Table of Contents






SS 3







  1. Factors affecting a population
  2. Dynamic population
  3. Family planning


In ecology, the interaction between plants and animals in an environment produces a stable and balanced system. A population is referred to as stable if the population size remains relatively constant.

There are many factors that contribute to the balance in nature; these factors are either abiotic or biotic

Abiotic factors these factors are:

(a) Availability of space

(b) Light

(c) Heat

(d) Oxygen (air)

(e) Water

(f) Food

Biotic factors are interactions between species, which can be either positive or negative depending on the circumstances. Common examples of biotic factors affecting a population include competition for resources, predation, and parasitism.

Biotic factors: these factors are

(a) Predation

(b) Competition

(c) Parasitism

(d) Diseases

(e) Natality

(f) Mortality

(g) Territorial behaviour

(h) Dispersal


  1. List five factors (abiotic) that contribute to the balance in nature
  2. List five factors (biotic) that contribute to the balance in nature.

Gaza, Gaza Strip

Palestinian fishers celebrate the Israeli withdrawal from the city of Gaza (background) and the Gaza Strip in 2005. Decades of political instability and a massive influx of Palestinian refugees have produced extreme overcrowding, large-scale unemployment, and poor living conditions in Gaza and the Gaza Strip. The Palestinian National Authority is headquartered in Gaza.


The biotic community in each ecosystem is composed of population of many species. An ecosystem with a stable climax community, the population sizes of the various species are constant. For each species the population size fluctuates around the carrying capacity of the habitat for that particular species. The various populations in a climax community are said to be in balance or in dynamic equilibrium. The equilibrium is said to be dynamic because though the population remain constant, new individuals are being added while the existing ones are removed all the time. Rate of addition is equal to the rate of removal.

Dynamic equilibrium is maintained by two major factors that relate to density

I. Density –independent factors: some factors affecting growth of a population irrespective of its population density.

Examples of such factors are:

  1. Sudden changes in weather conditions
  2. Natural disasters like flood, fire, drought, earthquakes, volcanic eruptions, tsunamis, storms, presence of pollutants, etc.
  3. These events may wipe out large number of individuals in a population and are independent of the number of individuals in a population

Density dependent factors: these are factors that affect the growth of a population and are dependent on the number of individuals in a population.

Examples Of density dependent factors are

  1. Diseases
  2. Predation
  3. Competition
  4. Parasitism

These factors have a greater effect on densely populated areas than sparsely populated area

For instance, as population increases food availability decreases and disease outbreaks occur more frequently. These may lead to low birth rate and high mortality rate, thereby reducing the population.

Therefore, as population size increases, density dependent factors cause population growth to slow down and help check definite population growth



Response of population to factors affecting them There can be fluctuations in population size.

This can be due to seasonal changes in the environment such as rainfall and temperature.

Seasonal population fluctuations

During favourable condition like dry seasons or extreme temperatures, the population size of a particular species usually decreases while favourable conditions like plenty of food supply during raining season lead to increase in population size,

During wet seasons in grassland, shrubs and grasses start to grow. The population of herbivores in the community also increases rapidly due to mating and return of migrated individuals. With the arrival if the dry season many plants wither and die off. This leads to the decrease in the population size of the herbivores that feed on the plants. Some of the herbivores migrate to other greener pastures. Low birth rate, high death rate and migration tend to reduce the population of the herbivores.

Fluctuation of predator – prey populations:

In nature the interactions between predators and preys may lead to regular cyclical fluctuation in population size. When predators feed on prey, the prey population decreases. This result in greater competition among the predators. Death rate of the weaker predators increases leading to the decline in the predators’ population with less predator, the birth rate of the prey population increases leading to increases in the size of the prey population. The cycle is then repeated.


  1. What is meant by dynamic equilibrium in ecosystem?
  2. Outline what happens in the situation of predator-prey population fluctuation.


  1. Dynamic equilibrium in an ecosystem refers to the balance that is maintained among different species within an ecosystem. This balance is often characterized by fluctuations in population size, as different species within the ecosystem are interdependent on each other.
  2. One example of population fluctuations is seen in the predator-prey relationship between predators and their prey. This relationship is characterized by cycles of predator population increases and decreases, which are driven by fluctuations in the prey population.


In nature, population of living things grows exponentially until they meet with environmental

resistance. Each population stabilizes at a certain size which can be supported by its habitats. The human population is, however, an exception to this usual situation. Through the effort of humans, the human population can be controlled through birth control and family planning. A couple can plan when to have children and when not to have children even if they have sexual intercourse.

Family planning is therefore the use of birth control methods to determine the number and timing of children born in the family.

Birth control methods are in four main groups.

  1. Natural method
  2. Mechanical Method
  3. Chemical Method
  4. Surgical Method

Natural method:

  1. Rhythm methods: This is also known as safe-period method. It based on the fact that in every menstrual cycle, there is a fertile period when ovulation is most likely to occur. For pregnancy to be prevented, sexual intercourse is to be avoided during the period. The fertile period may be from the 12th to the 16th day from the beginning of menstruation. This method however is most unreliable as many other factors such as emotional stress and illness may alter the length of the menstrual cycle.
  2. Withdrawal method: this is also known as coitus interruptus. It involves the withdrawal of the penis from the vaginal just before ejaculation. This method is unreliable as some viable semen may enter the vagina before ejaculation.

