need help in bio unit6

Vj Destiny

Vj Destiny

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bilquees said:
can anyone explain T test and ecological distribution experiment plz.
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PRACTICAL 5.1 ECOLOGICAL SAMPLING





INTRODUCTION:
The purpose of sampling is to obtain a series of independent estimates of a variable that is being measured. Sampling is the process of selecting organisms from a population in a region so that by studying the sample, one may generalize the results concerning the entire population from which they were chosen. The sample is the group of organisms or individuals that is selected to be in the study and can be a potentially infinite number. The greater the sample size, the closer the sample will be to the actual population. If the sample consists of the entire population, there will be no sampling errors as the entire population has been used instead of a sample.


Sampling methods may be divided into a few types:

Random selection (random sampling)

This method of sampling is applied in part A of this practical.

This is the best way to select a sample that is unbiased. In order to have a random selection method to be valid, a process that assures that the different units in a population have equal probabilities of being chosen must be used. The problem with random sampling methods for a population that is widely dispersed over a large geographic region is that the person doing the sampling will need to cover a lot of ground geographically in order to obtain sufficient number of samples in order to avoid bias in region selection.

The simplest form of random sampling is called ‘simple random sampling’ which makes use of the following procedures: a table of random numbers or a computer generated random number list, whereby each unit (individual, site or quadrat) in the population is identified by a certain random coordinate or identification number so that each unit has an equal chance of being part of the sample frame. The selection of each unit is independent of the selection of other units.

Random sampling is carried out with the assumption that the distribution of species within the area of interest is fairly uniform. Random sampling can be used to locate sampling positions independently of each other by marking out a sampling area using tape measures placed on the ground at right angles to each other and then selecting random coordinates from a table of random numbers. A random walking method may also be used for a small or less regular shaped area. To sample fixed objects within an area, for example the distribution of Pleurococcus on tree bark or the number of woodlice under rocks, all the units such as the trees and rocks may be numbered for random selection.

A random sample is thus defined as ‘a sample in which each individual measured or recorded (individual, site or quadrat) is independent of other individuals and of prominent features in the area or other units being sampled.’ In ecological field surveying, the random sampling methods generally used are (a) random quadrat sampling (used in this study) and (b) random-walk technique. In ecological field surveying, random quadrat sampling is most easily achieved by superimposing a grid over the sample area and then identifying a series of random coordinates using pseudorandom numbers.

Systematic sampling

This method of sampling is applied in part B of this practical.
Systematic sampling refers to samples taken at regular intervals along an environmental gradient. For example, when conditions gradually change across a habitat. Systematic sampling across a transect allows the environmental gradient to be studied. A transect is effectively a line laid out across a habitat along which samples are taken. The sample points may be at regular intervals or may be positioned in relation to a morphological feature such as on the ridges and in the hollows of a sand dune system. In other words, systematic sampling normally involves doing transects, where a sampling line is set up across areas where there are clear environmental gradients. For example you might use a transect to show the changes of plant species as you moved from grassland into woodland, or to investigate the effect on species composition of a pollutant radiating out from a particular source .

Sampling techniques that can be carried out along a specified transect are line transect (recording each organism that touches the line at fixed regular intervals along the line), belt transect (placing a series of quadrats along a line, sampling a 0.5m wide ‘belt’ along the transect) and interrupted belt transect (placing of quadrats at regular intervals along a transect discontinuously).

Systematic sampling is appropriate for observing changes across a habitat such as the zonation of plants and animals or successional changes in a developing community. Sampling needs to be carried out along some form of transect: (i) at fixed vertical or horizontal distances, (ii) according to a fixed morphological feature, or (iii) at fixed intervals of some other variable such as flow rate. Transects can be continuous(all possible locations lying along transect is analysed) or interrupted (samples taken at regular intervals with gaps in between).

Techniques for systematic sampling include:

1. Line transect
2. Continuous belt transect
This sampling technique is used when organisms are known to vary across some defined gradient to estimate percentage cover

3. Interrupted belt transect

Note: both types of belt transects are used to estimate the percentage coverage of the plant species. Cover is the area of the quadrat occupied by above-ground parts of a species when viewed from above.

For all the above methods of sampling:
a) As the plot size increases, accuracy of estimation increases but the sampling procedures will be more tedious and time consuming.
b) Smaller plot sizes or quadrat sizes are used for species that are presumably uniformly distributed. Larger plot sizes are for areas which are known to have uneven distribution of population. This prevents inaccuracy in sampling.
c) When sampling ‘clumped’ populations, it is best to use a belt transect.
d) As the number of replicates increases, the accuracy of the sampling estimation increases, but the sampling procedure is more demanding.
e) All the methods above are only used for sedentary animals and plants as they remain relatively stationary in the same area or locality.

Random Sampling used in PART A
  • This is used where the habitat being sampled is fairly uniform.
  • To remove observer bias in the selection of samples.
  • Where statistical tests are to be used which require randomly collected data.
  • Where a large area needs to be covered quickly.
  • If time is very limited. To show zonation of species along some environmental gradient.
  • e.g. down a sea shore, across a woodland edge.
Systematic Sampling (Transects) used in PART B

  • Where there is some kind of continuous variation along a line,
  • e.g. across the Centre's heathland strips.
  • To sample linear habitats, e.g. a roadside verge.
  • Where physical conditions demand it, e.g. sampling a vertical rock face, using a rope to climb it.

