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

This guide teaches sub-topic B4.2, Ecological niches, from the IB Biology guide, first assessment 2025. It covers statements B4.2.1 to B4.2.13. All of it is for SL and HL, and the guide lists no additional higher level content here. For condensed recall use the revision notes; to test yourself use the practice questions. Course hub: IB DP Biology. Printable list: checklist. Not sure where your gaps are? Try the free 10-minute diagnostics. The wider theme is in the Theme B study guide.

What this unit covers

Statement	What you must be able to do	Level
B4.2.1	Define a niche as a species’ role, including biotic and abiotic interactions and how it gets food	SL and HL
B4.2.2	Distinguish obligate anaerobes, facultative anaerobes and obligate aerobes by oxygen tolerance	SL and HL
B4.2.3	Describe photosynthesis as a mode of nutrition in plants, algae and photosynthetic prokaryotes	SL and HL
B4.2.4	Describe holozoic nutrition in animals	SL and HL
B4.2.5	Describe mixotrophy in protists, obligate and facultative	SL and HL
B4.2.6	Describe saprotrophic nutrition in fungi and bacteria	SL and HL
B4.2.7	Describe the nutritional diversity of archaea	SL and HL
B4.2.8	Infer the diet of hominids from dentition (skull skills)	SL and HL
B4.2.9	Explain herbivore adaptations and plant defences	SL and HL
B4.2.10	Explain predator and prey adaptations	SL and HL
B4.2.11	Explain plant form for harvesting light, using forest examples	SL and HL
B4.2.12	Distinguish fundamental and realized niches	SL and HL
B4.2.13	Explain competitive exclusion and the uniqueness of niches	SL and HL

B4.2.1 The ecological niche

A niche is the role a species plays in its ecosystem. It is not the same as a habitat, which is only where the species lives. The niche includes every biotic interaction (feeding, being eaten, competing, parasitism, mutualism, disease) and every abiotic factor (temperature, light, water, pH, salinity, oxygen) that affects the growth, survival and reproduction of the species. How a species obtains its food is a central part of its niche.

B4.2.2 Oxygen and nutrition: three tolerance groups

Limit your answer to how the organism copes with oxygen gas in its surroundings.

• Obligate anaerobes cannot use oxygen and are harmed or killed by it. They live where oxygen is absent, such as deep mud or the gut.
• Facultative anaerobes can grow with or without oxygen. They usually grow better, with more ATP per glucose, when oxygen is present.
• Obligate aerobes need oxygen for aerobic cell respiration and cannot grow without it.

Worked example 1. Relative growth of three bacterial cultures after 24 hours was measured at three oxygen concentrations (data supplied, arbitrary units).

Culture	0% oxygen	2% oxygen	21% oxygen
P	9	0	0
Q	6	9	10
R	0	4	10

P grows only without oxygen, so it is an obligate anaerobe. Q grows at every concentration but best with oxygen, so it is a facultative anaerobe. R cannot grow at 0%, so it is an obligate aerobe.

B4.2.3 to B4.2.7 Modes of nutrition

Autotrophs make organic compounds from inorganic ones. Heterotrophs take in organic compounds made by other organisms.

Photosynthesis (B4.2.3). Plants, algae and a number of prokaryote groups (cyanobacteria are one) use light energy to build organic compounds from carbon dioxide. You do not need the details of the different forms of prokaryote photosynthesis.

Holozoic nutrition (B4.2.4). All animals are heterotrophs. In holozoic nutrition the food is ingested, digested inside the body (the gut or a food vacuole), absorbed, and assimilated into the animal’s own tissues. Indigestible remains are egested.

Mixotrophic nutrition (B4.2.5). Mixotrophs combine autotrophy and heterotrophy. Euglena is a freshwater protist that photosynthesises in light and can also take in organic compounds. Many mixotrophs in oceanic plankton photosynthesise and also ingest prey. Obligate mixotrophs need both modes. Facultative mixotrophs can survive on one mode alone and switch to the other when conditions change.

Saprotrophic nutrition (B4.2.6). Some fungi and bacteria secrete digestive enzymes onto dead organic matter. They absorb the soluble products. This is external digestion, and the organisms are called decomposers. Hyphae give fungi a large surface area for secretion and absorption.

Archaea (B4.2.7). Archaea form one of three domains, alongside Bacteria and Eukarya. They are metabolically very diverse. Different archaea obtain energy for ATP production from light, from oxidising inorganic chemicals, or from oxidising carbon compounds. You need not name examples.

Worked example 2. A protist is grown for a week under four conditions (net change in cell number per day, data supplied).

Condition	Net change per day
Light, with prey	2.4
Light, no prey	1.1
Dark, with prey	0.9
Dark, no prey	-0.2

It grows in light without prey (photosynthesis) and in the dark with prey (ingestion), so either mode alone supports growth. It is a facultative mixotroph. Growth is greatest with both modes, 2.4 against 1.1 for light alone, an increase of 118% relative to light alone. If it had failed in the dark even with prey, it would not be a mixotroph that can switch.

B4.2.8 Dentition and diet in Hominidae

The family Hominidae includes humans and the great apes, and the fossil relatives of both. Theories linking teeth to diet came from observing living mammals: herbivores have broad cheek teeth for grinding tough plant matter, and meat-eaters have sharp teeth for cutting. These theories let you deduce the diet of extinct organisms. That is a good example of the nature of science point that deductions can be made from theories.

