Questions: Niche: Fundamental and Realized

Question 1

In a rocky intertidal zone, barnacle species A occupies the upper half of the shore and barnacle species B dominates the lower half. An experiment removes species B entirely. What outcome would the fundamental-realized niche distinction predict?

• A. Species A remains confined to the upper half — it is physiologically limited to those conditions
• B. Species A expands to occupy the full range it is physiologically capable of surviving, including the lower half
• C. A new competitor immediately fills the lower half before species A can expand
• D. Species A's population declines because it depends on species B for resources

Answer: (B)

This is a classic ecological release scenario. Species A's realized niche is restricted to the upper shore by competition from B. Its fundamental niche — the range it can physically survive — extends lower. When the competitor is removed, species A expands toward its fundamental niche. This experiment was actually conducted with Chthamalus and Semibalanus barnacles by Joseph Connell, and it directly demonstrated the gap between fundamental and realized niches.

Question 2

Invasive species often occupy broader habitat ranges in their introduced regions than in their native ranges. Which concept best explains this pattern?

• A. Invasive species evolve rapidly to exploit new resources after introduction
• B. Introduced environments have more diverse resources than native ones
• C. Without their native competitors and predators, invasive species can expand toward their fundamental niche
• D. Invasive species have larger fundamental niches than native species by definition

Answer: (C)

In native ranges, biotic interactions — competition, predation, parasitism — constrain species to their realized niches, which are subsets of their fundamental niches. When introduced to a new environment, these biotic constraints are absent (competitors and predators haven't co-evolved with them). The species can then occupy a broader range of conditions, approaching its fundamental niche. This is called ecological release. The invasive species hasn't changed genetically — the same fundamental niche is now less constrained by competitors.

Question 3

A species' realized niche is generally smaller than its fundamental niche.

• T. True
• F. False

Answer: False

In most cases the realized niche is a subset of the fundamental niche, because biotic interactions (competition, predation) exclude the species from parts of its tolerable range. However, mutualistic interactions can expand a species' realized niche *beyond* its fundamental niche — enabling it to persist in conditions it could not survive alone. For example, a plant that depends on a mycorrhizal fungus for nutrient uptake may colonize nutrient-poor soils it could not otherwise inhabit. The general rule is that competition contracts the realized niche; mutualism can expand it.

Question 4

Invasive species typically occupy a narrower ecological niche in their introduced range than in their native range, because they lack the co-evolutionary history to exploit new resources effectively.

• T. True
• F. False

Answer: False

This is the opposite of the observed pattern. Invasive species typically occupy a *broader* niche in their introduced range because they are freed from the biotic constraints (competitors, predators, parasites) that restricted them in their native range. Without these constraints, they expand toward their fundamental niche. This is called ecological release. The absence of co-evolutionary enemies — not the presence of new opportunities — is the primary mechanism. Climate models that predict invasion risk based on abiotic tolerances are essentially estimating fundamental niches, which is why they sometimes underestimate actual invasion breadth.

Question 5

Why do invasive species often occupy broader habitats in their introduced range, and what does this reveal about the relationship between fundamental and realized niches?

Model answer: In their native range, biotic interactions — competition from ecologically similar species and pressure from predators, parasites, and pathogens — restrict invasive species to a realized niche that is smaller than their fundamental niche. When introduced to a new region, these constraints are absent: co-evolved enemies have not followed them. The species can then expand into the full range of conditions it is physiologically capable of tolerating, approaching its fundamental niche. This reveals that the gap between fundamental and realized niche is primarily determined by biotic interactions, not abiotic conditions, and that the realized niche is a dynamic outcome of the ecological community rather than a fixed property of the species.

This insight has practical importance: predictive models of invasive species spread that use only climate data (abiotic factors) systematically underestimate invasion potential, because they ignore the biotic release effect.