Populations (CIE IGCSE Biology)

• A population is defined as a group of organisms of one species, living in the same area at the same time

Other important definitions

• A community is defined as all of the populations of different species in an ecosystem
• An ecosystem is defined as a unit containing the community of organisms and their environment, interacting together (eg a decomposing log, a lake)

• All living organisms compete with each other for food, water and living space
• Those which are the best adapted to their environments generally increase their populations at the expense of those less well adapted
• Population growth in most organisms is controlled by the following three factors:
  • Food supply
  • Predation
  • Disease

• If the growth of microorganisms in a fermenter is measured over time, the population growth looks like the graph below

A typical growth curve for a population in an enclosed environment

• The shape of this curve ( a little like an ‘S’), gives it its name - a sigmoid growth curve
• The curve has four distinct phases:
  • Lag phase - organisms are adapting to the environment before they are able to reproduce; in addition, at this stage there are very few organisms and so reproduction is not producing larger numbers of offspring
  • Log phase (aka exponential phase) - food supply is abundant, birth rate is rapid and death rate is low; growth is exponential and only limited by the number of new individuals that can be produced
  • Stationary phase - population levels out due to a factor in the environment, such as a nutrient, becoming limited as it is not being replenished; birth rate and death rate are equal and will remain so until either the nutrient is replenished or becomes severely limited
  • Death phase - population decreases as death rate is now greater than birth rate; this is usually because food supply is short or metabolic wastes produced by the population have built up to toxic levels

• Organisms in a natural environment are unlikely to show population growth like a sigmoid growth curve because they are affected by many other factors, including:
  • changing temperature or light
  • predators
  • disease
  • immigration (individuals moving into the area)
  • emigration (individuals moving out of the area)