Line Transect (not applied in this practical)

  • Where time is limited. A line transect can be carried out much quicker than a belt transect.
  • To visually illustrate how species change along the line (a transect diagram).
  • Keys can be chosen to represent individual species. Vegetation height can be drawn in choosing an appropriate scale. The slope of the line can also be measured when carrying out the transect and incorporated into the transect diagram.
  • To show species ranges along the line.
  • This will generally show only where the species occurs, not how much of it is present.
Belt Transect
  • A belt transect will supply more data than a line transect. It will give data on the abundance and distribution of individual species at different points along the line, as well as on their range, apart from their presence and absence.
  • As well as showing species ranges along the line, a belt transect will also allow bar charts to be constructed showing how the abundance of each individual species changes within its range.
  • Belt transect data will allow the relative dominance (percentage cover or relative distribution) of species along the line to be determined. Where a marked chemical gradient occurs, a belt transect gives rich results over a very short distance. In all of these cases, the belt must run parallel to the environment gradient if you are to gather meaningful data.
 
Vj Destiny

Vj Destiny

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bilquees said:
can anyone explain T test and ecological distribution experiment plz.
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Advantages of using random sampling are as follows:
Useful with large sample populations that have a uniform distribution in population and a homogeneous make-up.
Avoids bias in choosing sample units (for example, in determining location of quadrats)


Disadvantages of using random sampling are as follows:
Can lead to poor representation of the overall population if the random numbers generated miss large areas or areas of ecological importance or are clustered together instead of spread out uniformly. The results may not be fair.
There may be practical constraints in terms of time available and inaccessibility to certain parts of the study area.

Advantages of using systematic sampling
The main advantage of systematic sampling over random sampling is its simplicity. It allows the researchers to add a degree of system or process into the random selection of its subjects. There is an observable degree of evolution in the subject being studied.

Another advantage of systematic random sampling over simple random sampling is the assurance that the population will be evenly sampled. There exists a chance in simple random sampling that allows a clustered selection of subjects. This is systematically eliminated in systematic sampling.

Disadvantages of using systematic sampling
More biased since not all units have an equal chance of selection.
May lead to over - or under-representation of a particular pattern.

The process of selection can interact with a hidden periodic trait within the population. If the sampling technique coincides with the periodicity of the trait, the sampling technique will no longer be random and representativeness of the sample is compromised.


Some of the factors that affect precision are:
1) Measurement error. In the real world, it is important to count organisms carefully and lay out
plots accurately for good estimates of density. This is not a concern here however, because the computer will be laying out the plots and counting the plants.

2) Total area sampled. In general, the more area sampled, the more precise the estimates will be, but
at the expense of additional sampling effort.

3) Dispersion of the population. Whether the population tends to be aggregated, evenly spaced, or randomly dispersed can affect precision. Note that the dispersion pattern of the same population
may be different at different spatial scales (e.g., 1 x 1 m plots vs 100 x 100 m plots).

4) Size and shape of quadrats. The size and shape of the plots can affect sampling precision. Often, the optimal plot size and shape will depend on the dispersion pattern of the population.

METHOD:
PART A: STUDYING THE ABUNDANCE OF SPECIES 1 (Digitaria sanguinalis ) using random sampling

  1. The chosen field is marked out with 10 m tape measures placed on the ground at right angles to each other. This is the sampling area.
  2. The sampling area is then divided into a grid-structure with an x-axis and a y-axis with coordinate numbers 0 to 10.
  3. The quadrats used have a dimension of 0.5m x 0.5m, hence an overall area of covers one quarter of a square metre (0.25m2). The total area of the sampling area is 100m2.
  4. 10 random coordinates were selected from a table of random numbers, covering a total sample area of 2.5m2, which is 2.5% of the total sampling area.
  5. The respective quadrats are placed at the specific locations.
  6. At each quadrat, the number of individual plants of each species present in within the area encompassed by the quadrat is counted manually. The area covered by each individual plant is also estimated. It was standardized that the counting would include plants that are more half in and exclude ones less than half in.
  7. After that, the percentage coverage and the number of individual plants of ‘Large Crabgrass’ or Digitaria sanguinalis was recorded on a table.
  8. The abundance of was measured in terms of percentage frequency, frequency density and percentage cover.



PART B: STUDYING THE ABUNDANCE OF SPECIES 2 (Viola papilonacea) using systematic sampling (continuous belt transect)

  1. In this method, the transect line is laid out across the area to be surveyed (the slope) and a quadrat is placed on the first marked point in a horizontal line. The quadrats are numbered 1 to 10 in a continuous series with negligible gap in between.
  2. The sampling frame is the list of species found throughout the belt transect, particularly the number of individual Viola papilonacea. The sample size is fixed at 0.25m2 quadrats taken 10 at a time in a continuous belt. The species to be investigated is fixed as Viola papilonacea.
  3. The number of wild violet plants (Viola papilonacea) inside the quadrat are then identified and their abundance estimated.

The above are objective estimates of abundance of Viola papilonacea.

Any changes across the habitat (the slope) such as zonation or successional changes are observed.

Random sampling is used for the determination of abundance of a species (Digitaria sanguinalis) and is mainly used to investigate the species composition of a sample.

Systematic sampling(continuous belt transect method) is used to determine both abundance and distribution of a species (Viola papilonacea) in a habitat and to investigate the sample along gradual changes in environmental gradient, especially for populations in newly-cleared habitats.
 
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