IB Biology · Theme B: Form and function · SL and HL
B4.2 Ecological niches
A one-page summary of B4.2 Ecological niches, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What are the advantages of specialized modes of nutrition to living organisms?
How are the adaptations of a species related to its niche in an ecosystem?
What B4.2 covers
Niche, nutrition and the tools to get food
- B4.2.1A niche is a job, not an address
- B4.2.2How much oxygen can an organism tolerate?
- B4.2.3Three groups, one autotrophic strategy
- B4.2.4Four steps from meal to tissue
- B4.2.5Euglena runs both modes at once
- B4.2.6Digesting the meal outside the body
- B4.2.7Archaea: one domain, many strategies
- B4.2.8Teeth tell you what a hominid ate
Adaptation, competition and niche breadth
- B4.2.9Mouthparts built to break in
- B4.2.9Plants fight back, chemically and physically
- B4.2.10Finding, catching and killing prey
- B4.2.10Prey evolve just as many ways to survive
- B4.2.11Two ways to reach the canopy
- B4.2.11Living on another plant entirely
- B4.2.11Thriving where light barely reaches
- B4.2.12What a species could do vs what it actually does
- B4.2.13Two identical niches can't coexist
- B4.2.13Sharing a resource without competing
B4.2 Ecological niches: summary
Niche & nutrition modes
- Niche = full role, not just habitat.
- Photosynthetic, holozoic, mixotrophic, saprotrophic, archaeal.
Oxygen tolerance
- Obligate aerobe / facultative anaerobe / obligate anaerobe.
- One O₂ gradient, three tolerance strategies.
Dentition & diet
- Flat molars = herbivore/omnivore; sharp canines = carnivore-leaning.
- Diet deduced from fossil skulls (NOS).
Herbivory & predation
- Herbivore mouthparts vs. plant thorns/toxins.
- Predator claws/venom vs. prey camouflage/aposematism.
Harvesting light
- Canopy trees, lianas, epiphytes, stranglers, shade-floor plants.
- Five strategies, one scarce resource.
Niche breadth & competition
- Fundamental (potential) vs. realized (actual) niche.
- Competitive exclusion or niche partitioning.
Key terms
- Ecological niche
- The full role of a species in its ecosystem, including every biotic and abiotic interaction that shapes it.
- Obligate anaerobe
- An organism that survives only without oxygen, for which O₂ is often directly toxic.
- Holozoic nutrition
- Heterotrophic nutrition via ingestion, digestion, absorption and assimilation of food.
- Mixotrophic nutrition
- Nutrition combining autotrophic and heterotrophic modes within a single organism.
- Saprotrophic nutrition
- Extracellular digestion of dead organic matter, followed by absorption of the products.
- Fundamental niche
- A species' full potential niche, limited only by its own adaptations and abiotic tolerances.
- Realized niche
- The niche a species actually occupies once competition and other biotic interactions are factored in.
- Competitive exclusion
- The principle that two species with fully overlapping niches cannot coexist indefinitely.
- Niche partitioning
- Division of a shared resource along some dimension, allowing competing species to coexist.
Sample exam questions
Three of the 47 multiple-choice questions for B4.2. Try each one before opening the answer.
Question 1. An ecological niche is best defined as:
- The physical location where a species lives (its address)
- The role of a species within its ecosystem — including all the biotic and abiotic conditions it needs to survive, its habitat, its trophic position, and its interactions with other species
- The set of species that share the same habitat
- The total number of individuals of a species in a given area
Show the answer
Answer: B. An ecological niche is the full set of conditions and resources a species requires, plus the role it plays in the ecosystem — including its habitat, food sources, activity patterns, temperature/pH tolerance, and interactions (predation, competition, mutualism). It is the species' 'profession', not just its 'address'.
Question 2. The fundamental niche differs from the realized niche in that:
- The fundamental niche is the full range of conditions a species could theoretically occupy in the absence of competition/predation; the realized niche is the actual range it occupies when biotic interactions constrain it
- The fundamental niche is always smaller than the realized niche
- There is no difference — the terms are synonyms
- The fundamental niche is the geographical range; the realized niche is the trophic position
Show the answer
Answer: A. Fundamental niche = potential niche, defined by abiotic tolerances (temperature, pH, salinity) and resource requirements. Realized niche = actual niche, reduced by competition, predation, disease, and other biotic interactions. For example, a barnacle species may be physiologically capable of living across the entire intertidal zone (fundamental) but is competitively excluded from the lower zone (realized).
Question 3. Why is a species' realized niche always a subset of (or, at most, equal to) its fundamental niche, never larger?
- Because the fundamental niche is always smaller than what a species actually needs
- Because the realized niche adds biotic interactions such as competition and predation, which can only restrict, not expand, the theoretical range set by abiotic tolerances
- Because realized niches only apply to plants, not animals
- Because biotic interactions always expand a species' range beyond its physiological tolerances
Show the answer
Answer: B. The fundamental niche defines the maximum potential range based on abiotic tolerances and resource needs alone; adding real-world biotic interactions (competition, predation, parasitism) can only exclude the species from parts of that range, never expand it beyond its physiological limits, so the realized niche is a subset of (or equal to) the fundamental niche.
Linking questions
Questions that connect B4.2 to other parts of the course, the kind that come up in Paper 2.
- Resource partitioning reduces interspecific competition. How does this concept relate to the evolution of adaptations (B4.1) — specifically, how can competition act as a selective pressure that drives divergence in traits between species? (see B4.1)
- Population size is influenced by energy availability in ecosystems. Explain how energy transfer through food chains (C4.2) limits the carrying capacity for top predators. (see C4.2)
- Keystone species have disproportionate effects on their communities. Using a named example, explain how the removal of a keystone species can trigger a trophic cascade that alters the entire ecosystem structure. (see C4.2)
- Climate change (D4.3) is shifting the geographic ranges of many species. Using the niche concept, explain why some species can track their fundamental niche by shifting their range, while others cannot and face extinction. (see D4.3)
Practise B4.2
Study notes, every question and full markschemes for B4.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.