IB Biology SL B4.2 Ecological niches Questions

Practise IB Biology SL B4.2 by comparing niches, habitats, resource use, nutrition and environmental requirements with evidence.

71 questions in this syllabus scope, with question parts counted together.

Syllabus
First assessment 2025
Course
Biology SL
Level
SL

Exam analysis

SL · 2010–2025
Papers with this topic55 / 141in the analysed sample
Latest appearanceNovember 2025
Tagged question parts84
Paper types5

Most tested objectives

  1. B4.2.12—Fundamental vs. realized niche13 papers
  2. B4.2.7—Diversity in archaea12 papers
  3. B4.2.13—Competitive exclusion8 papers
  4. B4.2.1—Ecological niche7 papers
  5. B4.2.4—Holozoic nutrition6 papers

Common question formats

  • Comparison
  • Structured response
  • Process explanation
  • Data analysis
  • Definition or recall

Recent exam appearances

November 2025Paper 2 SL · TZ1Q7(c)[ 4 ]
November 2025Paper 1A SL · TZ1Q25[ 1 ]
May 2025Paper 2 SL · TZ2Q1(a)(ii), 1(a)(iii)[ 2 ]

141 papers analysed · Tagging incomplete

Exam points

  • Define an ecological niche as a species’ role, then compare species by habitat, resources and biotic or abiotic interactions.
  • Identify oxygen availability as a survival requirement for obligate aerobes in an environmental scenario.
  • Distinguish autotrophic from heterotrophic nutrition and explain how photosynthetic prokaryotes released oxygen into the early atmosphere.
  • Identify holozoic nutrition from ingestion and internal digestion, including evidence that distinguishes it from external saprotrophic digestion.
  • Classify Euglena or other named protists as mixotrophs when evidence shows both photosynthesis and heterotrophic feeding.
  • Identify fungi or bacteria as saprotrophs in food-web evidence, and explain external digestion, decomposition and nutrient recycling.
  • Identify methanogens as methane-producing Archaea and relate them to anaerobic conditions or habitats; identify halophile habitats from salt evidence.
  • Compare tooth-count, jaw and molar evidence across named Hominidae fossils, then relate dentition trends to dietary change.
  • Identify herbivore feeding adaptations such as chewing mouthparts, grinding teeth, long guts or cellulase-producing microbes.
  • Explain how prey camouflage, chemical mimicry or shell armour—and predator or prey morphology—change detection, attack or predation success.
  • Interpret chlorophyll and leaf-nitrogen data across light conditions, and explain how greater leaf chlorophyll supports light harvesting in low light.
  • Distinguish fundamental from realized niches and use interaction evidence to explain why a species occupies a narrower range.
  • Explain competitive exclusion and interpret Paramecium or other niche-overlap data to identify which species is excluded or restricted.

Covered syllabus points

B4.2.1—Ecological nicheB4.2.2—Oxygen toleranceB4.2.3—Photosynthesis as nutrition modeB4.2.4—Holozoic nutritionB4.2.5—Mixotrophic nutritionB4.2.6—Saprotrophic nutritionB4.2.7—Diversity in archaeaB4.2.8—Dentition and diet in Family HominidaeB4.2.9—Herbivore and plant adaptationsB4.2.10—Predator and prey adaptationsB4.2.11—Plant form for light harvestingB4.2.12—Fundamental vs. realized nicheB4.2.13—Competitive exclusion

Exam points

  • Define a niche as a species’ role, including habitat, feeding and interactions within its community.
  • Compare species niches by food, predators, activity timing, nest sites or environmental conditions.

Question 1

[Maximum number: 3]

Explain what is meant by the niche concept.

Exam points

  • Identify oxygen as a required survival condition for seaweed described as an obligate aerobe.

Question 1

[Maximum number: 1]

A student set up a 20 -metre transect line along a rocky shore and measured the abundance of seven species of seaweeds (A-G) at 21 different sampling stations at fixed distances. The kite diagram represents the species abundance data across the 20 metres.

Figure for Question 1 — IB Biology SL

Seaweeds are obligate aerobes. Describe an environmental condition required for seaweed survival.

Exam points

  • Distinguish autotrophic from heterotrophic nutrition by carbon source and how each obtains organic molecules.
  • Explain how photosynthetic prokaryotes released oxygen and helped form an oxygen-rich atmosphere.

Question 1

[Maximum number: 2]

Outline the contribution of prokaryotes to the origin of an oxygen-rich atmosphere.

Exam points

  • Identify holozoic nutrition as heterotrophic feeding with internal digestion.
  • Compare detritivores and saprotrophs by food source and digestion site.
  • Classify organisms from food-web or diet evidence as heterotrophs, consumers or detritivores.

Question 1

[Maximum number: 1]

Outline the method of nutrition carried out by P. caudatum.

Exam points

  • Identify mixotrophs from evidence of both photosynthesis and heterotrophic feeding.
  • Compare Euglena and Chlorella by autotrophic versus heterotrophic nutrition.

