Revision Notes

19.5 Populations

Definition of Population

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

Factors Affecting Population Growth

  • 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

Human Population Growth

  • Human population growth globally has been increasing exponentially for the last 150 years

Human population growth, IGCSE & GCSE Biology revision notesHuman population growth is growing exponentially

  • There are many reasons for this exponential growth, including:
  • Improved technology leading to an abundance of food = rapid increase in birth rate
  • Improved medicine, hygiene and health care = decrease in death rate

Exam Tip

There are many different ways of showing human population growth.

In an exam make sure you look carefully at the data to figure out the trend it is showing and think about how that links to social implications, locally and globally, of the growth of the human population.

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Growth Curves

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

Sigmoid growth curve, IGCSE & GCSE Biology revision notesA 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)

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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