Edexcel IGCSE Biology · Spec 4.2, 4.4B
Comparing Habitats and Biodiversity with Quadrats
Covers compare populations in two areas using quadrats, Measure abundance, frequency and percentage cover and Use quadrat data to compare biodiversity.
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Comparing Habitats and Biodiversity with Quadrats
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Explained
Comparing two places fairly
Comparing habitats is sampling done twice, and the whole difficulty is making sure the two sets of results can honestly be set against each other.
Use the same size of quadrat, the same number of quadrats, the same method of placing them, and the same person deciding what counts as present. Anything you change between the two areas becomes an alternative explanation for the difference you find.
Three ways to measure abundance
Counting individuals works for things you can distinguish, such as daisies or limpets. Record the number in each quadrat, find the mean per quadrat, then multiply by the number of quadrats that would fit the whole area.
Percentage cover works for things you cannot count, such as grass or moss. Estimate what proportion of the quadrat's area the species covers, which is easier with a gridded quadrat divided into 100 small squares, so each square is one per cent.
Frequency is the percentage of quadrats in which the species appeared at all. If a species turned up in 14 of 20 quadrats, its frequency is 70 per cent. This tells you how widespread a species is rather than how much of it there is, and the two can differ sharply.
Biodiversity
Biodiversity is the variety of living organisms in an area, and it has two components that questions distinguish.
Species richness is how many different species are present. Species evenness is how similar their numbers are. A field with five species in roughly equal numbers is more biodiverse than one with five species where one accounts for almost everything.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks what biodiversity means and credits the number or range of different species, the number or abundance of each species, and the variety of organisms in an ecosystem. It labels the first two in its margin as species richness and species evenness, which is a direct statement that both halves are wanted.
On placing the quadrats, it awards one mark for placing at random and one for using coordinates, then adds that the single phrase use random coordinates earns both. One precise phrase, two marks.
The comparison question that follows credits a mixture of data description and evaluation: more species present, a more even distribution with similar numbers of each, a named species absent from one field, more biodiversity, and that the result is reliable because more than one quadrat was used in each field.
Then it credits the limitations: that there is no information on water, sunlight, temperature, fertiliser or season, that the work was done in only one field of each type, and that nothing is known about the number or age of the animals grazing.
Notice how much of that is about what the study did not record. Naming a variable that was not controlled, and that could explain the difference instead, is worth as much as describing the data.
Drawing a conclusion properly
Describe the difference using the figures, not just the direction. Field A had a mean of 12 plants per quadrat against field B's 4, so three times as many.
Then suggest a biological reason, and then say what would make the conclusion safer: more quadrats, other fields, repeating in a different season, and recording the abiotic factors you could not otherwise rule out.
A conclusion that goes beyond the data is the commonest fault here. Two fields sampled once tell you about those two fields on that day, and saying so is a strength rather than a hedge.
Spec 4.2, 4.4B
What you need to know
- Compare populations in two areas using quadrats
- Measure abundance, frequency and percentage cover
- Use quadrat data to compare biodiversity
Active recall
Quick check
Answer each question before opening the answer.
Why are random coordinates used?
To reduce selection bias
How is frequency calculated?
Quadrats containing the species divided by total quadrats, multiplied by 100
What is species richness?
The number of different species recorded in an area
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