Fieldwork Techniques
These are outlines of some of the techniques that we will teach you over the next
three days. You will practise each technique in the field and will be given further
instructions. Thus only the briefest information is provided here. Experience
shows that the vast majority of student A2 projects will require a knowledge and
understanding of at least one of them. You will need to justify your use of any
chosen technique.
You will learn other techniques, such as methods of soil analysis and
measurement of Biological Oxygen Demand, that you will not find summarised
here. Relatively few people have neede to use these for their coursework in the
past. This doesn’t mean that if there’s no summary here, you cannot use a
technique! There is a great deal of information available to you – so ask.
SAMPLING ANIMALS
1. Soil:
Pooters, Pitfall traps, Tullgren funnels
2. Air or vegetation
Sweep nets, butterfly nets and beating trays
Light traps, sticky traps
3. Water
Plankton nets, drop nets, kick sampling
If you need to sample animals, check on the details of your chosen method from the
Curriculum Press software.
SAMPLING PLANTS
Estimating percentage cover
Calculate surface area of the plant, perhaps species, in question
e.g. Use a 10cm x 10cm grid drawn on an acetate sheet to measure the area covered
by pleurococcus on a tree
e.g. Use a 0.5m x 0.5m frame quadrat with grid to calculate the area covered by
plantains on a lawn.
Convert plantain area to percentage cover:
Area covered by plantain x 100
Area of surface
TECHNIQUES
SAMPLING VERY MOBILE ANIMALS
Mark-Release-Recapture
A technique used to estimate the population of mobile organisms such as sandhoppers,
woodlice or small mammals. If we want to measure how many sandhoppers there are on a
stretch of sandy beach, it’s impossible to count every one so we use this special sampling
technique.
How it works
1. A proportion of the sandhopper population is captured and marked in a fixed time.
2. They are then released, and allowed to mix freely with the other sandhoppers on the
beach.
3. A second sample is captured.
4. Some of the sandhoppers in the second sample will be those caught in the first
sample. The following assumptions are made to estimate the total population of
sandhoppers on the beach
No of sandhoppers caught in first sample (S1) = No. of sandhoppers in the second sample that had been marked (MR)
Total number of sandhoppers on beach (N) = No. of sandhoppers in second sample (S2)
N = S1 x S2
MR
Where
N = population of sandhoppers
S1 = number captured and marked in first sample
S2 = number captured in second sample
MR = number in second sample that bore markings (i.e. they’d been captured in S1)
This particular technique for Mark Recapture is called the Lincoln Index. There are others. It
makes these assumptions:
1. That births, deaths, immigration and emigration do not occur for the duration of the
experiment
2. That the marking lasts for the duration of the experiment
3. That the marking doesn’t affect the chances of the organism being killed by predators
or, if it is a predator, its chances of catching prey, or of it being recaptured.
4. That complete random mixing of the population had been achieved before
resampling occurred. This means sufficient time must be allowed at stage 2 (above)
TECHNIQUES
MEASURING DIVERSITY
The simplest measure of biodiversity is the number of species at a site or species richness.
However, species richness says nothing about the relative abundance of the different
species; we don’t know which species are dominant, which are rare etc.
Data 1
Grassland A (grazed)
Total no of species
25
Grassland B (ungrazed)
15
Species composition
24 species have 1 individual and 1
species has 150 individuals
40 individuals of each species
Grassland A is dominated by one species. It is considered to be richer.
Grassland B has fewer species but more of each so would be considered more diverse.
