IB DP Biology: Ecological niches: nutrition, feeding adaptations and competition (DP Biology) – Revision Notes

These notes condense sub-topic B4.2, Ecological niches, from the IB Biology guide, first assessment 2025 (statements B4.2.1 to B4.2.13, SL and HL, with no HL-only content). For full explanations and worked examples read the study guide. Test yourself with the practice questions. Course hub: IB DP Biology. Printable list: checklist. Unsure what to revise first? Take the free 10-minute diagnostics.

Key definitions

Term	Meaning
Ecological niche	The role of a species in its ecosystem: its biotic and abiotic interactions, including how it gets food
Habitat	Where a species lives (part of the niche, not the whole)
Fundamental niche	Potential niche set by adaptations and tolerance limits, with no competitor
Realized niche	The niche actually occupied when competing species are present
Competitive exclusion	One species eliminated, or both restricted to part of their fundamental niche
Autotroph / heterotroph	Makes its own organic compounds / takes in organic compounds

Oxygen tolerance (B4.2.2)

Group	With oxygen	Without oxygen
Obligate anaerobe	Harmed or killed	Grows
Facultative anaerobe	Grows, usually better	Grows
Obligate aerobe	Grows	Cannot grow

Modes of nutrition (B4.2.3 to B4.2.7)

Mode	Who	Key feature
Photosynthesis	Plants, algae, photosynthetic prokaryotes	Light energy builds organic compounds
Holozoic	All animals	Ingest, digest internally, absorb, assimilate
Mixotrophic	Some protists (Euglena, oceanic plankton)	Autotrophic and heterotrophic; obligate need both, facultative can switch
Saprotrophic	Some fungi and bacteria	Secrete enzymes onto dead matter, absorb products; decomposers
Archaea	One of three domains	Energy for ATP from light, inorganic oxidation or carbon-compound oxidation

Method for a mixotroph data question:

1. Find which conditions give growth.
2. If light alone and prey alone both work, it is facultative.
3. If growth needs both, it is obligate.
4. Quote values from the table.

Dentition (B4.2.8)

• Large, flat molars and premolars suggest grinding of plant foods.
• Reduced canines and moderate molars are consistent with an omnivorous diet, as in humans.
• A sagittal crest points to powerful chewing muscles.
• Theories built on living mammals let us deduce the diet of extinct hominids. Quote this for the nature of science point.
• Examples in the guide: Homo sapiens, Homo floresiensis, Paranthropus robustus.

Calculation reminder: ratio = molar area ÷ incisor area. A skull with molar 180 mm² and incisor 45 mm² gives 180 ÷ 45 = 4.0.

Herbivory (B4.2.9)

Herbivore	Plant
Chewing mouthparts (mandibles, as in a leaf-eating insect)	Thorns, spines, tough or hairy leaves
Piercing mouthparts (stylet into phloem)	Toxic secondary compounds in leaves and seeds
Detoxifying enzymes (metabolic adaptation)

Percentage damage example: 36 leaves damaged out of 240 gives 36 ÷ 240 × 100 = 15%.

Predator and prey (B4.2.10)

• Chemical: venom in predators; poison, bad taste or spray in prey.
• Physical: claws, teeth, speed, depth vision; camouflage, spines, shells, warning colours.
• Behavioural: ambush, group hunting; herding, alarm calls, fleeing, freezing.

Plant form and light (B4.2.11)

Strategy	How it reaches light
Canopy tree	Tall woody trunk
Liana	Roots in soil, climbs on other plants
Epiphyte	Grows on branches of trees, high up
Strangler epiphyte	Starts on a branch, roots reach soil, may kill the host
Shade-tolerant shrub or herb	Survives on very little light on the forest floor

Light calculation: incident light 2200 units with 2% reaching the floor gives 2200 × 0.02 = 44 units.

Niches and competition (B4.2.12 and B4.2.13)

Fundamental niche is larger than or equal to the realized niche. Example: a species tolerates 10 to 34 °C alone, but with a competitor is found only at 16 to 28 °C. Fundamental width 24 °C, realized width 12 °C, so the realized niche is 50% of the fundamental niche.

Outcomes of competition between two species with the same limiting resource:

1. One species eliminated.
2. Both survive, each limited to part of its fundamental niche.

Because no two species can occupy exactly the same niche in the same place, each species has a unique niche.

Magnification reminder

Magnification = image size ÷ actual size. A drawing of 36 mm for a mouthpart of actual length 0.9 mm gives 36 ÷ 0.9 = ×40.

