ECOLOGICAL SURVEY FOR TERRESTRIAL HABITATS
Ecological
surveys are carried out when studying the vegetation of a given area. They are
directed towards the description of the vegetation of a given area. The purpose
of such description is to enable people other than the observer to build a
mental picture of the area and its vegetation. It also allows the comparison
and classification of different plant units.
Before any
serious or detailed work is done in the area, it is necessary to know the
species present, their distribution and relative degree of abundance of each
species. An account of the environmental factors is also important. Therefore the
floristic composition expressed as a list of species, life-form composition and
structure of vegetation are a necessary basis for all ecological surveys.
The vegetation
of an area is studied using several methods and some of the commonly used
sampling techniques include the following:
quadrat method, transect method, distant
method, point method.
1.
QUADRAT METHOD
The most
accurate way to obtain quantitative information about the plants on a
particular site is to record every
individual on the whole site. This is clearly impractical except for very
small areas. Therefore samples are
taken, usually by selecting small parts of the site for intensive study. A
quadrat is a sample area of defined size and shape and it is assumed that the
contents of the quadrat will be representative of the whole site. A quadrat is
therefore the smallest or minimum area within which the species of a community
are adequately represented.
The minimum area
(quadrat size) may be determined by a species-area
curve. A species area curve is compiled by placing quadrats of varying
sizes on the ground in such a way that each larger quadrat includes all the
smaller ones, an arrangement known as nested
quadrats. Initially a quadrat of a small size is laid on the ground and all
the species occurring in this quadrat are counted. The quadrat size is then
increased and newly encountered species are recorded separately. The plot area
is increased in this manner and every time the number of new species
encountered is listed. A point of diminishing
return is eventually reached, beyond which increasing the quadrat size
results in the addition of only very few new species.
Quadrat Number Species Recorded Additional
Species
1. 5 -
2. 10 5
3. 17 7
4. 26 9
5. 38 12
6. 51 13
7. 65 14
8. 67 2
9. 69 1
10. 70 1
11. 71 1
12. 71 0
The number of
increasing species versus the quadrat area is plotted. From the graph it will
be noted that the increase in the number of species is rapid at the initial
stages but gradually the rate of increase in the number of new species falls
and the curve tends to become horizontal. The point on the curve where the
slope becomes horizontal is taken as the minimum area (quadrat size) for
sampling the particular vegetation.
The shape of the
quadrat is usually square but may be of different shapes – rectangular or
circular. The size of quadrat depends upon the type of vegetation to be
studied. Quadrats can be made of strings, wood or metal. They can also be made
by markings on the ground using pegs and strings; or just a point like in the
point quadrat.
Sampling Procedure
In the field the
quadrats are either distributed randomly
or systematically but the aim should
be to cover the entire range of vegetation. The number of sampling quadrats
should be so much as to cover 5-10% of the area under study.
Systematic
sampling procedure
Quadrats are
evenly spaced through out the study area i.e. quadrats are placed at regular
intervals. To achieve this pattern, quadrats may be located at uniform
intervals along evenly spaced transects throughout the study area. However
systematic sampling may lead to biasness and may give a data that is not
suitable for statistical analysis.
Random
sampling procedure
Strict
application of random sampling means that every point within the survey area
should have an equal chance of being chosen on each sampling occasion. To take
a random sample, a grid of coordinates is set up over the survey area and pairs
of random numbers are taken to locate each quadrat. Quadrats can be located
randomly by constructing two imaginary axes along the edges of the large area
being sampled, dividing the axis into units and picking pairs of units from a
table of random numbers. Each quadrat is located using the random numbers as x
and y co-ordinates. The bottom left-hand corner of the quadrat is placed where
the co-ordinates meet.
Stratified
random sampling
Sometimes random
quadrats might be clustered in one section of the area. To avoid this, the area
can be subdivided into plots and each plot is randomly sampled with fewer
quadrats. This sampling procedure is important when environmental parameters
vary systematically across the sampled area e.g. upper and lower sections of a
slope.
