Quadrat Sampling

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Quadrat Sampling

Count all the individuals on several quadrats of
known size and then extrapolate the average
count to the whole area.

Estimates require:
• The area of each quadrat must be known
• The population of each quadrat examined

must be determined accurately
• The quadrats must be representative of the

whole area i.e. achieved by random
sampling

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Quadrat Sampling

1 Select quadrat shape
2 Select quadrat size
3 Determine number of quadrats
4 Decide placement of quadrats
5 Quantify organisms in quadrats

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1. Select

Square Oblong Circular
a a r

Area = b b πr2
axb axb

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1. Select

Edge effect

Low High

• Circular

• May be chosen where edge effects are important
• Occurs when researchers must make subjective decisions on when a species is considered
“in” or “out” of the quadrat
• If all organisms touching the edge are counted as being inside, this may overestimate
the cover. Conversely, ignoring all those on the edge may result in an underestimate.

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1. Select

• Square and oblong
• Both are easier to define since tape
measures can be strung through
dense vegetation stands
• Oblong is better than circular or
square quadrates because it covers
habitat heterogeneity i.e. tends to
cross more patches

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2. Select

The recommended quadrat sizes for various organisms being sampled.

Vegetation / Habitat type Quadrat size
Algae, mosses and lichens 1.0m x 1.0m to 0.5m x 0.5m
Seaweeds, grasslands, small heathers and aquatic plants 0.5m x 0.5m to 2m x 2m
Large heathers or long tall vegetation 1m x 1m to 4m x 4m
Shrub and woodland vegetation 10m x 10m
Woodland canopies 20m x 20m to 50m x 50m
Habitats (e.g. badger setts, ponds, woodland blocks) 1km x 1km to 10km x 10km

Static animals Quadrat size
Small animals (e.g. barnacles, Bryozoa) where cover >50% 2cm x 2cm to 5cm x 5cm
Small animals (e.g. barnacles, Bryozoa) where cover <50% 0.1 m x 0.1m
Large animals (e.g. perwinkles, limpets) 0.5m x 0.5m
Ants’ nest 20m x 20m to 50m x 50m

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3. Determine

1 Draw grid of the study area based on the selected
quadrat size. Each square represents a quadrat

2 Assign number to each quadrate
3 Measure the size of study area

Sample 10% of the total area size 1m
1m
4 • E.g. Total area is 29 m2, therefore sample 2.9 m2

(2.9 quadrat ≈ 3 quadrats)

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4. Decide

Random Sampling: Use random number

From Fisher and Yates; courtesy from Longman Ltd., London

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4. Quantify

• Quadrat is used to simply define an area within which you can estimate density, frequency,
and cover of the organisms being studied.

a. Density

• Density is simply the count of the number of
individuals found inside the quadrat area

• It is expressed as the number of individuals per m2

• Low density indicates that individuals are relatively
rare or cover larger areas

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Density

= Species abundance x unit area
Number of quadrats

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4. Quantify

• Quadrat is used to simply define an area within which you can estimate density, frequency,
and cover of the organisms being studied.

b. Frequency

• Frequency is the chance of finding a species in a
given area

• To estimate frequency, a species is counted as
present or absent in each of a series of quadrats

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Frequency

= Number of quadrats where individuals present x 100 (%)
Total number of quadrats

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4. Quantify

• Quadrat is used to simply define an area within which you can estimate density, frequency,
and cover of the organisms being studied.

c. Cover

• Is based on the visual estimation of the quadrat area occupied by a species

• It is expressed as a percentage

• Cover is of use when
• Species are not easily identifiable (eg. algae, moss, lichens)
• Individuals are not easy to distinguish (as in many grasses)
• It is used to estimate canopy closure in woodlands

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Cover

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Activity:

Estimate density, frequency, and cover of the following vegetation types.

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WOODLAND CANOPIES

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SHRUB

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