Theory & Procedure, Study of Plant Population Frequency by Quadrat Method | Additional Study Material for NEET PDF Download

Aim

Our aim is to study plant population frequency by quadrat method.

Theory

In ecology it is useful to know the frequency of certain plant species in a certain place, or at a certain time. Frequency can be defined as the degree of uniformity of the occurrence of individuals of a species within a plant community.  However, finding the frequency of plant species is very difficult for a large populations or extensive habitats. A widely used method for plant frequency sampling is by quadrat method.

Quadrat Method

Scientists usually calculate the plant population frequency using the quadrat method. A quadrat is a sample plot of a specific size used for the study of population or a community. Quadrats are used in many different scientific disciplines like vegetation assessment, including plant density, plant frequency and plant biomass. Frequency is highly influenced by the size and shape of the quadrats used. The area that is chosen for study must not be so big that it cannot be sampled adequately, or so small that the habitat is difficult for sampling. For herbaceous vegetation a metre square quadrat is normally used.

Frequency

Determining plant population frequency using quadrat method has become popular primarily because it is relatively simple and objective. Frequency indicates the number of times a plant species is present within a given number of sample quadrats. It is measured by noting the presence of a species in random sample areas which are distributed as widely as possible throughout the area of study. Once analyzed, the sample data enables the scientist to calculate population frequency of the entire population by the following method:
%Frequency=(Number of sampling units in which the species occurs)/(Total number of sampling units employed for the study)*100

How to calculate population frequency?

The image below depicts a plot which is divided into seven quadrats.

Observe the presence sunflower plant in each quadrat.  In this case, sunflowers are present in five quadrats among the seven.

Hence, the percentage frequency of sunflower is:

%Frequency=5/7*100=71.42

Variations in Plant Distribution

Variation in distribution is caused by several factors like soil conditions, vegetative propagation, quantity and dispersal of seeds, grazing or other biotic activities and predation by insects or diseases. Some species abundantly spread all over the area have a chance of occurring in all the sampling quadrats and therefore, its frequency will be 100. The plants with high frequency are wide in distribution.

Learning Outcomes:

• Students understand the term quadrate method.
• Students understand how to calculate plant population frequency.
• Students do the experiment better in the real lab having gone through the animation and simulation.

Materials Requires

Real Lab Procedure

• In the selected site of study, hammer the nails firmly in the soil without damaging the vegetation.
• Fix four nails to make a square.
• Tie each end of the nails using a thread, to make a 1 m X 1 m quadrat.
• Similarly, make nine more quadrats randomly in the site of study.     
• Select the plant species for study of the population frequency.
• Observe the presence of species “A” in the first quadrat and mark it in the table.
• Similarly, check for the presence of species “A” in other quadrats respectively and record the data in the table.
• Observe the presence of species “B” in all quadrats and mark it in the table.
• Repeat the same procedure for species C and record the data in the table.
• We can calculate the frequency of plant populations by this equation:
• Percentage Frequency=(Number of sampling units in which the species occurs)/(Total number of sampling units employed for the study)*100

Observations

Frequency value indicates the number of times a plant species is present within a given number of sample quadrats.

Simulator Procedure (as performed through the Online Labs)

• Click on the popup corresponding to each quadrat to enlarge the view.
• You can select the environment for study from the ‘Select the environment’ drop down list.
• Observe the presence plant species in the quadrat.
• Enter 'Y’ in the worksheet if a particular plant species is present or ‘N’ if it is not present.
• Click on the back button to show normal view of the quadrat.
• Based on the values entered, the worksheet gives the plant population frequency for each species.
• You can redo the experiment by clicking on the ‘Reset’ button.