Every organism has a role within its environment, a specific function that defines its way of life. This role, known as an ecological niche, is the sum of all its relationships with the living and non-living elements it needs to survive. An ecosystem can be compared to a large company, where each species is an employee with a particular job description. This “job” includes how a species’ population grows with available resources and how it affects its environment by consuming resources or becoming food for other organisms. This web of interactions means each species has a unique niche that guides its existence.
The Niche and The Habitat
A common point of confusion is the difference between a niche and a habitat. A simple way to distinguish them is with the analogy: a habitat is an organism’s address, while a niche is its profession. The habitat is the physical place where a species lives, providing the conditions needed to find food and reproduce. A niche describes the organism’s functional role within that address, including its diet, activity patterns, and interactions.
For example, a squirrel’s habitat might be an oak forest. Its niche includes eating acorns, dispersing seeds by burying them, and serving as a food source for predators like hawks. While multiple species can share the same habitat, each has its own distinct niche.
Fundamental vs. Realized Niches
The concept of a niche is divided into two types: fundamental and realized. A species’ fundamental niche represents the full range of environmental conditions and resources it could use without competition or predation. It is the ideal scenario where a species lives at its full potential. In reality, interactions with other organisms force species into a more restricted lifestyle. The actual space and resources a species uses in the presence of these pressures is its realized niche, which is a smaller portion of the fundamental niche.
A classic illustration involves two barnacle species on the rocky coasts of Scotland. Ecologist Joseph Connell observed that Chthamalus lived in the upper tidal zone, while Semibalanus occupied the lower zone. When he removed Semibalanus, he found that Chthamalus could thrive in the lower zone, showing its fundamental niche included both areas. The more competitive Semibalanus naturally crowded it out, restricting Chthamalus to its smaller, realized niche in the upper zone.
Niche Overlap and Species Interaction
When two or more species require the same limited resources, their niches overlap, leading to competition. This pressure shrinks a fundamental niche into a realized one and can have significant consequences.
Competitive Exclusion
One possible outcome of severe niche overlap is competitive exclusion. This principle states that two species cannot occupy the exact same niche in the same habitat indefinitely. One species will be more efficient at acquiring resources, eventually outcompeting the other and leading to its local elimination. A famous laboratory example involved two species of Paramecium, where one always outcompeted the other for a limited food supply when grown together.
Resource Partitioning
A more common result of niche overlap is resource partitioning. This occurs when competing species evolve to use a shared resource in different ways, minimizing direct conflict. For example, several species of warblers that live in the same spruce trees avoid competition by foraging for insects in different parts of the tree. Some feed in the upper canopy, others in the middle branches, and some near the trunk, allowing them to coexist.
Specialist and Generalist Species
The breadth of a species’ niche classifies it as either a specialist or a generalist. Specialists are organisms with very narrow niches and specific needs, such as a limited diet or a particular habitat. The koala is a prime example, as its diet of eucalyptus leaves makes it very efficient in a stable environment but dependent on those trees.
Generalists, by contrast, have broad niches. They are adaptable organisms that can thrive in a wide variety of environmental conditions and use many different resources. Raccoons are a classic generalist, able to eat everything from fruits to garbage and live in habitats ranging from forests to cities.
This difference in niche breadth affects a species’ ability to cope with environmental change. Specialists are vulnerable to disturbances; if their specific food source or habitat is altered, they may struggle to survive. Generalists are much more resilient, as their flexibility allows them to switch resources or habitats when faced with new challenges.