Population, community, habitat, ecosystem
Four definitions, each needing two elements. Population = all individuals of one species in one area at one time. Community = all the populations of different species in the same area. Habitat = the place where an organism lives, e.g. a rock-pool. Ecosystem = a community together with the non-living (abiotic) environment it interacts with — self-sustaining through energy flow and nutrient cycling.
Sampling with quadrats and transects
Quadrats estimate numbers of non-motile organisms: place at random coordinates, count the species inside, take the mean per quadrat, then scale up. Random placement avoids bias; more placements raise reliability. Transects show how distribution changes along a gradient: a belt transect counts abundance in quadrats along a tape; a line transect records only the species touching the tape — quicker but less detailed.
Lincoln index; abiotic and biotic factors
For motile animals, use capture-mark-recapture: catch and mark , let them mix, then catch and count the marked ones ; estimate . It assumes the mark is harmless, mixing is random, and the population is closed. Population size is limited by abiotic factors (temperature, light, pH, water, oxygen, minerals) and biotic factors (food, competition, predation, disease).
Drawn from real examiner reports.
Random gives validity, not reliability
Random placement gives validity — it avoids bias (e.g. choosing lush patches) so the sample represents the whole area. Taking more quadrats and a mean gives reliability. So "why place at random?" = reduce bias / improve validity; "improve reliability?" = more quadrats and a mean. Swapping the two loses the mark.
June 2023 Paper 1BR Q3(b)(i–ii): quadrats are placed at random to reduce bias / improve validity (not reliability); reliability comes from more placements and a mean.
Population ≠ community
A population is all the individuals of one species in an area — the "one species" part wins the mark. Writing "all the living things" or "all the animals" describes a community (many different species). Keep it sharp: population = one species; community = many species in the same area.
IV given instead of the DV
Asked for the dependent variable, candidates often give the independent variable. The IV is what the investigator changes; the DV is what is measured in response. In a transect study of light and plant cover: IV = distance along the transect; DV = mean plants (or % cover) per quadrat.
June 2024 Paper 2B Q2(a)(i) and November 2024 Paper 2B Q4(b)(i): candidates gave the independent variable when asked for the dependent variable — a recurring error across recent sittings.
Abiotic vs biotic factors
Abiotic factors are non-living physical/chemical conditions — temperature, light, pH, water, oxygen, minerals, salinity. Biotic factors are living interactions — food, competition, predation, disease, parasitism. "Soil pH" is abiotic; "a predator" is biotic. Naming the wrong type earns no credit.
Quadrats vs capture-mark-recapture
Match the method to the organism. Use quadrats only for non-motile organisms (plants, limpets) that stay still to be counted. Use capture-mark-recapture for motile animals (woodlice, snails, fish) that would escape a quadrat. Counting woodlice with a quadrat is invalid because they move away before they are counted.
Reliability vs accuracy
These are different. Reliability is improved by repeating and taking a mean (results agree). Accuracy is improved by using a better measuring instrument (e.g. a more precise balance). More repeats do not make a reading more accurate, and a finer instrument does not by itself make results reliable.
Scaling up a quadrat estimate
Estimated total = mean per quadrat × (total area ÷ quadrat area). State the mean, substitute, and give a whole number. The common error is forgetting to divide the total area by the quadrat area before multiplying.
Using the Lincoln-index formula
Write , label , and from the question, substitute, then round to a whole number. Common errors: putting on top instead of underneath, or using alone without multiplying by .
Irregular plots: add every shape
If the study area is an L-shape or irregular polygon, split it into simple shapes (a rectangle plus a triangle) and add the areas. Many candidates computed the rectangle but forgot to add the triangle, losing the mark.
Describe a method in full steps
For "describe how you would estimate the population", give every step: random placement → count → repeat → mean → scale up. "Use quadrats" alone scores zero — state the randomisation, counting and scaling explicitly.
These four terms are defined by the spec (statement 4.1) and appear frequently in 2-mark definition questions. Each needs two elements to score full marks.
| Term | Mark-scheme definition (two elements) | Do NOT write |
|---|---|---|
| Population | All individuals of one species living in the same area at the same time | "All living things in an area" — that is a community |
| Community | All populations of different species living in the same area at the same time | "All plants and animals" — too vague; must say different species |
| Habitat | The place where an organism lives, characterised by its physical and biological conditions | "Where an animal comes from" — no mention of environment |
| Ecosystem | A community of living organisms interacting with the non-living (abiotic) environment | "A place with lots of species" — misses the abiotic component |
Nesting relationship: Individuals of one species form a population -- populations of different species share the same space as a community -- the community plus its physical surroundings form an ecosystem -- the physical location itself is the habitat.
Abiotic factors (non-living) that affect population size and distribution (statement 4.5): Temperature, light intensity, water/moisture availability, pH, mineral ion concentrations, oxygen concentration, salinity.
Biotic factors (living) that affect population size and distribution (statement 4.5): Food supply, competition (intraspecific = same species; interspecific = different species), predation, disease, parasitism.
Define the term ecosystem.
A student surveys the plants in a meadow.
(a) Define the term population. (2)
(b) Define the term community. (2)
