A fundamental niche includes the full range of abiotic conditions and resources under which a species could survive and reproduce. A realized niche is the narrower range it actually occupies after biotic interactions restrict its distribution.
This is HL-only content in IB ESS subtopic 2.1, Individuals, populations, communities and ecosystems.
How the Difference Arises
A species' niche describes the abiotic and biotic conditions and resources on which it depends. Its fundamental niche is determined mainly by its physiological tolerances, including suitable temperature, moisture, salinity, pH, food and shelter.
Within an ecosystem, the species also experiences biotic interactions such as competition, predation, parasitism and disease. These interactions may prevent it from occupying some otherwise suitable habitats, producing its realized niche.
| Feature | Fundamental niche | Realized niche |
|---|---|---|
| Meaning | Full potential range of conditions and resources | Range actually occupied and used |
| Main controls | Physiological tolerance and resource requirements | Abiotic limits plus biotic interactions |
| Relative size | Usually broader | Usually narrower or equal |
| Evidence | Predicted from tolerances or tested by removing constraints | Measured from the species' observed distribution |
Joseph Connell's barnacle study on the Isle of Cumbrae, Scotland, provides a named example. Chthamalus stellatus could survive in both upper and lower intertidal zones, but competition from Semibalanus balanoides excluded it from much of the lower zone. Its fundamental niche therefore extended farther down the shore than its realized niche.
The common misconception is that the realized niche is simply the species' preferred habitat. It is instead the habitat actually occupied after ecological constraints operate. Changes such as removing a competitor or introducing an invasive species can therefore alter a realized niche without changing the species' physiological tolerances.
Exam Technique
For a distinguish question, define both terms and make the contrast explicit: potential range versus actual range. For an explain question, identify a biotic interaction, describe its mechanism and apply a named example. This distinction is also important when evaluating conservation strategies because observed distribution may underestimate suitable habitat.