Exercise 23

Principle: Density represents the numerical strength of a certain plant species in the community

per unit area. The number of individuals of the species in any unit area is its density. The unit area

may be as small as 5 square cm to as large as 10 square metre depending on the size and nature of the plant community under study. For herbaceous vegetation a metre square quadrat is normally

used. Density which gives an idea of degree of competition is calculated as follows.

The value thus obtained is then expressed as number of individuals per unit area. When the

measured unit area is divided by the number of individuals the average area occupied by each

individual is obtained.

Requirement: Cotton/nylon thread (five meters), 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 the same species if seen in other quadrats assign the same alphabet).

3. Count the number of individuals of each species present in the quadrat and record the data as shown in the table.

4. Similarly make nine more quadrats randomly in the site of study and record the names and number of individuals of each species.

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.

Discussion:

Plants growing together exhibit mutual relationships among themselves and also with the environment. Such a group of plants in an area represent a community. The number of individuals of a species varies from place to place, making it necessary to take many random sample areas for reliable results. Density values are significant because they show relative importance of each species. With increasing density the competition stress increases and the same is reflected in poor growth and lower reproductive capacity of the species. Data on population density are often very essential in measuring the effects of reseeding, burning, spraying and successional changes. Discuss the vegetation composition of the area (herbs/shrubs) and comment on the dominant component species.

Questions

1. What factors influence the population density?

2. What is the significance of quadrat method?

3. What conclusion can be drawn if density of a plant species is low?