Introduction


General Background:

Within an ecosystem the organisms are rarely distributed evenly and therefore it can be very difficult to count their numbers accurately. Scientists therefore need to adopt an appropriate sampling technique to achieve a realistic estimate of population numbers.

Sampling Methods:

Quadrat sampling

A quadrat is a sampling unit of known area and are usually square frames. They are of various sizes and the size depends very much on the particular habitat and type of organisms likely to be encountered. Plant quadrats for use on grasslands are usually 0.5m x 0.5m but larger scale types are employed for larger ecosystems.

There are two types of quadrats:
i. point-quadrats (usually ten pins in a metal frame)
point_transect.jpg












Figure 1 : Point-quadrat frame typically used to estimate the percentage cover of species in a grassland or in this case in the salt marsh. The percentage cover/abundance is calculated as the number of pins that touch a particular species out of a total of ten pins. E.g. if species X touches 3 pins out of the ten pins, percentage cover is 30%.




ii. quadrats of different sizes
grid_quadrat.jpgFigure 2 : Standard metal quadrat used to assess the percentage cover of species within a community. Typically the quadrat is thrown at random within the study community and the percentage of area covered by a species estimated (by eye), hence the percentage can exceed 100% due to species overlapping other species. Accuracy of the percentage cover depends on a range of methodological and ecological factors.
The photo here shows the students using a grid quadrat in Wales






Quadrat siting:

The siting of the quadrats is of crucial importance and for the results to be valid some objective approach is required. A true random selection is difficult to achieve but three methods of sample site selection are in common use.

1. Simple random sampling
The area under study is divided up into a grid system of boxes and using random numbers for the coordinates a number of boxes can be selected.
quadrat.jpg

2. Transect sampling
The use of transects constitutes a form of systematic sampling but in this case the samples are arranged in linear fashion. This approach is useful when recording changes in the species composition where some sort of transition exists eg. A sea shore or transition from an esturine to land community.
marking_transect.jpg


Figure 3: RGS students laying down a baseline horizontal to the river before plotting out a transect perpendicular to it at the Gann salt marsh in Dale, Wales.
The are seen here mapping out the profile of the land.
Most transect work also requires the recording of a profile which indicates the changes in height or nature of the substrate.







Two types of approach are in use.
transects.jpg

a) Belt transect
This is a strip usually 0.5m in width that is located across the study area in such a way as to highlight any transition. A quadrat is laid at regular intervals along this belt and those organisms within each quadrat recorded. Taking samples at regular intervals should give a good idea of the numbers and distribution of the plant.

b) Line transect
This is a quicker though far less quantitative and less representative method. A line is laid across the area and marked off at regular intervals. A lot of organisms are missed out and the results are only likely to give a rough impression of the community.












In summary:
systematic_sampling.jpg

WHAT CAN WE MEASURE?

Measurements of abundance:

Quadrats are useful when attempting to determine the species composition of an area and also when assessing the abundance of those species in the study area. The four measurements of abundance most often used are;
  1. Cover - The percentage of ground covered by that species in the sample zone.
  2. Frequency - The number or percentage of sampling points in which that species occurs.
  3. Density - The mean number of individuals per unit area
  4. Biomass - The measure of the weight of that species in the sample zone.

1. Cover

1a. Abundance scale (Qualitative)
A variety of methods are available to an ecologist to assess the abundance of plant species, these can be broadly divided into qualitative and quantitative techniques. Qualitative techniques are infrequently used, as the technique is descriptive and comparisons between population, communities or between studies are not possible. The most commonly used qualitative method in plant abundance studies is ACFOR, where species are simply listed into one of five types of abundance;
  • Dominant (D)
  • Abundant (A)
  • Common (C)
  • Frequent (F)
  • Occasional (O)
  • Rare (R)

Wherever possible you should use a quantitative method for assessing plant abundance. Different quantitative techniques in estimating plant abundance can be used.

1b. Percentage abundance (Quantitative)
The most common method for assessing plant abundance is the percentage of ground covered by the species being investigated. Rather than simply quoting the percentage cover of a species, species can be grouped into categories or scales of abundance. This, in part, overcomes the problem of accuracy that is especially important when combining data collected by more than one observer when individual biases may occur in estimating percentage cover. DACFOR Scale is a common type of scale that is widely used across Europe.

