What is population density? Describe the quadrat method to find the density and frequency of a plant species.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define in one line. Population density (N) is the size of a population, usually the number of individuals of a species per unit area.
2 · Say how it can be measured.
Direct count for large, visible organisms.
Per cent cover or biomass when sizes differ greatly, e.g. one banyan and 200 Parthenium plants.
Relative density, e.g. fish caught per trap.
Indirect signs, e.g. pugmarks and faecal pellets in a tiger census.
3 · Describe the quadrat method.
Lay 10 quadrats of 1 m × 1 m at random.
Count individuals of the species in each; note presence or absence.
Record the data in a table.
4 · Write both formulas. Density = total individuals ÷ total quadrats studied (per m²). Frequency (%) = quadrats with the species ÷ total quadrats × 100.
5 · Sketch the layout. [Sketch: a field with 10 square quadrats placed at random, each marked 1 m × 1 m]
6 · Link to real life. Tiger reserves in India and lake fisheries use these ideas to track their populations.
7 · Close in one line. So density tells how many individuals there are, and frequency tells how widely the species is spread.
Scoring tip: Show one worked calculation with numbers. Examiners give marks for both formulas and the unit per m².
