area" scores zero — it drops the defining element (one species) and wrongly excludes plants and microbes. A community is the opposite: all the populations of the DIFFERENT species in a habitat. Calling a community a population, or vice versa, is a frequent error.
Quadrat calc: mean, then per m^2, then total area
For a population estimate, work in steps: mean per quadrat (total counted / number of quadrats), then per square metre (divide by the quadrat area), then multiply by the total habitat area. Show each step so an arithmetic slip still earns the method marks.
Explain means give the reason, not the trend
"Describe" wants the pattern; "explain" wants the reason with a direction ("more light → faster photosynthesis → more growth → bigger population"). The commonest command-word error is describing a trend when asked to explain it — for "explain", name the factor and its mechanism.
Use percentage cover for mat-forming plants
When plants form a mat and cannot be counted individually, record PERCENTAGE COVER: the fraction of the quadrat's small squares the species covers, times 100. The denominator is the number of squares in THAT quadrat (e.g. 25), not 100 — dividing by 100 is a common slip.
State random, repeats and mean every time
When describing or evaluating the quadrat method, always include: place it AT RANDOM (reduce bias / valid); take SEVERAL readings and a MEAN (improve reliability); use the SAME size quadrat and method in both areas (fair test). These phrases are the discriminating marks.
Ecology studies how organisms interact with each other and with their surroundings. Learn these four terms in their exact mark-scheme form (each earns its mark on one key element):
| Term | Mark-scheme definition | The mark-earning element |
|---|---|---|
| Population | All the organisms of one species living in a habitat at a given time | "one species" |
| Community | All the populations of the different species living together in a habitat | "different species / all populations" |
| Habitat | The place (area) where an organism lives | "place / area" |
| Ecosystem | A community of organisms together with the non-living (abiotic) parts of their environment, interacting | "living and non-living" |
Two more definitions you need:
Define the term population.
A pond and the plants, fish, insects and water around it can be studied by ecologists.
Define the terms population, community and ecosystem. (3 marks)