DACFOR scale associated with some percentages.
Abundance scale
DACFOR
Coverage of Organisms
+
Rare (R)
<1% cover
1
Occasional (O)
1 - 5%
2
Frequent (F)
6 - 25%
3
Common (C)
26 - 50%
4
Abundant (A)
51 - 75%

2. Frequency

Frequency is simply the probably of encountering a particular species per unit area of study. For example, Polygala vulgaris is found in three out of twenty quadrates within a heath land vegetation type compared to only one of twenty in grassland vegetation. This provides a quick method for assessing the relatively abundance of plant species within a vegetation type. But clearly does not provide an absolute measure of abundance. Wherever possible either plant density or percentage cover estimates of abundance should be used.

3. Plant density

Plant density is defined as the number of individual plants per unit area (e.g. ten plants per Metre Square, or twenty per hectare). This is of course simple to calculate when individual plants can be separated, i.e. species exhibit a discrete growth form, and such as an individual tree or rosette forming herbaceous species. However, a continuous, carpet-forming growth form of some plant species (such as grasses), means that plant density is almost impossible to estimate.


Methodological considerations in assessing plant abundance

Plant density is always the best method to use in assessing plant abundance, as it gives the observer a direct measure of plant abundance, and quantifiable for the unit of study (i.e. two plants per metre square, etc.). Percentage cover is, however, highly dependent on the precise methodology you use. The accuracy and therefore reliability of percentage cover estimates is dependent upon five interactive factors;
  • the size of the plant you are studying,
  • the size of the quadrat you are using,
  • the spatial distribution of the plant your studying,
  • the spatial distribution of the quadrats, and;
  • the number of quadrats you use in estimating the species abundance.



Practical on Quadrat Sampling in RGS


Objective:
You will be using quadrats to evaluate the percentage cover of grass at two sites in RGS; a disturbed and an undisturbed site.

Locations:
Disturbed site:
Grass patch located between D-block and E-block, facing J-block.
(It is the patch where the dental container is usually laid)

Undisturbed site:
Grass patch infront of block J (in the school field) between the 2 Eucalyptus trees.

Apparatus:
Clipboard
30m long measuring tape
Grid quadrat of 0.5m

Instructions:
1. A 20 metre line transect has been set up at the two designated areas.
2. You are going to use quadrats to carry out a transect of the designated areas.
3. Estimate what percentage of the ground is covered with grass and with bare soil and the number of different types of species of plants found.
4. Use the measuring tape provided as a guide to lay down your quadrat along the transect line.

Recording of Results
Please upload your results in the space below.
You may want to use the following headings for your table:
Distance from start (m), Disturbed Site, ‘Undisturbed’ Site, % cover of grass, % cover of bare ground, No. of different species of plants
Distance from the start (m)
Disturbed Site
Undisturbed Site
% cover of leaf litter
% cover of grass
No. of different species of grass
% cover of leaf litter
% cover of grass
No. of different species of grass

0
0
18
0
4
96
1
1
0
99
0
5
96
1
2
0
100
0
0
97
2
3
0
99
3
2
100
3
4
0
45
4
2
100
6
5
6
60
3
4
100
4
6
7
23
4
7
93
3
7
9
31
5
5
95
3
8
2
48
6
2
98
4
9
4
21
6
2
100
4
10
12
46
3
1
99
2
11
6
60
3
1
95
3
12
0.5
61
4
4
95
3
13
0
97
4
1
100
5
14
0.5
61
5
4
100
4
15
1
79
4
0
100
3
16
0
83
5
0
98
3
17
2
81
4
1
99
3
18
0
91
4
0
100
3
19
0
85
4
0
100
3
average
2.5
64.4
3.55
2.25
98.05
3.15








Displaying Results
Draw a bar chart to show the percentage cover of grass. Draw a line graph to show the number of different species of plants observed at the various intervals. Please refer to Biozone if you are unsure as to what a bar chart should look like.

Upload your graphs below.

-Bar chart-
bar_graph_(3).jpg

-Line Chart-

bar_chart-3.jpg


What was the independent and dependent variable in this investigation?


Discussions and Conclusions
  1. Describe what your graph shows.
  2. Where was the percentage grass cover highest and lowest?
  3. What pattern can you see between grass cover and the number of different species of grass in a given area?
  4. Is there a scientific idea or theory to explain your observations? If yes, please elaborate.

Evaluation
Suggest how the investigation could be improved, and how you may extend the investigation if you had more time.