B4.2 Ecological niches

The three lizard species share the same desert habitat but differ in when and where they hunt — this is temporal and spatial partitioning of resources.

Niche partitioning (also called resource partitioning) occurs when species divide up available resources — in time, space, or food type — so that their niches do not fully overlap. Each lizard uses a different microhabitat at a different time of day, reducing direct competition.

Competitive exclusion would result in one species being driven to local extinction, which is not the case here — all three species coexist. The scenario describes the mechanism that prevents competitive exclusion.

Because the species have partitioned their niches through temporal and spatial differences, the concept illustrated is niche partitioning.

The question asks which ecological concept explains coexistence despite shared habitat, given that each species forages at different times and in different microhabitats.

Competitive exclusion predicts that one species drives another to local extinction when niches are identical. Here all three species coexist, so this concept does not apply.

Fundamental niche expansion is not a standard ecological term describing coexistence through resource division. It is a distractor and can be eliminated.

Ecological succession refers to the sequential change in community composition over time following a disturbance — it has no relevance to the simultaneous coexistence of species described here.

The remaining option, niche partitioning, correctly describes how species subdivide resources in time and space to reduce competition and allow coexistence.

The fundamental niche is the full range of conditions and resources a species could use in the absence of competitors. The realized niche is the portion of the fundamental niche actually occupied, after biotic interactions (especially competition) reduce it.

Chthamalus can physiologically survive in both zones (fundamental niche spans both zones), but Balanus competitively excludes it from the lower zone. Therefore, Chthamalus is only found in the upper zone in practice — this is its realized niche.

Because competition from Balanus restricts Chthamalus to the upper zone, the realized niche is smaller than the fundamental niche. Biotic interactions, not abiotic limitations, are responsible for this reduction.

The correct statement is that the realized niche is smaller than the fundamental niche because competition limits where Chthamalus actually lives.

The question tests understanding of fundamental versus realized niche, specifically which is larger and what causes the difference.

The fundamental niche is defined by abiotic tolerances in the absence of biotic interactions — it is not restricted by competition. This option reverses the definitions and is incorrect.

If the niches were identical, Chthamalus would occupy both zones — but the scenario states it is excluded from the lower zone by Balanus. This option is clearly incorrect.

A realized niche can never be larger than the fundamental niche — the realized niche is always a subset of or equal to the fundamental niche. This option is biologically impossible.

The correct option — the realized niche is smaller than the fundamental niche because competition limits where Chthamalus actually lives — accurately describes how biotic interactions reduce the occupied niche.