Sampling Techniques

Quadrats

Sampling

• The best way to get information about a particular ecosystem would be to count every individual of every species

• This would take unfeasibly long

• So we sample a small part of the ecosystem we are studying

Quadrats

• A square of known size used to sample the ground-living (sessile) organisms in an ecosystem

• Traditionally used to count plants in a particular area

• Can be used to study changes over distance

Counting with quadrats

• Best done in reasonably uniform ecosystems, e.g. grassland

• Many need to be placed randomly• Averages worked out• Multiplied by area/quadrat size• Works well for common species• Rare species are usually underestimated or

occasionally grossly overestimated• Suggest why? (Imagine using quadrats to count trees on the

grass outside business and no, it’s not because they wouldn’t fit over the top.)

Placing quadrats randomly

• Throwing is not random• Measure out an area

using tape measures as axes

• Use random number tables to select coordinates at which to place quadrats

• Be consistent about how quadrats are positioned in relation to coordinates

10 m0 m

10 m

0 m

Measuring changes

• Studying how the environment changes over a distance is done using a transect

• A tape measure is placed in a line• Quadrats are placed at

standardized distances along the line

• Quadrats laid adjacent to each other along the line is called a belt transect

• Quadrats at intervals (e.g. 5 m) is called an interrupted transect

• Again, placement of the quadrat in relation to the measurement on the tape needs to be standardized beforehand, e.g. bottom left corner of quadrat on tape.

Ways of counting what’s in a quadrat

1. Subjective rating

2. Percentage cover

3. Percentage frequency

4. Species density

• Increasing accuracy• Decreasing speed

Subjective rating• ACFOR• Look at whole quadrat and

decide if species is– Abundant– Common– Frequent– Occasional– Rare

• If you want to be a bit quantitative you can assign each a score (A=5, R=1)

• Quick and easy• Subject to innaccuracy due to

subjectivity• May lack reliability between

samples/samplers

Percentage cover

• Estimate, to nearest 5% how much of the quadrat is covered by each species

• Doesn’t need to add up to 100%

• Still open to problems of subjectivity

• Data is more quantitative which is useful for analysis

• The average percentage cover of a particular species in all quadrats is called the species cover in the area being sampled

Species frequency/percentage frequency

• Needs a grid-type quadrat (25 mini squares)

• Count how many squares the species appears in

• Multiply by 4 to get %• Much more objective and

therefore reliable• Tends to produce an

overestimate, therefore inaccurate

Species density

• Count every individual of every species in the quadrat

• Objective– Accurate– Reliable

• Time consuming• Useful for larger (taller)

species with more distinct individuals

Point quadtats

• Most objective of quick techniques – increases reliability

• Frame placed on ground• Pins dropped into holes in

frame• The first species the pin

touches is the only one counted

• May over/underestimate cover of rare/small species – problem with accuracy