Must-know distinctions

• Niche or habitat. Habitat is an address. Niche is the job, including diet, predators, competitors and the tolerances for temperature, light, water and oxygen.
• Saprotrophic or holozoic. A saprotroph digests outside the body and absorbs. A holozoic animal ingests first and digests inside.
• Heterotrophic or autotrophic. Animals are always heterotrophic. A mixotroph is both.
• Physical or chemical plant defence. Thorns, spines and tough leaves are physical structures. Toxic secondary compounds in leaves and seeds are chemical.
• Chewing or piercing mouthparts. Chewing mouthparts remove pieces of leaf. Piercing mouthparts reach the sap inside a vessel.
• Fundamental or realized. Fundamental is what the species could use. Realized is what it does use because of competitors.
• Elimination or restriction. In elimination one species disappears. In restriction both persist in part of their fundamental niche.

Method boxes

Explaining an adaptation. Name the feature, say what it does, then link it to survival or feeding. Example: a stylet reaches phloem, so the aphid obtains sugar-rich sap without chewing tough tissue.

Reading a competition graph.

1. Find the population of each species grown alone. This shows the level reached in the absence of the competitor.
2. Compare with the populations grown together.
3. If one line reaches zero, that species was eliminated.
4. If both fall but stay above zero, both were restricted.
5. Calculate percentage change for each, and say which species is affected more.

Inferring diet from a skull.

1. Compare molar size and shape with the incisors and canines.
2. Look for muscle attachment features on the skull.
3. State the inference using words such as “suggests” and “consistent with”.
4. Say the deduction rests on theories developed from living mammals.

Comparing the oxygen groups from a table. Look at the 0% column first. If growth is zero, the culture is an obligate aerobe. If growth is highest at 0% and falls to nothing in oxygen, it is an obligate anaerobe. If growth appears everywhere, it is facultative.

Prey and predator examples to quote

• Warning colouration paired with poison: a visual signal advertising a chemical defence.
• Camouflage: a physical colour match to the background.
• Group living: a behavioural defence that raises the chance that a predator is seen early.
• Ambush: a behavioural predator strategy that reduces the energy spent in chasing.
• Venom: a chemical adaptation that helps to kill or immobilise prey.

You may use any suitable named organism, as long as you state the type of adaptation correctly.

Quick self-test

1. Distinguish a niche from a habitat.
2. State the oxygen requirement of an obligate aerobe.
3. A bacterium grows with and without oxygen, faster with it. Name its group.
4. Outline why a decomposing fungus is not described as holozoic.
5. Give two ways archaea can obtain energy for ATP production.
6. A mixotroph grows only when it has both light and prey. Is it obligate or facultative?
7. A species falls from 80 individuals to 62 when a competitor arrives. Calculate the percentage change.
8. A fundamental niche of 10 to 34 °C is reduced to 16 to 28 °C. Calculate the realized niche as a percentage of the fundamental.
9. Outline the difference between a liana and a strangler epiphyte.
10. Light above a canopy is 2200 units and 2% reaches the floor. Calculate the light at the floor.

Answers

1. A niche is the whole role of a species (feeding, interactions, abiotic tolerances); a habitat is only where it lives.
2. It needs oxygen for aerobic respiration and cannot grow without it.
3. Facultative anaerobe.
4. It digests food outside its body by secreting enzymes and absorbs the products; holozoic nutrition involves ingestion and internal digestion.
5. Any two of: light, oxidation of inorganic chemicals, oxidation of carbon compounds.
6. Obligate mixotroph, because it cannot survive on one mode alone.
7. (62 - 80) ÷ 80 × 100 = -22.5%, a fall of 22.5%.
8. Widths are 24 and 12, so 12 ÷ 24 × 100 = 50%.
9. A liana roots in soil and climbs on other plants for support. A strangler epiphyte starts on a branch and sends roots down, which may engulf and kill the host.
10. 2200 × 0.02 = 44 units.

Where marks are usually lost

• Describing the niche as only a place.
• Mixing up facultative (can use either) and obligate (needs one).
• Writing that facultative anaerobes cannot use oxygen.
• Calling saprotrophic digestion internal.
• Naming a defence without saying whether it is chemical, physical or behavioural when asked.
• Describing dentition as proof of diet instead of evidence for an inference.
• Giving only one outcome of competition, or forgetting that restriction to part of the fundamental niche is an outcome.
• Quoting percentage changes without the sign or direction.
• Using data in a mixotroph question without quoting numbers.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Sub-topic B4.2, Ecological niches.