19.4) Population size

19.4) Population size


Population: is a group of organisms of one species, living and interacting in the same area at the same time.

Community: is all of the populations of different species in an ecosystem.

Ecosystem: is a unit containing the community of organisms and their environment, interacting together, eg. a decomposing log or a lake.


Factors affecting the rate of population growth for a population of an organism:

  • Food supply
  • Predation
  • Disease – when a disease spreads globally it is called a pandemic.


Human population:

  • About 20 years ago, the human population was increasing at the rate of 2% a year, this means the world population was doubling every 35 years.
  • This doubles for demand for food, water, space and other resources.
  • Infant mortality: the death rate for children less than 1 year old.
  • Life expectancy: the average age to which a newborn baby can be expected to live.
  • Fertility rate: the average number of children a woman would have.
  • Agricultural development and economic expansion led to improvements in nutrition, housing and sanitation, and to clean water supplies.
  • These improvements reduced the incidence of infectious diseases in the general population, and better-fed children could resist these infections when they did meet them.
  • The social changes probably affect the population growth more than did the discovery of new drugs or improved medical techniques.
  • Longer and better education: marriage is postponed and a better-educated couple will have learned about methods of family limitation.
  • Application of family planning method: either natural methods of birth control or use of contraceptives is much more common.
  • Because of these techniques – particularly immunisation -diphtheria, tuberculosis and polio are now rare, and by 1977 smallpox had been wiped out by the World Health Organization (WHO)’s vaccination campaign.


Sigmoid population growth curves:

  1. Lag phase. The population is small. Although the numbers double at each generation, this does not result in a large increase.


  1. Exponential(log) phase. Continued doubling of the population at each generation produces a logarithmic growth rate (eg. 64-128-256-512-1024). When a population of 4 doubles, it is not likely to strain the resources of the habitat, but when a population of 1021 doubles there is likely to be considerable competition for food and space and the growth rate starts to slow down.


  1. Stationary phase. The resources will no longer support an increasing population. At this stage, limiting factors come into play. The food supply may limit further expansion of the population, diseases may start to spread through the dense population and overcrowding may lead to a fall in reproduction rate. Now the mortality(death) rate equals the reproduction rate, so the population number stays the same.


  1. Death phase. The mortality rate is now greater than the reproduction rate, so the population numbers begin to drop Fewer offspring will live long enough to reproduce. The decline in population numbers can happen because the food supply is insufficient, waste products contaminate the habitat or disease spreads through the population.


Abiotic and biotic limiting factors:

  • Plant populations will be affected by abiotic ( non-biological) factors such as rainfall, temperature and light intensity.
  • Biotic (biological) factors affecting plants include their leaves being eaten by browsing and grazing animals or by caterpillars and other insects, and the spread of fungus disease.
  • Animal populations will to be limited by these factors.
  • The size of an animal population will also be affected by immigration and emigration.
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