Revision notes for Edexcel A A Level Biology Ecosystems, niches and ecological sampling. Open the guide for explanations and worked examples. Written against the Edexcel A A Level Biology (9BN0) specification, so the content matches what's examinable rather than general Biology background.
Ecosystems, niches and ecological sampling
What you'll learn
- How ecological terms such as population, community, ecosystem, habitat and niche fit together.
- How biotic and abiotic factors affect where organisms live.
- How to use quadrats and transects to collect valid ecological data.
- How to estimate abundance, density, frequency and percentage cover from samples.
The ecological “levels” you need first
Ecology is the study of how organisms interact with each other and with their environment. Before sampling a habitat, you need to be clear about what level you are studying.
Definition
Individual, population, community and ecosystem
An individual is one organism. A population is all the organisms of one species living in the same area at the same time. A community is all the populations of different species living and interacting in an area. An ecosystem is the community plus the non-living, abiotic environment.
Habitat and environmental factors
A habitat is the place where an organism lives. For example, a rocky shore, woodland floor, pond or sand dune can all be habitats.
Factors affecting organisms are grouped into two types:
- Abiotic factors are non-living factors, such as light intensity, temperature, soil pH, water availability, wind speed and mineral ion concentration.
- Biotic factors are living factors, such as predation, competition, disease and food availability.
Key Idea
Organisms are not randomly distributed
Organisms are found where their adaptations allow them to survive and reproduce. Their distribution is often explained by abiotic conditions, biotic interactions, or both.
Ecological niche
An organism’s ecological niche is more than just where it lives. It describes its role in the ecosystem.
Definition
Ecological niche
An ecological niche is the role of a species within an ecosystem, including the resources it uses, the conditions it can tolerate, and its interactions with other organisms.
A niche can include:
- what the organism eats
- when it is active
- where it feeds, nests or shelters
- the temperature, pH or moisture range it tolerates
- its predators, parasites and competitors
Two species with very similar niches may compete strongly because they need the same resources.
Example
Comparing two niches
Two insect species live on the same tree. Species A feeds on aphids on the upper leaves during the day. Species B feeds on aphids on the lower leaves at night.
- Compare the resource used: both species feed on aphids, so there is potential competition for food.
- Compare the place used: Species A mainly uses upper leaves, while Species B mainly uses lower leaves, reducing direct overlap.
- Compare the time of activity: Species A feeds by day and Species B feeds by night, further reducing competition.
- Conclude that their niches overlap partly, but differences in feeding position and time reduce competition.
Common Mistake
Habitat is not the same as niche
A habitat is the place an organism lives. A niche is its role in that place. “Woodland” could be a habitat; “nocturnal seed-eating small mammal that disperses seeds and is prey for owls” is closer to a niche.
Why ecologists sample
In most ecosystems, it is impossible to count every organism. Instead, ecologists take a sample, which is a smaller part of the habitat used to estimate the whole.
Definition
Sampling
Sampling is collecting data from part of a population or habitat in order to estimate patterns in the larger area.
Good sampling should be:
- representative: it reflects the habitat fairly
- unbiased: the investigator does not choose “interesting” spots only
- repeatable: someone else could follow the method and collect comparable data
- large enough: enough samples are taken to reduce the effect of chance
Random sampling with quadrats
A quadrat is a square frame of known area used to sample organisms, usually plants or slow-moving animals. A common size is 0.25 m² or 1 m².
Random sampling is used when you want an unbiased estimate across a habitat.
Typical method:
- Lay out two tape measures at right angles to make a coordinate grid.
- Use random numbers to generate coordinates.
- Place the quadrat at each coordinate.
- Record abundance, percentage cover or frequency.
- Repeat many times and calculate a mean.
What you can record in a quadrat
Abundance is the number of individuals of a species. For small plants, you may count each individual.
Density is abundance per unit area, often in individuals per m².
density=number of individualsarea sampled\text{density} = \frac{\text{number of individuals}}{\text{area sampled}}density=area samplednumber of individualsFrequency is the proportion or percentage of quadrats in which a species is present.
percentage frequency=number of quadrats containing the speciestotal number of quadrats×100\text{percentage frequency} = \frac{\text{number of quadrats containing the species}}{\text{total number of quadrats}} \times 100percentage frequency=total number of quadratsnumber of quadrats containing the species×100Percentage cover is an estimate of the percentage of the quadrat area covered by a species. It is useful for grasses, mosses or plants that overlap and are hard to count individually.
Example
Estimating population size from quadrats
A student uses ten 0.25 m² quadrats in a field. They count a total of 80 daisy plants. The whole field has an area of 200 m².
- Calculate the total sampled area:
ten quadrats each have area 0.25 m², so sampled area = 2.5 m². - Calculate the mean density:
- Estimate the population in the whole field:
- State the answer as an estimate because only part of the field was sampled.
