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. Population characteristics such as size, density, frequency, percentage cover, growth rate and distribution describe it. Population size changes with birth, death, immigration and emigration, and ecological factors such as light, wind and temperature are measured with instruments.

Behavioural objectives

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

• define population size, density, frequency, percentage cover, growth rate and distribution;
• state the formula for population density;
• outline the steps in a population study;
• list the factors that affect the size of a population;
• name the instruments used to measure ecological factors and state their uses.

What are the characteristics of a population?

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

1. Population size: the number of organisms of the same species living together in a given area at a particular time.
2. Population density: the number of a particular species per unit area or volume of the habitat. Therefore, population density = population size ÷ area of habitat.
3. Population frequency: the number of times an organism occurs within a given area or habitat.
4. Percentage cover: how much space or area an organism occupies in its habitat.
5. Population growth rate: the net result of the influence of natality (birth rate) and mortality (death rate) of organisms in a given habitat.
6. Population distribution: how individuals of the same species are arranged in a given habitat, either randomly, evenly or clumped.

How is a population studied?

1. Choose the habitat.
2. Select the sampling method to be used.
3. Collect, count and record the different types of organisms present. Sampling using a quadrat, or a transect using a tape, can be adopted.
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 factors affect population?

1. Natality: the ability of organisms to reproduce to increase their population.
2. Mortality: the death rate.
3. Immigration: the movement of organisms from different habitats into a new habitat. This increases the population of the new area and decreases the population of the former habitat.
4. Emigration: the movement of organisms out of their habitat due to unfavourable conditions, e.g. food scarcity. This reduces the population of the habitat.
5. Availability of food: animals tend to migrate to where there is plenty of food, thereby increasing the population of such places.
6. Seasonal climatic changes: whether the change is favourable or not determines the stay or migration of organisms into or out of a habitat.
7. Breeding season: some organisms move out of their habitats during this season (e.g. fishes), so the population drops.
8. Natural disasters: fire, drought, floods, earthquakes etc. lead to a decrease in population through the death of organisms in such habitats.

How are ecological factors measured?

Ecological factors can be measured using the instruments shown below.

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

1. Climatic factors in an ecosystem include the following except A. light B. wind C. temperature D. oxygen.
2. The organic part of the abiotic component of an ecosystem includes the following except A. water B. carbohydrate C. protein D. lipids.
3. The ratio of population size to the area of the habitat is called A. population size B. population density C. population frequency D. population cover.
4. Habitat factors include all the following except A. biotic B. abiotic C. edaphic D. nature.
5. The total number of organisms of the same species in a habitat at a particular time is called A. population B. community C. census D. all of the above.
6. List five ecological instruments and state their uses.
7. State five factors affecting a population size.