Basic Ecological Systems III – SS1 Biology Lesson Note (Second Term)

A population is the number of organisms of the same species living together in a given area at a particular time.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define population size, density, frequency, percentage cover, growth rate and distribution;
2. state the formula for population density and use it in a simple calculation;
3. outline the steps in a population study;
4. list the factors that affect the size of a population;
5. name the instruments used to measure ecological factors and state their uses.

What are the characteristics of a population?

The characteristics of a population are size, density, frequency, percentage cover, growth rate and distribution.

• Population size: the number of organisms of the same species living together in a given area at a particular time.
• Population density: the number of a particular species per unit area or volume of the habitat.
• Population frequency: how often a species occurs in the samples taken from a habitat, e.g. found in 6 of 10 quadrats.
• Percentage cover: how much of the space or area of a habitat an organism occupies, e.g. grass covering half of a quadrat has 50% cover.
• Population growth rate: the net result of natality (birth rate) and mortality (death rate) of organisms in a given habitat.
• Population distribution: how individuals of the same species are arranged in a given habitat.

How is population density calculated?

Population density is the population size divided by the area (or volume) of the habitat.

• Formula: population density = population size ÷ area of habitat.
• Unit: number of organisms per square metre (per m²) for land, or per cubic metre for water.
• Worked example: 120 grasshoppers counted in a field of 60 m² give a density of 120 ÷ 60 = 2 grasshoppers per m².

What are the types of population distribution?

There are three types of population distribution: random, even and clumped.

• Random: individuals are scattered without a pattern, as with many weeds on a farm.
• Even (uniform): individuals are spaced at about equal distances, as with crops planted in rows.
• Clumped: individuals gather in groups, as with herds of cattle or colonies of ants.

How is a population studied?

A population is studied by sampling a habitat, counting the organisms found and calculating the population characteristics.

1. Choose the habitat.
2. Select the sampling method to be used.
3. Collect, count and record the different types of organisms present, using a quadrat or a transect.
4. Identify the dominant species, that is, the species that has greater influence on the other members of the community.
5. Repeat the population studies at intervals.
6. Determine the population characteristics.

What are a quadrat and a transect?

• Quadrat: a square frame of known area, often 1 m by 1 m, placed on the ground to count the organisms inside it.
• Transect: a line of tape or rope laid across a habitat, along which the organisms are recorded.

How is a quadrat used?

1. Mark out the area to be studied.
2. Place the quadrat at random positions in the area.
3. Count and record each species inside the quadrat.
4. Repeat several times to get a fair sample.
5. Add the counts and divide by the total area sampled to find the density.

What factors affect the size of a population?

Population size is affected by natality, mortality, immigration, emigration, food supply, climate, breeding season and natural disasters.

Direct factors

• Natality: the ability of organisms to reproduce to increase their population.
• Mortality: the death rate.
• Immigration: the movement of organisms from other habitats into a new habitat.
• Emigration: the movement of organisms out of their habitat due to unfavourable conditions, e.g. food scarcity.

Other factors

• Availability of food: animals tend to migrate to where there is plenty of food, so the population of such places increases.
• Seasonal climatic changes: whether the change is favourable or not determines the stay or migration of organisms into or out of a habitat.
• Breeding season: some organisms, e.g. fishes, move out of their habitats at this time, so the population drops.
• Natural disasters: fire, drought, floods and earthquakes reduce population through the death of organisms.

What is the difference between immigration and emigration?

• Immigration: organisms move into a habitat from elsewhere.
• Emigration: organisms move out of a habitat to live elsewhere.

How can population change be expressed?

• Increase: births and immigration add organisms.
• Decrease: deaths and emigration remove organisms.
• Net change: (births + immigration) − (deaths + emigration).

How are ecological factors measured?

Ecological factors are measured with instruments such as the photometer, anemometer, rain gauge and thermometer.

Evaluation

1. Define population density and state how it is calculated.
2. Distinguish between immigration and emigration.
3. List the steps in a population study.
4. Name the instrument used to measure (a) the speed of wind, (b) the amount of rainfall and (c) relative humidity.
5. State the three ways in which individuals of the same species may be distributed in a habitat.

Assignment

(a) List five ecological instruments and state the use of each. (b) State five factors that affect the size of a population.