Interspecific Competition (SL IB Biology)

• 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