Tip
Choose the right measure
Use abundance or density when individuals are easy to count. Use percentage cover when organisms are too numerous, overlapping or mat-forming. Use frequency when presence or absence is more reliable than counting.
Systematic sampling with transects
Random sampling estimates what is present across an area. Systematic sampling is used when you expect organisms to change along an environmental gradient, such as distance from the sea, light intensity under trees, or soil moisture down a slope.
A transect is a line across a habitat along which samples are taken.
Definition
Transect
A transect is a measured line used to study how the distribution of organisms changes across a habitat or environmental gradient.
There are two common types:
- A line transect records organisms touching or occurring along a line.
- A belt transect uses quadrats placed at intervals along a line, giving data from a strip or “belt” of habitat.
Example
Interpreting a transect pattern
A belt transect runs from a shaded woodland edge into open grassland. Moss percentage cover falls from 70% near the woodland to 5% in the open area, while grass percentage cover increases.
- Identify the pattern: moss decreases as distance from the woodland increases, while grass increases.
- Link the pattern to an abiotic factor: moving into open grassland likely increases light intensity and may reduce moisture.
- Explain the distribution: moss may be better adapted to shaded, damp conditions, while grass may compete better in brighter, drier conditions.
- Recognise a limitation: the transect shows correlation, so extra measurements such as light intensity and soil moisture would strengthen the explanation.
Common Mistake
Correlation is not proof of cause
If plant abundance changes along a transect and light intensity also changes, that suggests a link. To support causation, measure other variables and consider controls, repeats and possible confounding factors.
Practical skills: making ecological data reliable
This topic links strongly to fieldwork practical skills. In Pearson Edexcel Biology A, you should be able to describe and evaluate methods investigating how environmental factors affect organism distribution.
Variables
The independent variable is the factor you choose or compare, such as distance along a transect.
The dependent variable is what you measure, such as percentage cover of a plant species.
Control variables are factors kept as consistent as possible, such as quadrat size, sampling time, identification method and counting rules.
Measuring abiotic factors
Depending on the investigation, you may measure:
- light intensity using a light meter
- soil pH using a pH probe or indicator kit
- temperature using a thermometer or data logger
- soil moisture using a moisture meter
- wind speed using an anemometer
Tip
Improve reliability
Use many quadrats, standardise the method, sample at the same time of day where relevant, and calculate a mean. Repeats reduce the effect of random variation.
Bias and uncertainty
Bias happens when the method consistently favours some results over others. For example, placing quadrats only where flowers are visible would overestimate flower abundance.
Uncertainty is unavoidable in measurements. Percentage cover is especially subjective, so using the same observer, a gridded quadrat, or agreed cover categories can improve consistency.
Common Mistake
Random does not mean haphazard
Random sampling means using a random number method to choose coordinates. Throwing a quadrat “wherever” is not truly random and may introduce bias.
Sampling mobile animals
Quadrats are best for plants and slow-moving organisms. For mobile animals, ecologists may use traps and a capture-mark-release-recapture method.
The population estimate is:
population estimate=number caught first time×number caught second timenumber marked recaptured\text{population estimate} = \frac{\text{number caught first time} \times \text{number caught second time}}{\text{number marked recaptured}}population estimate=number marked recapturednumber caught first time×number caught second timeThis assumes marking does not harm the animals, marks do not rub off, enough time is allowed for mixing, and the population is closed, meaning no major births, deaths, immigration or emigration.
Example
Estimating animals using mark-release-recapture
A student captures, marks and releases 40 woodlice. The next day, they capture 50 woodlice, of which 10 are marked.
- Substitute into the formula:
- Calculate the estimate:
- Interpret the result: the estimated population is 200 woodlice in the sampled area, assuming the marked woodlice mixed randomly back into the population.
Linking sampling back to ecosystems and niches
Ecological sampling gives evidence about where organisms live and how abundant they are. You can then use ecological ideas to explain the pattern.
For example, if a plant is found only in high-salt areas of a salt marsh, its niche likely includes tolerance of high salinity. If another species disappears where a competitor becomes abundant, biotic competition may be involved.
Key Idea
Data first, explanation second
In ecology, describe the distribution pattern using the data, then explain it using abiotic and biotic factors. Avoid jumping straight to a story without evidence.
Exam technique
In the exam
- Define ecological terms precisely: population is one species; community is many species; ecosystem includes abiotic factors.
- For quadrat calculations, always use total sampled area before scaling up to the whole habitat.
- When explaining distributions, include both the observed pattern and a biological reason linked to an abiotic or biotic factor.
Self review
Check yourself
- What is the difference between a habitat and an ecological niche?
- When would you use random quadrats rather than a belt transect?
- Why does increasing the number of quadrats usually make a population estimate more reliable?
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