There are TWO important components to biodiversity
1. The species richness, i.e. number of different species
2. The number of individuals of a species relative to the total number of all species.
We CALCULATE biodiversity using a SIMPSON’S DIVERSITY INDEX (SDI)
Data 2
Species 1
Species 2
Species 3
Species 4
Species 5
Total no of individuals
SDI = N(N - 1)
n(n-1)
Meadow C
Meadow D
496
1
1
1
1
500
100
100
100
100
100
500
N = Total number of individuals of all species (‘hits’ with a point quadrat)
n = number of individuals of each species (‘hits’)
Meadow C
1. N = 500, so N(N – 1) = 500 (499).
So N(N – 1) = 249500
2.  n(n -1) = 496 (495) + 1(0) + 1(0) + 1(0) + 1(0) = 245520
3. SDI = N(N - 1) =
 n(n-1)
THINK! This is
where everyone
goes wrong
249500/245520 = 1.01
Calculate the SDI for Meadow D. Which is more diverse? (The bigger the number, the
greater the diversity)
TECHNIQUES
QUADRAT SAMPLING
-
Quadrats are SAMPLING AREAS.
They can therefore be circular, (circles have areas too!) but are usually square frames.
There is a great range of size, and choosing the appropriate size is very important. It does, of course,
depend on what you want to sample
Quadrat size
Area
When it might be used
10cm x 10cm
0.01m2
Mosses, algae or lichen on rocks, trees or gravestones
50cm x 50cm
0.25m2
1m x 1m
1.00m2
The commonest size.
Short grassland, some rocky shore. heathland, sand dunes
Taller grassland, small shrubs , some rocky shore
50m2 +
50.0m2
Woodlands
HOW BIG A SAMPLE DO I NEED?
You need the minimum number of samples that provides you with confidence that your
sample data is representative of the population. It is estimated by plotting this type of graph.
The graph shows that in the first quadrat 9 species were found, in the second quadrat an
additional 4 species were found, giving 13 in total, etc. After the fifth sample (i.e. fifth
quadrat) there was a cumulative total of 21 species. No more new species were being found
after the seventh sample so further sampling is becoming unnecessary. The graph therefore
shows that for this particular habitat, at least 7 samples should be taken in each sampling
period. Further sampling will waste time and duplicate results.
So, how big should the sample be?
1. Big enough to be representative of the population.
2. Big enough to let you to use your chosen statistical test.
TECHNIQUES
HOW BIG SHOULD MY QUADRAT BE? What’s the minimum area I need?
Quite simply, this is done by counting occurrences of species in quadrats of increasing size. Quadrat
size is increased until you stop finding new species
1. Start by finding a random point in the community to be sampled. (random numbers best).Take
the smallest quadrat available, eg 50cm x 50cm, and place it where coordinates intersect.
Count and record the number of species found in it.
2. Now sample a quadrat twice the size (50cm x 50cm) - count the number of new species found
and add it to the number found in the smaller quadrat, i.e. calculate a cumulative total.
3. Now sample using a quadrat twice the size of the last one - 1m x 1m etc.
Minimum area
estimate
Draw a line from the origin to the last point. Now draw a line parallel to this so that it makes a tangent
with the curve of the graph. Drop a perpendicular from this to obtain an estimate of the minimum area
needed.
The optimum quadrat size here would be 2.0m2 as the doubling of size from 2.0m to 4.0m, and then
from 4.0 to 8.0 only yielded one more species. Note: in a species rich area, such as a tropical
rainforest, the graph doesn’t level off as it does here. Your sample area would be huge!
WHAT ABOUT POINT QUADRATS?
A point quadrat is a metal or wood bar with ten holes in it. The bar is held horizontally above
the vegetation and metal pointers are pushed through the holes one by one. Each plant ‘hit’
by the metal pointer is recorded.
When would you use it?
-
In areas where vegetation is short.
You can use point quadrats to record the proportion of points which hit a particular
species (percentage cover).
Advantages?
- Quick, without losing much accuracy. This is one of the few situations where you can
make this point.
- Allows multiple ‘hits’ and accounts for different layers of vegetation.
Disadvantages?
- A point quadrat can completely miss rare or occasional species.
- Species with vertical leaves, e.g. monocotyledons may be underestimated.
TECHNIQUES
WHAT TYPE OF SAMPLING?
RANDOM SAMPLING
Use this technique when
-
the area under study is fairly uniform, (no environmental gradient)
very large
there is limited time available
comparing two habitats.