Family Planning Class

A volunteer working for the United States Agency for International Development (USAID) gives a class in family planning to women in New Delhi, India. Family planning provides reproductive health services, contraception, and counseling on fertility control choices and is the principal

method used for stabilizing population growth.

Mechanical methods:

  1. Condom: this is also known as a sheath. It is a thin rubber tube that is used to cover the erect penis before intercourse. A small teat at the end of the sheath collects the semen. If used properly, this method is quite reliable. It also helps to protect the user against diseases. The condom is impermeable and prevents germs from semen to set in contact with the vaginal wall and likewise germs from the vaginal wall of an infected female from entering the urethra of the male.
  2. Diaphragm: this is dome –shaped rubber cap with an elastic rim. It is inserted into the top of the vaginal and placed over the cervix, thus preventing sperms from getting into the uterus. A spermicidal cream may be used together with the cap to kill the sperms.
  3. Intra-uterine device (IUD): this is a coil or loop of plastic inserted into the uterus to prevent the implantation of the fertilized egg into the uterine wall.


Tubal Sterilization

Tubal sterilization, or female sterilization, is an operation to prevent pregnancy in which the fallopian tubes in the female reproductive system are closed off so that the eggs produced by the ovaries cannot be fertilized by sperm after sexual intercourse. Surgeons typically use a laparascope, a thin, hollow tube inserted via a tiny incision in the abdominal wall, in this procedure. The tubes may be closed off by a variety of methods including cauterization, searing the tubes closed with burning heat; clamping using plastic clips that remain in the body; constriction using a plastic band; or cutting away a section of the tube and tying off the severed ends.

Chemical methods:

  1. Spermicides: these are chemical agents which are used to kill sperms. They may be in the form of a cream, jelly, foam or tablet. The spermicides are placed high into the vagina before intercourse. When used in conjunction with the diaphragm or condom, it offers greater protection
  1. Contraceptive pills: these pills contain female sex hormones (estrogen and progesterone like hormone) that prevent ovulation. One pill must be taken every 24 hours from the 5th to the 25th day from onset of menstruation. She then stops taking the pill for menstruation to occur. She repeats the cycle again from the 5th day. This method is very reliable if instruction are duly followed
  2. Depo-Provera: this contraceptive is injected into the body of the female to prevent ovulation. The injection is effective in preventing pregnancy for three months.

While there are many methods that can be used to prevent pregnancy, none is completely reliable. For this reason, it is important for individuals and couples to discuss their contraceptive options and choose the method that works best for them. Whether choosing a mechanical, chemical, or surgical method of birth control,

Family Planning in The Gambia

A health worker gives family planning advice to a group of women in The Gambia, in western Africa. The country has a high birth rate, and large families are the norm.

Surgical methods:

  1. Vasectomy: this sterilization procedure is carried out on males through a minor surgical operation. The sperm duct is tied and cut thus preventing sperms from reaching the penis so that no sperms are discharged during sexual intercourse. His sexual characteristics are not affected by this procedure.
  2. Tubal ligation: in the female, both oviducts (fallopian tubes are cut and tied back. This prevents the sperms in the uterus from reaching the egg. The above sterilization procedure is the most reliable contraceptive method but it is irreversible.


  1. What is family planning? Why do humans plan their families
  1. Mention the four major groups of human birth control. Give one example for each of the groups.



  1. Which of the following contribute to an increase in population size?(a) increased mortality rate(b) increased emigration (c) increased competition (d) increased natality rate (e) decreased reproductive capacity
  2. Which one of these is a density -independent factor? (a) competition (b) predation (c) chemical pollution (d) spread of disease (e) parasitism
  3. The following are examples of temporary forms of contraception except — (a) condom (b) rhythm method (c) ligation (d) diaphragm (e) contraceptive pills
  4. Abiotic factors that affect population include these — (a) temperature and water (b) light and food (c) space and predation (d) competition and parasitism (e) air and food
  5. Droughts, fires, floods and earthquakes are all —- (a) population increasing factors (b) density-independent factors (c) population decreasing factors (d) density-dependent factors (e) population fluctuation factors
  6. The following are examples of selective forces except — (a) competition (b) predation (c) parasitism (d) pollution (e) disease
  7. Which of the following is not a density dependent factor? (a) competition (b) parasitism (c) predation (d) disease (e) pollution
  8. The following are examples of temporary contraception except — (a) condom (b) rhythm method (c) Tubal ligation


  1. (a) What are the factors that affect the size of a population
  2. Give an example of a density-dependent factor and explain how it affects population growth
  3. With typical examples predict the response of population fluctuation due to population fluctuations due to
    • i. Seasonal changes
    • ii. Predator-prey relationship
  4. Outline the major advantages and disadvantages of various methods employed in family planning
  5. What is birth control?
  6. Describe any 2 major groups of birth control methods
  7. Distinguish between population and community
  8. Explain the terms (with named examples): intra-specific competition and inter-specific competition
  9. What is meant by dynamic equilibrium?


List 4 biotic factors that contribute to balance in nature and show how they affect population in your locality