Download Sampling Techniques

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
10 m
• 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
0m
relation to coordinates
0m
10 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