Population Studies

A population is defined as the total number of organisms of the same species living together in a given area at a particular time. In any ecosystem, the community is made up of many populations of different species.

To study a habitat’s populations, the following are usually investigated.

1. The type of organisms in the habitat: This involves listing all the different types of populations found in that particular habitat. This helps to determine the relationships that exist between the different organisms in the habitat.
2. The dominant species: This refers to the species of organisms in a community which exert a great influence on the habitat and on the other populations. Dominance may be expressed in terms of their number, size, the portion of space occupied and contribution to the energy flow of the habitat.
3. The characteristics of the population: This refers to;

(a) Population size: The total number of individuals of the same species in the habitat  (the total numbers of individuals in a population). A large population stands a better chance of surviving unfavourable conditions such as fires, diseases, harsh climate changes, while a small population can be easily wiped out. A large population also has the advantage of increasing its vigour through breeding which invariably increases its ability to withstand adverse conditions.

(b) Population density: This is defined as the number of individual organisms per unit area or volume of the habitat.

Example: If an area of land of 100m² has an elephant grass population of 1000 plants, the density of elephant grasses will be;

Total number of individualsTotal area=1000100m2 elephant grasses=10∅∅1∅∅m2 elephant grasses=10 elephant grasses/m2

Population density can be used to estimate the total number of individuals of a population i.e. population size.

(c) Population frequency: This refers to how often the species occurs at different sites in its habitat. It is recorded as the number of times the organism is sited (seen).

(d) Population growth rate: This refers to the total and final effect of birthrate and death rate of organisms in the habitat.

(e) Percentage cover: This is the area of ground or space covered (or occupied) by a given species its habitat. It is expressed in percentage.

(f) Distribution: This refers to the way in which individuals of a particular population are arranged in a given habitat. The individuals may live in clumps, they may be evenly spaced or randomly spaced.

Example: If the western half of the habitat contained ¾ of the elephant grasses, then,

Density =34×1000=75Ø5Ø=15 grasses/m^2

Methods of Studying Populations

To conduct population studies the following procedure is used;

1. Choose the habitat to be studied
2. Choose a sampling method
3. Identify the species in the habitat
4. Collect, count and record the different types of organisms present.
5. Repeat the population studies at different periods.

The following methods can be used to study specific populations:

1. Collection of plants: In a small area plants are easy to count and their distribution can be recorded on a map or scale diagram of the area, however for larger areas quadrats or transects are used.

(a) Quadrat Sampling: A quadrat is made of a square or rectangular piece of wire, plastic, wood or metal frame with predetermined area. E.g. the area of a quadrat  may be 25cm². A quadrat is used to sample the number of plant species in a habitat. It is not suitable for sampling animals because they move around so much. A quadrat is used by throwing it over the shoulder at random several times and on each landing, the area covered/enclosed by it is observed. The type of plant species and their number within the quadrat are recorded. From the results, the average number of plant per m² is calculated. If the area of the habitat is known, the total number of plants it contains can be estimated.

Permanent quadrats, with mapping grids attached can be made to study seasonal variations of plants. These quadrats are sturdier, larger and remain permanently fixed on a marked area.

(b) Transect method: A marked tape is used in this method. The tape is marked at convenient intervals and then stretched across the area to be studied. The plants encountered at the interval marks are counted and recorded. This procedure is repeated a few times. In this way, a fairly accurate estimate of the number and types of plants in the habitat are obtained. Plants are usually collected in plastic bags and then pressed and dried in a plant press. The dried plants are mounted on stiff paper, fixed in position with masking tape/cellotape and labeled with both the scientific and common names.

2. Collection of Animals: Animals are more difficult to collect than plants; however their presence can be inferred by looking for signs of their presence such as nests, eggs, feaces, tracks, feathers, etc and by studying the vegetation with which they are associated. To study animals different types of nets and traps are used in capturing them. The following methods can be used.

(a) Capture – Recapture method: In this method animals of one type in a particular area are caught, counted and marked with ink and released. Their number is recorded as A1. The following day another set is captured and the number recorded as A2.This second batch may include animals which had been caught and marked the previous day, their number is recorded as A3. The population of animals present in the area is found using the formula;

Population in area A1×A2A3

This method is based on the assumptions that;

1. Individuals do not move out of or into the ecosystem
2. The marked individuals are randomly distributed in the population.
3. The marked individuals are a random sample.
4. The initial capture and markings do not influence recapture.
5. That none of the marks have worn off during the interval between the two catches.

NB: The results obtained by this method are approximations.

(b) Collection of soil animals with quadrats: The soil animals in an area can be studied by collecting samples of soil from several sites chosen by tossing a quadrat randomly. A 25% sodium chloride solution is added to each soil sample and the animals are collected as they float in the solution.

For earthworms, the sites chosen with the quadrat are irrigated with 25% formalin solution and the earthworms are collected as they move to the surface of the soil.

EVALUATION

1. What are the major investigations carried out when studying a habitat?
2. A pond with an area of 200m² has a duckweed population of 1,500 plants. Find

(a) The population density

(b) If the western half of the habitat contained ¾ of the duckweed. What is the density of the eastern half?

3. Explain what a Tullgren’s funnel is and what it is used for.
4. Make a list of traps used for capturing animals for study and describe how captured animals are kept or preserved.