A number of factors referred to as environmental resistance control the population of organisms in a particular habitat. These factors are classified as abiotic and biotic factors.


Abiotic Factors Affecting a Population

  1. Heat: The degree of hotness of a place can determine the size of population. Hot environment generally is not conducive for habitation as organisms tend to run away from such areas. Heat can cause stress and death of individuals.
  2. Water: Availability of water in a habitat determines the rate of survival of the population. While availability of water can cause increase in population lack of it can cause decrease in population.
  3. Space: Space is very important to all organisms for normal growth and development. Lack of space leads to overcrowding and competition among organisms.
  4. Light: Light is an important abiotic factor especially in plant community. Without light, green plants (producers) may not be able to manufacture the food needed in a habitat. Low light intensity also can result in weak plant growth and development.
  5. Nutrients: Plants require nutrients in the soil to synthesize their various food substances. Lack of nutrients can result in stunted growth and poor yield of crops.


Biotic Factors Affecting a Population

  1. Food: Lack of food in a habitat can adversely affect the population of organisms. Food is the basic material required for survival of the organisms as it is required for growth, development and reproduction.
  2. Competition: Competition will arise among organisms when there is lack of space, food and other resources within the environment. Competition at its highest level usually leads to fighting and cannibalism among animals.
  3. Natality: Natality refers to the rate at which organisms give birth to young ones. High birthrate increases population and can result in overcrowding.
  4. Mortality: Mortality is the rate at which organisms die in a habitat. While low death rate increases population, high death rate reduces it. Low death rate can cause food shortage and overcrowding.
  5. Dispersal: Dispersal is a natural means of avoiding overcrowding especially in plants. The dispersal of seeds and fruits either by water, wind, animals, insects etc reduces the chances of overcrowding and so overpopulation is prevented or avoided.
  6. Parasites: Parasites are organisms which feed on or in another organism called the host, In the process, the host is harmed, damaged or even killed thereby reducing the population of such organisms.
  7. Pathogens: Pathogens are diseases causing organisms such as viruses, and bacteria which can affect the well-being of other organisms. High level of pathogenic attack invariably reduces the population of the host organisms.
  8. Predators: These are organisms which prey or feed on weaker ones. High level of predations in a habitat can reduce the population of prey.



Factors which affect population size include abiotic factors such as temperature, water, space etc and biotic factors such as food, competition, parasite etc. When these factors are favourable, growth is promoted but when they are scarce or unfavourable, growth is retarded.

A factor which limits population growth is called a limiting factor and the sum of all the limiting factors is known as environmental resistance. The net effect of these abiotic and biotic factors is that at a point, the population size of living organisms tends towards a dynamic equilibrium known as balance in nature.

When the population increases, the environmental resistance increases too.

This means that when population is on the increase, the available food tends to decrease. This calls for competition which will later lead to death of the weaker organisms, thereby keeping the population relatively constant.

Human beings are able to control their population by family planning and birth control whereas in nature, biological equilibrium is attained by predator prey relationship.



Family planning is a device by which a couple (husband and wife) determine the number of children they want and when they want them. Birth control on the other hand, refers to the method used to prevent a woman from becoming pregnant, for as long as she wishes.

Many parents decide to have a few number of children so that they can afford to eater for their feeding, clothing, housing, education and medical care. Without family planning, the population of a nation can rise indiscriminately and may not be able to match the available food and other resources, hence famine and death can result. It is very important to point out that family planning is centred on prevention of pregnancy rather than termination of life. When family planning is carefully carried out, the issue of unwanted pregnancy is prevented.


Birth Control or Family Planning

Devices Birth control or family planning devices include

  1. Withdrawal method: By this method, the man pulls his penis out of the woman’s vagina before the ejaculation of spermatozoa occurs. This method is not very reliable because some sperm cells may be released without the man knowing.
  2. Rhythm method or safe period: This involves the calculation of safe periods for each woman. To avoid getting pregnant, a woman should not have sex with a man during the fertile days, i.e ., days mid way between her menstrual periods which usually starts ten days after the first day of menstruation. This method is also not reliable as ovulation can take place at irregular intervals.
  3. Use of condom (Sheath): This is a rubber sheath which the man wears over the penis before sexual intercourse. When he ejaculates spermatozoa, they are collected within the condom thereby preventing them from entering into the vagina of the female. This works very well in preventing pregnancy and at the same time protecting the man against veneral diseases such as syphilis, gonorrhoea and AIDS.
  4. Cap (Diaphragm): This is a rubber protective cap worn by women. It is inserted into the vagina so as to prevent spermatozoa from entering the uterus and into the fallopian tube where they may bring about fertilisation of the ovum. The diaphragm is best used in combination with spermicidal cream or tablets.
  5. Spermicidal cream or tablet: A spermicidal cream or tablet is one that kills spermatozoa, A woman applies such a cream or tablet right inside the vagina 510 minutes before sexual intercourse. During ejaculation by the man, the spermatozoa discharge are killed by the spermaticide, hence pregnancy is prevented. This method is not very reliable except it is used with condom.
  6. Intra-uterine device (IUD): This may be a metal or plastic coil or loop that is inserted into the uterus ofthewoman.Itis usually done by a doctor. This device prevents fertilisation or the implantation of the zygote in the uterus. The intra-uterine device is left in the uterus until it is necessary to replace or remove it. When properly inserted, it is reliable and economical as well.
  7. Contraceptive pill: This is a tablet which is taken daily by the woman. The pill contains hormones which prevent ovulation or the release of egg from the ovary. The tablet must be prescribed by a qualified doctor. The pill is effective and must be used according to prescription to obtain good result.
  8. Injection: An injection is usually given to the woman every three or six months intervals. The injection contains hormones which prevent ovulation. Such injection must be given to the woman by a qualified medical doctor because it has some side effects. It is a sure way of family planning and it is very useful to women who have stopped child bearing.
  9. Sterilisation: Sterilisation is done by couples who do not want more children. It is an irreversible birth control method. In other words, once it is done, one cannot reverse it. It is the surest method. It is done by cutting the sperm duct (spermatic cord) In men (vasectomy) and the cutting of the oviduct (fubal ligation) in women. This method requires an operation to be done by a qualified medical doctor.
  10. Abstinence: This is when both the male and female stay way completely from having sex.


Importance of family planning to a nation

  1. It promotes maternal or mother’s health.
  2. It equally promotes the health of the child.
  3. Family planning prevents population explosion.
  4. It reduces maternal or infant mortality rate.
  5. Family planning makes child spacing possible.
  6. Family planning prevents indiscriminate abortion in women.
  7. Family planning increases or promotes the standard of living of the people.