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
Niches and oxygen
- B4.2.1The niche of a species
- B4.2.2Tolerance of oxygen
Modes of nutrition
- B4.2.3Nutrition by photosynthesis
- B4.2.4Holozoic nutrition in animals
- B4.2.5Mixotrophs: both modes
- B4.2.6Saprotrophs: decomposers
- B4.2.7The diversity of archaea
Teeth, herbivores and predators
- B4.2.8 · Nature of scienceTeeth and diet
- B4.2.8 · Application of skillsWhat hominids ate
- B4.2.9Insects that eat plants
- B4.2.9Plants resist herbivores
- B4.2.9Herbivores that detoxify
- B4.2.10Adaptations of predators
- B4.2.10How prey resist predators
Light, competition and niches
- B4.2.11Reaching the canopy
- B4.2.11Living on other plants
- B4.2.11Life in the shade
- B4.2.12Fundamental and realized niches
- B4.2.13Competitive exclusion
- B4.2.13Every niche is unique
B4.2 Ecological niches: summary
Niche
- The role of a species: biotic and abiotic interactions, and how it feeds.
- Habitat is where; niche is how.
Oxygen
- Obligate aerobe: needs O₂. Facultative anaerobe: with or without.
- Obligate anaerobe: only without; O₂ is toxic.
Nutrition
- Photosynthesis (plants, algae, cyanobacteria); holozoic (all animals: ingest, digest, absorb, assimilate).
- Mixotrophs (Euglena; obligate or facultative); saprotrophs; varied archaea.
Teeth
- Flat molars grind plants; pointed canines kill and tear.
- P. robustus: crest, huge molars, plants; H. sapiens and H. floresiensis: omnivores.
Feeding and defence
- Chewing and sucking mouthparts; thorns and toxins; koala detoxifies.
- Predators and prey: chemical, physical and behavioural adaptations.
Light and competition
- Canopy trees, lianas, epiphytes, stranglers, shade plants.
- Fundamental vs realized; exclusion (Gause) or sharing (warblers).
Key terms
- Ecological niche
- the role of a species in an ecosystem.
- Obligate anaerobe
- an organism that can live only without oxygen.
- Facultative anaerobe
- an organism that can live with or without oxygen.
- Autotroph
- an organism that makes its own organic compounds.
- Holozoic nutrition
- ingestion, internal digestion, absorption and assimilation.
- Mixotroph
- an organism that is both autotrophic and heterotrophic.
- Saprotroph
- a decomposer that digests dead matter outside its body.
- Fundamental niche
- the potential niche, set by adaptations and tolerance.
- Realized niche
- the actual niche, when in competition with other species.
- Competitive exclusion
- two species cannot share the same niche for long.
Sample exam questions
Three of the 47 multiple-choice questions for B4.2. Try each one before opening the answer.
Question 1. What is an ecological niche?
- The place where a species lives
- A species’ role in its ecosystem
- All the species sharing one habitat
- The number of individuals in an area
Show the answer
Answer: B. A niche is the role of a species, including its biotic and abiotic interactions and how it obtains food.
Question 2. How does the fundamental niche of a species differ from its realised niche?
- It is the potential niche, based on tolerances.
- It is the actual niche, with competitors present.
- It is always smaller than the realised niche.
- It is the geographical range of the species.
Show the answer
Answer: A. The fundamental niche is the potential niche based on adaptations and tolerance limits; the realised niche is the actual extent with competitors.
Question 3. Why is the realised niche of a species never larger than its fundamental niche?
- The fundamental niche is set by competitors.
- Competitors can only restrict where it lives.
- Realised niches apply only to plants.
- Competition always enlarges the niche.
Show the answer
Answer: B. Competition can restrict a species to part of its potential niche, but cannot extend its tolerances.
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)
- Energy is lost at each step of a food chain. Explain how energy transfer through food chains (C4.2) limits the number of top predators an ecosystem can support. (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 D4.2)
- 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.