A quadrat frame is most often used for this type of sampling.
What you must NOT do is throw the quadrat: you may ‘choose’ areas into which to throw,
you are almost certainly not ambidextrous, making throwing almost always biased. This
means your sampling will be systematic not random, and if it’s not random, almost any stats
test you’ll want to use will not be valid. A better approach is to …
.
- Draw an outline map of the area being studied.
- Lay a numbered grid over the map
- Use random number tables (computer generated) or a random number-generating
calculator (you could even use your mobile phone), to generate pairs of random
numbers that act as the co-ordinates at which the quadrat will sample
- Random number tables and an excellent description of how to use them at
http://www.offwell.free-online.co.uk/howto.htm#RANDOM%20SAMPLING
Another method, only useful if the area is small, is for two people working
together to place measuring tapes, A and B, along two sides of the area. After
generating the pairs of numbers, person one paces out the first number in one
direction, and person two paces out from the second number in the other
direction. They walk at right angles into the sampling area until they meet. This is
the sampling point at which the quadrat can be placed.
Remember that for A2 coursework, you have to work entirely on your own. You
cannot therefore use this method without some modification.
SYSTEMATIC SAMPLING
This involves taking samples at fixed intervals along a line or belt.
LINE TRANSECT
Line transects are used when you wish to sample down a particular environmental gradient
or linear pattern along which communities of plants and, or animals change.
The position of the line transect is very important and should be thought about carefully
before it is placed. A transect line, usually a rope marked and numbered at 0.5m, or 1m
intervals, is laid across the study area. The line is put along the gradient identified. Any
species touching the line may be recorded (continuous sampling). Alternatively, the absence
or presence of species at particular marked intervals (e.g. 2m) is recorded (interrupted
sampling). http://www.offwell.free-online.co.uk/wetland_survey/trans1a.htm has a good
example of a continuous line transect.
Environmental gradient
B
A
A
A
A
A
G
G
C
C
C
S
S
T
L
B = bare ground
A = green algae
G = glasswort
C = cordgrass
S = seablight
T = thrift
L = sea lavender
BELT TRANSECT
This is similar to a line transect but gives information on abundance as well as presence, or
absence of species. The ‘belt’ is a continuous sequence of quadrats across the area to be
sampled. The transect line is laid out across the survey area and a quadrat is placed on the
first marked point on the line. The plants and/or animals inside the quadrat are then
identified and their abundance estimated, usually by percentage cover. Quadrats are
sampled all the way down the transect line, at each predetermined interval or marked point
on the line.
When is it used?
- The technique is important if you are investigating how organisms or conditions vary
across an area, e.g. down a slope or across a sand dune.
- A belt transect shows not only where a particular species occurs (as does a line
transect), but also how much of it is present at any point along the line.
- Many other details can be recorded, such as soil characteristics, or height and
biomass of the plants.
STRATIFIED SAMPLING
Stratified sampling is used to take into account different units that are identified in the main
part of a habitat. Sometimes random samples may not cover all areas of a habitat equally.
E.g. if an area of grassland with heather patches was random sampled, it is possible that no
samples might fall within the heather patches. Results would then not show any heather in
the habitat. So in stratified sampling the sample area is broken into relatively homogeneous
areas which are then sampled separately, either randomly or systematically, within each,
thus ensuring that overall the sampling becomes more representative.
Fieldwork Techniques
SAMPLING ANIMALS
-
Soil
Air
Water
SAMPLING PLANTS
-
Estimating percentage cover
SAMPLING VERY MOBILE ANIMALS
-
Mark-Release-Recapture
MEASURING DIVERSITY
-
Simpson’s diversity index
QUADRAT SAMPLING
-
How big a sample do I need?
- How big should my quadrat be?
POINT QUADRATS?
-
Advantages/disadvantages
WHAT TYPE OF SAMPLING?
-
RANDOM SAMPLING
What? When? How?
- SYSTEMATIC SAMPLING



What?
Line Transect When? How?
Belt transect When? How?
Stratified sampling