Exercise 24

Principle: Frequency is concerned with the degree of uniformity of the occurrence of individuals

of a species within a plant community. It is measured by noting the presence of a species in random sample areas (quadrats) which are distributed as widely as possible throughout the area of study.

Frequency is the number of sampling units (as %) in which a particular species (A) occurs. The

frequency of each species (sps. A or sps. B or sps. X etc) is expressed in percentage and is calculated as follows.

Requirements: Cotton/nylon thread of 5 metres, 4 nails and a hammer

Procedure:

1. In the selected site of study, make a 1 m X 1 m quadrat with the help of nails and thread. Hammer the nails firmly and make sure that the vegetation is not damaged while laying the quadrat.

2. List the names of the plant species seen in the quadrat (if the name is not known mark these as species A or B etc. and if the same species is seen in other quadrats assign the same alphabet)

3. Similarly lay nine more quadrats randomly in the site of study and record the names of individuals of each species.

4. Calculate the percentage frequency of occurrence using the formula given.

Observations:

Record the total number of species seen in the ten quadrats. This will give an idea about the composition of the vegetation. There will be difference in the species composition in the quadrats made in shady areas, exposed areas with bright sunlight, dry or wet areas etc. Observe that the frequency of occurrence is not the same for all species.

Discussion:

Variation in distribution of a species is caused by factors like soil conditions, quantity and dispersal of gemmules, vegetative propagation, grazing, predation, diseases and other biotic activities. Also frequency values differ in different communities. They are influenced by micro-habitat conditions, topography, soil and many other environmental characteristics. Thus unless frequency is not correlated with other characters such as density, frequency alone does not give correct idea of the distribution of a species.

Frequency determinations by means of sample areas are often needed in order to check general impressions about the relative values of species. Many species having low cover or population density also rate low in frequency, but some may have high frequency because of their uniform distribution. Usually if the cover and population density are high, the frequency will be high. The plants with high frequency are wide in distribution.

Questions

1. If frequency of a plant is high, what will be your interpretation?

2. Can many micro-habitat in an area affect frequency of a species? Comment.