Edexcel IGCSE Biology · Spec 4.2-4.5
Studying Ecosystems
How to study ecosystems using quadrats and transects to estimate the size of populations.
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Studying Ecosystems
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Explained
Sampling a habitat, and the words that describe it
You cannot count every organism in a field, so ecology works by sampling: count a small part properly and scale up. The techniques are simple, and the marks are in doing them without bias.
The vocabulary, smallest to largest
A habitat is the place where an organism lives. A population is all the organisms of one species living there. A community is all the populations of all the species. An ecosystem is that community together with the non living surroundings and the interactions between them.
Those definitions are recall marks, and the distinction that matters is that a population is one species and a community is all of them.
Quadrats
A quadrat is a square frame of known area. Place it, count the organisms of interest inside, and repeat in several places. Then find the mean number per quadrat and multiply by the total area to estimate the population.
Placing it randomly is essential. If you choose where to put it, you will choose where the plants look interesting, and your estimate will be wrong. Generate random coordinates and use those, and say so in the answer.
Quadrats suit organisms that do not move, which in practice means plants and slow animals such as limpets and barnacles.
Transects
A transect is a line across a habitat, used when you expect a gradual change rather than an even spread, such as going up a beach or from open field into woodland.
Lay a tape along the line and place quadrats at regular intervals along it, recording what you find at each. Now the placing is deliberately not random, because the question is how distribution changes with position, and the interval is what keeps it fair.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for estimating a population credits five things: using a quadrat, placing it randomly or using random coordinates, repeating, counting the organisms inside, and scaling up by multiplying by the area.
Two notes beside it are worth having. The first says to ignore quadrant. A quadrant is a quarter of a circle and has nothing to do with ecology, and the two words are close enough that the error is common.
The second is more interesting. Writing quadrats, in the plural, earns two of the marks at once, because the plural implies repeating. One well chosen letter carries a whole marking point.
A following part asks for a variable that should be kept the same, and credits the time of day, the location or position on the shore, the species being counted, or the same day. Sampling is an experiment like any other, and the fair test rules still apply.
Abiotic and biotic factors
Abiotic factors are the non living conditions: light intensity, temperature, water availability, soil pH, oxygen or carbon dioxide concentration, and wind.
Biotic factors involve other organisms: predation, competition for food, water, light or space, disease, and the availability of food.
If a question asks why a species is more common in one part of a habitat, name a factor from one of those lists and say how it affects that species. Naming light intensity is half an answer; saying that more light means more photosynthesis, so the plants grow better, is the whole one.
Why estimates vary
Sampling gives an estimate, not a count, and its reliability depends on how many quadrats you used and how evenly the organisms are spread.
More samples give a better estimate. If a question asks how to improve the method, more quadrats, random placement and repeating on different days are all creditworthy, and each addresses a different source of error.
Spec 4.2-4.5
What you need to know
- Use quadrats to estimate population size
- Use transects to study distribution
- Recall abiotic and biotic factors
Active recall
Quick check
Answer each question before opening the answer.
What is a quadrat used for?
To estimate the abundance or distribution of organisms in an area.
How do you estimate a population size using quadrats?
Find the mean number per quadrat, then multiply by the total area (in quadrats).
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