B4.2 Ecological niches

No two species can make a living in exactly the same way in the same place — if they tried, one would always lose. This simple but powerful idea sits at the heart of B4.2. An ecological niche describes everything about how a species fits into its ecosystem: not just where it lives, but how it obtains energy, what conditions it tolerates and how it interacts with other organisms. Understanding niches explains why biodiversity is so high, how similar species manage to coexist, and why introducing a new competitor can drive a native species to local extinction.

What an ecological niche is

The ecological niche of a species is its particular role within an ecosystem — the full set of abiotic conditions it can tolerate and the way it interacts with other species and obtains its resources. A helpful distinction is between habitat and niche: the habitat is the place an organism lives (its address), whereas the niche is its way of life (its profession) within that place.

The syllabus separates the fundamental niche — the full range of conditions and resources a species could use in the absence of competitors — from the realised niche, the narrower range it actually occupies once competition and other interactions are taken into account. The realised niche is therefore usually smaller than the fundamental niche.

Modes of nutrition: how organisms obtain energy

A central part of any niche is how an organism obtains carbon and energy. The syllabus requires you to know the main modes of nutrition:

• Autotrophs make their own organic compounds from simple inorganic substances. Photoautotrophs (such as plants and algae) use light energy; chemoautotrophs (some bacteria) use energy from chemical reactions.
• Heterotrophs obtain organic compounds from other organisms. These include consumers that ingest other organisms, saprotrophs (decomposers) that secrete enzymes onto dead matter and absorb the products externally, and detritivores that ingest dead organic matter internally.
• Mixotrophs can use both autotrophic and heterotrophic nutrition; some, like Euglena, switch depending on conditions.

Holozoic feeders (most animals) take food into the body and digest it internally, whereas saprotrophs digest externally — a distinction examiners like to test.

Adaptations that define the niche

An organism’s niche is reflected in its adaptations, particularly those for feeding. The dentition of mammals is a classic example: herbivores tend to have broad, ridged molars for grinding plant material and continuously growing teeth to cope with wear, while carnivores have sharp canines for gripping and carnassial teeth for shearing flesh. The structure of an organism therefore reveals the niche it occupies.

In plants, adaptations to different light environments illustrate the same point. Plants of the shaded forest floor and those of open, sunlit habitats differ in leaf size and arrangement, and obligate shade plants are adapted to photosynthesise efficiently at low light intensities but may be damaged by full sun, whereas sun plants show the reverse. These contrasting tolerances are part of what separates their niches.

Competition, exclusion and coexistence

When two species require the same limiting resource, they compete. The competitive exclusion principle states that two species cannot occupy the same niche in the same place indefinitely: the better competitor will eventually exclude the other from that niche. Gause demonstrated this using two species of Paramecium — grown separately each survived, but grown together one species outcompeted and eliminated the other.

How, then, do so many similar species coexist? Through resource partitioning, where competing species divide a resource and use slightly different parts of it, reducing the overlap of their realised niches. A celebrated example is MacArthur’s study of North American warblers, which feed in different zones of the same conifer trees and so avoid direct competition. Coexistence is possible precisely because their niches differ rather than match exactly.