Why Species Occupy Only Part of Their Possible Niche
Every species has the potential to live in a wide range of environmental conditions, but in reality, it only occupies a portion of that range. This distinction is captured in two essential ecological concepts: the fundamental niche and the realized niche. Understanding what determines each niche is crucial for IB Biology students learning about community structure, competition, and species interactions.
The fundamental niche represents the full range of environmental conditions and resources a species could use if no competitors, predators, or limiting factors were present. It reflects the biological and physiological capabilities of the species—temperature tolerance, dietary range, habitat type, and reproductive conditions. This niche is determined primarily by the species’ genetic traits and evolutionary history.
In contrast, the realized niche represents the narrower set of conditions a species actually occupies in nature. This niche is influenced by interactions with other organisms and environmental pressures that limit access to parts of the fundamental niche. As a result, the realized niche is almost always smaller than the fundamental niche.
One of the most influential factors in shaping the realized niche is competition. When two species compete for the same resource, one may outcompete the other, forcing the weaker competitor into a narrower niche. This process is described by the competitive exclusion principle. Resource partitioning may occur as species adjust to avoid conflict, further shaping their realized niches.
Predation and herbivory can also restrict a species’ realized niche. For example, a prey species might avoid otherwise suitable habitats because predators are present there. Herbivores may not feed on certain plants if predators frequently patrol those areas, even if the plants fall within their physiological capabilities.
Abiotic factors, such as soil pH, light availability, temperature, and water supply, also shape the fundamental niche by defining the environmental limits a species can tolerate. However, microclimates, seasonal changes, or local disturbances may constrain the realized niche even if the species could theoretically survive more broadly.
Additionally, mutualistic relationships influence niche occupation. For instance, plants that rely on specific pollinators may not be able to spread into areas where those pollinators are absent, reducing their realized niche despite suitable abiotic conditions.