When you examine skulls or models, look at:

• the size and flatness of the molars and premolars (large and flat suggests grinding of plant material),
• the incisors (large incisors suggest biting off or processing food at the front),
• the canines (long and projecting suggests tearing, short suggests mixed feeding),
• the jaw and the crests of bone for muscle attachment (a sagittal crest suggests very strong chewing muscles).

The guide gives Homo sapiens, Homo floresiensis and Paranthropus robustus as examples. Humans have reduced canines and moderate molars, consistent with an omnivorous diet. Paranthropus robustus had very large, flat cheek teeth and a heavy jaw, which fits a diet needing heavy grinding of plant foods.

Worked example 3. Two skulls are measured (data supplied). Skull A: molar occlusal area 215 mm², incisor area 48 mm². Skull B: molar 148 mm², incisor 66 mm².

Ratio A = 215 ÷ 48 = 4.48. Ratio B = 148 ÷ 66 = 2.24.

A has the larger relative molar area, so it points to more grinding of plant material. B has relatively larger front teeth and smaller cheek teeth, which fits a more mixed diet. State the limits: two teeth measurements suggest diet, they do not prove it.

B4.2.9 Herbivores and plants

Herbivore adaptations. Leaf-eating insects such as grasshoppers have chewing mouthparts: strong mandibles that cut and grind leaf tissue. Aphids have piercing mouthparts: a stylet pushed into a phloem sieve tube, where the sugar-rich sap flows in under pressure. Some animals also have metabolic adaptations: enzymes that detoxify plant poisons. Koalas, which eat eucalyptus leaves, are often cited.

Plant resistance. Physical structures include thorns, spines, tough leaf cuticles and hairs. Chemical defences are toxic secondary compounds in seeds and leaves, such as alkaloids and tannins.

Worked example 4. Five plants of a variety with low toxin content and five with high toxin content had the percentage of leaf area eaten recorded (data supplied). Low: 18, 25, 12, 30, 22. High: 4, 6, 3, 8, 5.

Mean low = 107 ÷ 5 = 21.4%. Mean high = 26 ÷ 5 = 5.2%. Reduction = (21.4 - 5.2) ÷ 21.4 × 100 = 75.7%.

High toxin content is associated with much less herbivory, so toxins protect the plant. The ranges do not overlap (3 to 8 against 12 to 30), which strengthens the conclusion. Plants were not shown to differ only in toxin, so say “associated with”.

B4.2.10 Predators and prey

Predators and prey show chemical, physical and behavioural adaptations.

Type	Predator	Prey
Chemical	Venom to subdue prey	Distasteful or poisonous tissues, spray defences
Physical	Claws, sharp teeth, speed, forward-facing eyes for depth perception	Camouflage, spines, shells, warning colours, fast escape
Behavioural	Ambush, hunting in groups	Herding, alarm calls, freezing, fleeing

Warning colouration advertises a chemical defence, and prey that is unpalatable benefits when predators learn to avoid it.

B4.2.11 Plant form and light

Light is often the limiting resource in a forest. The guide asks for these strategies:

• Canopy trees invest in a tall woody trunk to reach the light above others.
• Lianas root in the soil and climb other plants, so they reach the canopy with little investment in a thick trunk.
• Epiphytes grow on branches of trees, high in the light, without roots in soil.
• Strangler epiphytes begin on a branch, send roots down to the soil, and may eventually engulf and kill the host tree.
• Shade-tolerant shrubs and herbs on the forest floor survive on a small fraction of the light. Large thin leaves and efficient use of dim light are typical.

Worked example 5. Light above the canopy is 1800 µmol photons m⁻² s⁻¹ (data supplied). Only 1.5% reaches the forest floor. Light at the floor = 1800 × 0.015 = 27 µmol m⁻² s⁻¹, which is 1/66.7 of the canopy value. Only plants that can photosynthesise at this level will grow there.

B4.2.12 and B4.2.13 Fundamental and realized niches, competition

The fundamental niche is the full potential of a species, set by its adaptations and tolerance limits, with no competitors. The realized niche is the part actually occupied when competing species are present. The realized niche is usually smaller.

Two species cannot have exactly the same niche in the same place. If they compete for the same limiting resource, there are two outcomes (competitive exclusion):

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

So every species has a unique niche. Niches may overlap, but not completely.

Worked example 6. Species X reaches a density of 480 per tank alone. Species Y reaches 520 alone. Together X falls to 150 and Y to 410 (data supplied).

X change = (150 - 480) ÷ 480 × 100 = -68.8%. Y change = (410 - 520) ÷ 520 × 100 = -21.2%.

Neither is eliminated, so both are restricted to a smaller realized niche. X is affected much more, so Y is the stronger competitor in these conditions.

Worked example 7. Magnification. A drawing of an aphid stylet is 54 mm long. The magnification is ×30. Actual length = 54 ÷ 30 = 1.8 mm.

Common errors

• Writing “habitat” when the question asks for niche.
• Saying a facultative anaerobe “cannot use oxygen”. It can, and usually does better with it.
• Saying saprotrophs ingest food. They digest externally and absorb.
• Treating all mixotrophs as the same. Check the data for which modes support growth.
• Calling the realized niche larger than the fundamental niche.
• Claiming competitive exclusion always eliminates a species.

Next steps

Condense this in the revision notes, then try the practice questions. Return to the course hub or the checklist.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Sub-topic B4.2, Ecological niches (Form and function: Ecosystems).