Random sampling Stratified
random sampling
Diagram showing
the location of a quadrat for random coordinates 0203
Data Collection, Recording and Analysis
In each quadrat
the species are identified, the number of individuals per species is recorded
and the area covered or occupied by each species in the qaudrat is also
recorded. The data obtained above is used to calculate the density, frequency
and cover of each species. The relative value of each of the above parameters
is also calculated.
No. and
cover of individual species per quadrat
|
Species
|
Quadrat
Number
|
|||
|
1
|
2
|
3
|
Total no. and
cover
|
|
|
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
|
2
(70%)
4
(10%)
8
(50%)
0
0
0
0
0
0
0
|
2
(5%)
6
(25%)
0
2
(20%)
1
(5%)
5
(60%)
2
(20%)
0
0
0
|
0
4
(30%)
5
(40%)
3
(15%)
0
0
10
(25%)
50
(55%)
10
(35%)
9
(20%)
|
4
(75%)
10
(65%)
13
(90%)
5
(35%)
1
(5%)
5
(60%)
12
(45%)
50
(55%)
10
(35%)
9 (20%)
|
Density
Density indicates
the numerical strength of a species in an area. It is obtained by counting the
number of individuals of each species in the sample area and the dividing it by
the total area sampled. (Number of individuals of a species per unit area)
Relative density
is the density of one species as a percent of total plant density
Frequency
Refers to the
degree of dispersion of individuals of a species in an area and it is usually expressed
in terms of percentage occurrence. It is obtained by noting whether a species
is present or not in a series of randomly placed quadrats.
Relative
frequency is the frequency of one species as a percentage of total plant
frequency.
Cover
It is an
estimation of the area covered by a given species and it is usually expressed
as a percentage of the total area. It may be studied at the canopy level or
ground level.
Relative cover
is the cover of a species as a percentage of total plant cover (not ground
cover).
2.
TRANSECT METHOD
A transect is a
sampling strip (line) extending across a vegetation or habitat and along which
observations are made in a systematic or random way. The use of transects is of
greater importance where the variation of vegetation in response to a changing
environmental factor is well marked. Therefore transects are used to describe
the zonation of vegetation in relationship to their environmental factors.
There are tw types of transects: line and belt transects.
(a)
Line transect/intercept
A line transect
is a single line running across a vegetation or habitat, and is usually a tape
measure. Plants touching the line are identified and recorded. The lengths of
plant canopies of all the species touching the tape measure are recorded. The
total decimal fraction of the line covered by each species multiplied by 100 is
equal to its percent cover. Disadvantage of the method is loss of density and
frequency measures because there is no area involved. However, if the line is
broken up into segments, linear cover, linear density and linear frequency can
be obtained.
(b)
Belt transect
It is a transect
with a wide breadth. All species between the two lines are identified and
recorded. A belt transect can be subdivided into plots and random quadrats are
placed in the plots. Density, frequency and cover noted within the quadrats and
are calculated as in the quadrat method.
3.
POINT QUADRAT
Involves
reducing the quadrat into no dimension and it becomes a minute point. Metal
pins with sharp tips serve as the points and they are fixed on a metal frame.
As the frame is lowered the plants touched by the pins are recorded. The frame
is lowered until the ground is reached.
The data
obtained is used in the calculation of:
Disadvantage of
the method:
- Density and frequency cannot be used
- It is limited to low vegetation.
4.
DISTANT METHODS
The distance between
point to plant or plant to plant is measured. There are four distant methods:
(a)
The Nearest individual Method –
The distance measured is between random points to nearest plant.
(b)
Nearest Neighbour Method –
distance measured is between plant close to random point and its nearest
neighbour
(c)
Point Centered Quarter Method
(PCQ) – Area around random point is divided into four quarters at 90o. Distance
measured is between random point and nearest plant in each quarter.
(d)
Random Pair’s method – A line
is drawn between nearest plant and random point. Perpendicular to this line and
passing through the random point, an exclusion line is drawn at right angle to
the first one. Distance measured is the one between the plant and its nearest
neighbour but on the other side of the exclusion line.
-