Populations and population density
A population is a group of one species in one area. Its size, called population density (N), is measured by counting, per cent cover, biomass or indirect signs.
What is a population?
In nature we rarely find a single, isolated individual of a species. Most live in groups. A population is all the individuals of one species living in a defined area at the same time. They share or compete for the same resources and can interbreed. A group formed by asexual reproduction is also treated as a population in ecology. Examples: all the cormorants in a wetland, all the teak trees in a forest patch, all the lotus plants in a pond, or all the bacteria in a culture plate. A population has features that one individual cannot have, such as a birth rate, death rate, sex ratio and age distribution.
Ways to measure population size
| Measure | When it is used | Example |
|---|---|---|
| Total count | Large, easily seen organisms in a small area | Counting the neem trees on a school campus |
| Per cent cover or biomass | When individuals differ hugely in size, so numbers mislead | One huge banyan tree beside 200 Parthenium plants: a count would underrate the banyan's role |
| Biomass or per cent cover | When counting is impossible or too slow | A dense culture of bacteria in a petri dish |
| Relative density | When the exact total is not needed, only a comparison | Number of fish caught per trap in a lake |
| Indirect signs | When animals are shy, spread out or hard to see | Tiger census from pugmarks (footprints) and faecal pellets (droppings); camera traps are also used today |
Activity: plant population density by the quadrat method (Experiment 2)
A **quadrat** is a square frame that marks a fixed sample area, usually **1 m × 1 m** for herbs and grasses. Counting plants in a few quadrats lets you estimate the whole area.
- 1Choose a grassy patch, such as the school lawn or a field edge. Take four pegs and a string, or a ready 1 m × 1 m wooden or metal frame.
- 2Lay out 10 quadrats at random. For example, toss a small stick and place the quadrat where it lands, so you do not choose only the thick patches.
- 3In each quadrat, identify the species present and count every individual of the species you are studying, for example Tridax, a common daisy-like weed.
- 4Record the counts in a table: one row for each quadrat, one column for each species.
- 5Add the counts of each species across all the quadrats.
- 6★Calculate: Density = total number of individuals of the species in all quadrats ÷ total number of quadrats studied. Write the unit, individuals per m².
- 7Record: the species with the highest density is the most numerous plant in that patch. Write the density of each species with its unit.
★ key step
Activity: plant population frequency by the quadrat method (Experiment 3)
- 1Use the same 10 random quadrats, or lay out a fresh set in the same way.
- 2For each species, note only whether it is present or absent in each quadrat. You do not count individuals.
- 3Count the number of quadrats in which each species occurs.
- 4★Calculate: Frequency (%) = (number of quadrats in which the species occurs ÷ total number of quadrats studied) × 100.
- 5Record and conclude: frequency shows how evenly a species is spread over the area. A species found in 9 of 10 quadrats (90%) is widespread; one found in 2 of 10 (20%) is patchy.Extra reading (not in your textbook): some lab manuals then sort species into Raunkiaer's frequency classes: A (1 to 20%), B (21 to 40%), C (41 to 60%), D (61 to 80%), E (81 to 100%)
★ key step
Worked example: density and frequency from quadrat data
A student laid 10 quadrats of 1 m × 1 m in a lawn. Tridax was found in 6 quadrats, with 4, 2, 5, 3, 6 and 4 plants. The other 4 quadrats had none. Find the density and the frequency of Tridax.
- Total Tridax plants = 4 + 2 + 5 + 3 + 6 + 4 = 24.
- Density = total individuals ÷ total quadrats studied = 24 ÷ 10 = 2.4 plants per m².
- Frequency = (quadrats with Tridax ÷ total quadrats) × 100 = (6 ÷ 10) × 100 = 60%.
- Check: density divides by all 10 quadrats, including the 4 empty ones; frequency uses only presence or absence.
Density = 2.4 plants per m²; frequency = 60%.
Forest officials estimate tiger numbers in reserves such as Corbett and Kanha from pugmarks, droppings and camera traps. Fisheries officers judge fish stocks in a lake from the catch per net, a relative density.
1 marks: “Name two indirect signs used to estimate the number of tigers in a reserve.” Pugmarks (footprints) and faecal pellets (droppings).
2 marks: “Why is per cent cover or biomass sometimes a better measure of population size than the number of individuals? Give an example.” When individuals differ greatly in size, numbers mislead. With one banyan tree and 200 Parthenium plants, a count makes the banyan look minor, though its canopy and role are far larger. Per cent cover or biomass shows this.
3 marks: “How would you find the density and frequency of a plant species in a field by the quadrat method?” Lay 10 random quadrats of 1 m × 1 m; count individuals of the species in each and note presence or absence; record in a table. Density = total individuals ÷ total quadrats studied (per m²). Frequency = quadrats with the species ÷ total quadrats × 100.
Density asks how many; frequency asks in how many quadrats. Density divides by all the quadrats.