Interactions Between Population & Environment (WJEC GCSE Biology)

• Organisms rely on resources from their environment for survival, e.g.
  • Animals require:
    • Food
    • Water
    • Mates
    • Territory (that contains food, water and mates)
  • Plants require:
    • Water
    • Light
    • Minerals
    • Space to grow (that provides access to water, light and minerals)
• Organisms that require the same resources as each other will compete for survival
  • Intraspecific competition occurs between individuals of the same species
  • Interspecific competition occurs between individuals of different species

Intraspecific and interspecific competition diagram

Individuals compete for resources in order to survive; competition can be intraspecific (top) or interspecific (bottom)

Factors affecting population size

• The size of a population can be affected by factors in the environment, e.g.
  • Competition
    • Competition for resources, either within a species or with another species, limits the maximum size of a population
  • Predation
    • Predators will increase the death rate in a population, slowing population growth
    • A lack of predators may result in the rapid growth of a population
      • In a situation like this, the resulting large population will compete more for resources, and may be more susceptible to disease
  • Disease
    • Disease may kill some individuals, slowing population growth
    • The effect of disease is likely to be greater in a large population as infection may spread more quickly in crowded conditions
  • Pollution
    • Pollution may kill some individuals, altering the structure of an ecosystem, e.g.
      • A build-up of heavy metals in a food chain may affect top predators more than lower trophic levels, reducing predation
      • Fertiliser run-off into water bodies may cause algal overgrowth, providing food for algae feeders but reducing light and oxygen levels deeper under the water