Competition Between Species (SL IB Biology)

Niche

• The niche of a species can be defined as follows:

The role of a species within its habitat

• A species' role takes into account
  • The biotic interactions of the species, e.g. the organisms it feeds on and the organisms that feed on it
  • The abiotic interactions, e.g. how much oxygen and carbon dioxide the species exchanges with the atmosphere

Fundamental vs realised niche

• The fundamental niche of a species is:

The full range of conditions and resources in which a species could survive and reproduce, based on its adaptations and tolerance limits

• The realised niche is:

The actual conditions and resources in which a species exists, due to biotic interactions

• An example of a fundamental niche compared to a realised niche can be seen in the case of the barnacle species Chthamalus dalli
  • Its fundamental niche includes a wide range of rocky intertidal areas in the Pacific Northwest, where it can attach to a variety of substrates and tolerate a wide range of temperature and salinity conditions
  • However, in reality, the realised niche of this species is much smaller due to competition with other barnacle species, such as Balanus glandula, for space and resources
  • As a result the actual range of Chthamalus dalli is restricted to areas where Balanus glandula is absent or scarce, such as higher up on the shore, where it is exposed to air for longer periods of time and can avoid competition with Balanus glandula for space and resources
  • Therefore, the realised niche of Chthamalus dallia is smaller than its fundamental niche due to the biotic interactions with other species

Fundamental niche vs realised niche table

• A niche can only be occupied by one species, meaning that every individual species has its own unique niche
• If two species try to occupy the same niche, they will compete with each other for the same resources
• The eventual result of this competition will be that:
  • One of the species will be more successful and out-compete the other until the second species is either:
    • Forced to occupy a new, slightly different niche
    • Made locally extinct
  • Both species are forced into a smaller part of their fundamental niches
• The elimination of a competing species from its niche is known as competitive exclusion

A possible consequence of different species of warbler competing for the same resources is that the most successful competitor will exclude other species from their niche, pushing them into slightly different niches, e.g. the location in which they feed may change

Competitive exclusion example

• A classic example of competitive exclusion is between two species of single-celled free-living protozoans
  • Paramecium aurelia
  • Paramecium caudatum
• When each species is grown separately from each other both species will thrive
• When the two species are grown in the same habitat they compete for resources and eventually P. aurelia outcompetes P. caudatum for these resources, which leads to P. caudatum's elimination 
  • It is worth noting that this example involves both species being grown under laboratory conditions; it is rare in nature that two species will occupy exactly the same niche