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First teaching 2023

First exams 2025

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Interspecific Competition (HL IB Biology)

Revision Note

Naomi H

Author

Naomi H

Last updated

Tests for Interspecific Competition

  • If two species occupy very similar niches, then competition can exist between them for resources; this is interspecific competition
  • One species may be slightly better adapted to compete than the other, so the second species may be outcompeted
  • The result of the interspecific competition could be that the second species is forced to alter its distribution within a habitat so that it no longer directly competes with the first species 
    • If this is not possible then the second species could become locally extinct
  • The second species has been forced out of its niche into an alternative niche due to competition; this is known as competitive exclusion
    • The ideal niche is known as the fundamental niche while the new niche is known as the realised niche

Testing for interspecific competition

  • This competitive exclusion effect can be used to test for the presence of interspecific competition
    • If the removal of a competitor species results in a change in species distribution then interspecific competition is likely to have been taking place, but if the removal of a competitor has no effect then distribution is likely to be the result of another factor
      • Note that this effect does not prove the presence of interspecific competition, but does indicate that it could be occurring
  • There are different ways of carrying out such a test for the presence of interspecific competition, e.g.
    • In the lab
      • E.g. by culturing bacteria species on their own or together and measuring how this affects population size or colony distribution
    • In the field with random sampling and then with manipulation, e.g.
      • By first carrying out random quadrat samples and recording the presence/absence of one or both species at different locations around a habitat
      • By then removing one species from a small area and measuring the effect that this has on distribution of the second species

NOS: Students should recognize that hypotheses can be tested by both experiments and observations and should understand the difference between them

  • A hypothesis (plural hypotheses) is a proposed explanation for an observation, that can be tested by scientific investigation
  • There are different ways of carrying out such tests, e.g. as described above, hypotheses can be tested either
    • In a laboratory
    • In the field
  • Laboratory investigations are carried out under controlled conditions and on a small scale, e.g. growing bacteria in a lab, or plants in a greenhouse
    • Laboratory tests allow a high level of control, so only the independent variable is changed while other variables are carefully controlled
    • Laboratory experiments are designed to represent real life, but the results can never be directly applied to a real life situation
    • Organisms may not always behave in the same way in a lab as they do in their natural environment
  • Field tests are carried out in a real-life setting, and can be carried out on a large scale e.g. observing the growth of plants in an area of forest, or the distribution of species on a rocky shore
    • It is not possible to control factors beyond the independent variable, so field experiments may not provide a perfectly valid set of results, and are very hard to replicate exactly
    • Field experiments may provide a more realistic representation of the real world

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Naomi H

Author: Naomi H

Expertise: Biology

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.