Ecological Niches and Competition

Core Concepts

Habitat vs. Niche

A habitat is the physical location where an organism lives — a pond, a forest understory, a desert. A niche is the organism’s ecological role: what it eats, where and when it feeds, what conditions it tolerates, what eats it, and how it interacts with other species. If the habitat is the address, the niche is the job description.

Fundamental vs. Realized Niche

The fundamental niche is the complete set of environmental conditions and resources a species could theoretically occupy in the absence of competitors, predators, and other biotic constraints. The realized niche is the portion of the fundamental niche a species actually occupies, constrained by interactions with other species. Competition typically causes the realized niche to be smaller than the fundamental niche. A classic experiment with barnacles (Chthamalus and Balanus) in the intertidal zone showed that the upper limit of Balanus is set by desiccation tolerance (abiotic), while the lower limit of Chthamalus is set by competition with Balanus (biotic). Removing Balanus allowed Chthamalus to expand into the lower intertidal — its fundamental niche was larger than its realized niche.

Types of Competition

Intraspecific competition occurs between individuals of the same species. It is a major density-dependent factor regulating population size. Intraspecific competition drives natural selection for traits that reduce competition (e.g., dispersal, territoriality).

Interspecific competition occurs between individuals of different species that share a limiting resource. It can reduce the population size of both competitors and, over evolutionary time, drive niche differentiation.

Competitive Exclusion and Resource Partitioning

The competitive exclusion principle (Gause’s principle) states that two species competing for exactly the same limiting resource cannot coexist indefinitely — the species with even a slight competitive advantage will eventually exclude the other. This was demonstrated in laboratory experiments with Paramecium species: P. caudatum and P. aurelia grown separately thrived; grown together, P. aurelia outcompeted P. caudatum to extinction.

In nature, similar species often coexist by resource partitioning — dividing the limiting resource by using different portions of it. Five species of warblers in a spruce forest coexist by feeding in different parts of the tree canopy, reducing competition. This is an example of niche differentiation.

Character displacement occurs when species diverge in morphology or resource use in areas where they coexist (sympatry) compared to areas where each lives alone (allopatry). Darwin’s finches show character displacement in beak size: where different species coexist, their beak sizes diverge, reducing dietary overlap.

Limiting Resources and Coexistence

A limiting resource is any resource whose scarcity constrains population growth. Competition is most intense for limiting resources. Coexistence is possible when: species use different resources (resource partitioning), the environment is heterogeneous (providing different microhabitats), disturbance prevents competitive exclusion, or the competitively inferior species is a superior colonizer (competition-colonization tradeoff).

ELI-10

A niche is not just where an animal lives — it is what it does for a living. A habitat is the street address. A niche is the job description — what it eats, when it works (day or night), where it shops (what part of the tree it feeds in), and who its coworkers and competitors are.

Every species has a “dream job” niche (fundamental niche) — all the conditions it could handle if no one else was competing. But in the real world, competitors push you into a smaller “actual job” niche (realized niche). You could work anywhere in the building, but someone else already has the corner office, so you take what is left.

The competitive exclusion principle is brutally simple: two species cannot have exactly the same job in the same place at the same time. One will eventually push the other out — not necessarily by fighting, but by being slightly better at using the shared resource. If two species do coexist, it is because they have divided the resources somehow — different food, different feeding times, different places. Five species of warblers in the same tree avoid competing by feeding in different parts of the tree — like five workers sharing the same office but working on different floors.

ELI Example

Think of a niche as a restaurant menu. The fundamental niche is the complete menu — everything the restaurant could theoretically serve. The realized niche is what actually gets ordered — constrained by what ingredients are available, what the competition down the street is serving, and what the customers want. Two restaurants serving exactly the same menu in the same neighborhood cannot both survive — one will get more customers and the other will close (competitive exclusion). If both survive, it is because one serves breakfast and the other serves dinner (resource partitioning), or one is cheap and fast while the other is fancy (niche differentiation).

Do Not Confuse

• Habitat vs. Niche: Habitat = physical location. Niche = ecological role (resource use, interactions, conditions tolerated).

• Fundamental vs. Realized Niche: Fundamental = theoretical, without competitors. Realized = actual, with competitors present.

• Intraspecific vs. Interspecific Competition: Intraspecific = same species. Interspecific = different species. Both reduce growth and reproduction, but intraspecific is a density-dependent regulator; interspecific can drive niche differentiation or exclusion.

High-Yield Memory Anchors

• Niche = ecological role (job description). Habitat = physical location (address).

• Fundamental niche = potential. Realized niche = actual (constrained by competition).

• Competitive exclusion: two species cannot share exactly the same limiting resource. One wins.

• Coexistence requires resource partitioning or environmental heterogeneity.

• Character displacement: species diverge where they coexist.

Quick Check

Q1: The realized niche of a species is typically smaller than its fundamental niche because of:

A) Physiological tolerance limits

B) Abiotic factors such as temperature

C) Biotic interactions such as competition and predation

D) The fundamental niche being measured incorrectly

Q2: Two species of seed-eating birds coexist in the same grassland. Describe three ways they could partition resources to avoid competitive exclusion.

Q3: In an experiment, a scientist removes a dominant competitor from experimental plots. Predict what happens to the realized niche of a subordinate competitor species, and explain why.

Quick Check Answers

A1: C. Biotic interactions such as competition and predation. The fundamental niche is defined by abiotic tolerances alone. The realized niche is the fundamental niche constrained by biotic interactions. Competition, predation, and parasitism typically shrink the realized niche.

A2: Spatial partitioning: One species feeds on the ground, the other in shrubs. (2) Dietary partitioning: One species eats large seeds, the other eats small seeds (different beak sizes). (3) Temporal partitioning: One species feeds at dawn and dusk, the other feeds during midday. Any of these would reduce overlap in resource use, allowing coexistence.

A3: The subordinate competitor’s realized niche would expand — it would occupy a broader range of habitats, use a wider range of resources, or achieve higher densities. This is because the dominant competitor was excluding it from portions of its fundamental niche. Removing the dominant competitor releases the subordinate from competitive pressure, allowing it to realize more of its fundamental niche. This is exactly the barnacle experiment: removing Balanus allowed Chthamalus to expand into the lower intertidal zone.

Chapter Summary

A niche is an organism’s ecological role — its resource use, habitat requirements, and interactions. The fundamental niche is constrained by abiotic factors; the realized niche is further constrained by biotic interactions. Competitive exclusion drives niche differentiation and resource partitioning, allowing species coexistence. Competition is a major force structuring communities and shaping evolutionary divergence.

Common Mistakes

• “Two species with similar niches cannot coexist anywhere.” They can coexist if resources are partitioned, the environment is heterogeneous, disturbance prevents equilibrium, or if they are not limited by the same resource. Competitive exclusion is a principle, not an absolute law that applies in all situations.

• “Competition always involves direct fighting.” Competition can be exploitative (indirect — one species uses a resource, leaving less for another) or interference (direct — one species actively prevents another from accessing a resource). Both are competition.