Question 1

[Maximum number: 1]

The image shows Prorocentrum, a coastal dinoflagellate that obtains energy from sunlight and from other organisms.

Figure for Question 1 — IB Biology SL

What best describes the mode of nutrition of Prorocentrum?

A

Mixotrophic

B

Autotrophic

C

Holozoic

D

Heterotrophic

Exam points

  • Identify saprotrophs by enzyme secretion onto dead matter and absorption of digestion products.
  • Compare saprotrophs and detritivores by external digestion versus ingestion and internal digestion.
  • Interpret fungal nutrition charts and food webs to identify saprotrophic feeding roles.

Question 1

[Maximum number: 1]

The pie chart shows the modes of nutrition of fungi in Huahu Lake wetland in China.

Figure for Question 1 — IB Biology SL

What is the most common mode of nutrition of fungi in this wetland?

A

Heterotrophic fungi that feed on living organisms by ingestion

B

Autotrophic fungi that obtain organic nutrients from detritus by internal digestion

C

Fungi that have either an autotrophic or heterotrophic method of nutrition

D

Heterotrophic fungi obtaining nutrients from dead organisms by external digestion

Exam points

  • Explain biogas production from biomass by bacteria and methanogenic Archaea in anaerobic fermenters.
  • Identify methanogenic Archaea and the anaerobic habitats or conditions they require.
  • Outline how methanogens increase methane in biogas and the atmosphere.

Question 1

[Maximum number: 4]

Many countries are considering the use of biofuels to reduce their dependence on fossil fuels. Discuss the production of biogas.

Exam points

  • Compare Hominidae skulls by jaw projection, tooth size and brow ridge shape.
  • Relate fossil tooth-structure and canine changes to a shift from soft plant foods towards omnivory.

Question 1

[Maximum number: 3]

Outline the trends illustrated by the fossils of Ardipithecus and Australopithecus.

Exam points

  • Identify herbivore feeding adaptations such as mouthparts, teeth, digestive systems and detoxifying gut microbes.

Question 1

[Maximum number: 4]

Convergent evolution increases similarities, despite differences in origins.

Suggest examples of adaptations in herbivores for feeding on plants.

Exam points

  • Explain how camouflage, mimicry, wing traits or palatability affect prey detection and survival.
  • Relate prey size, shell thickness, claws or speed to predator choice and feeding success.

Question 1

[Maximum number: 2]

The Asian shore crab (Hemigrapsus sanguineus) is an invasive invertebrate species found along the French coast. It preys on another invasive species, the amphipod Ptilohyale littoralis, and also on the blue mussel (Mytilus edulis), a native species. Scientists studied the relationships between the species.

Figure for Question 1 — IB Biology SL

In order to study which prey is preferred by the crabs, each crab was put into a tank for 12 hours containing only one species of prey at a time (no choice) or containing both species at the same time (choice). 15 crabs of each sex were tested. The graphs show the mean number of prey consumed by male and female crabs.

Figure for Question 1 — IB Biology SL

Question (a)

(a)

Suggest a reason for the prey preference of the male compared to the female.

[ 1 ]

Question (b)

(b)

Suggest a characteristic of the prey that could affect the feeding choice for either sex of crab.

[ 1 ]

Exam points

  • Use data to compare chlorophyll and leaf-nitrogen responses to high and low light intensity.
  • Explain how more chlorophyll per leaf area in low light helps plants absorb a similar amount of light.

Question 1

[Maximum number: 6]

The nitrogen content of a leaf is mainly due to the proteins contained in the chloroplasts. These proteins are either in the thylakoids or in the stroma, where most enzymes are found. The quantity of nitrogen from the thylakoids is directly proportional to the amount of chlorophyll; a ratio of approximately 50 mmol nitrogen : 1 mmol chlorophyll would represent 100 % of the leaf nitrogen content. Scientists hypothesized that the higher leaf percentage nitrogen content resulting from a decrease in light intensity is due mainly to an increase in chlorophyll in many plant species, three of which are represented in the following graph.

Figure for Question 1 — IB Biology SL

Question (a)

(a)

State the difference in chlorophyll content for Phaseolus between high and low light intensity, giving the units.

[ 1 ]

Question (b)

(b)

State the percentage value of total leaf nitrogen in chlorophyll for Spinacia at low light intensity.

[ 1 ]

Question (c)

(c)

Suggest one advantage for plants to increase their leaf chlorophyll content per surface area when light intensity is lower.

[ 1 ]

Question (d)

(d)

Evaluate the hypothesis that lower light intensity increases thylakoid nitrogen.

[ 3 ]

Exam points

  • Distinguish fundamental niche as potential conditions from realized niche as the conditions actually occupied.
  • Explain how competition, predation or environmental limits can restrict a species’ realized niche.

Question 1

[Maximum number: 1]

Which statements necessarily describe competitive exclusion between two species?

I. They compete for the same resources so cannot coexist in the same niche.
II. Competing species are regulated by predators in the ecosystem.
III. Both species are restricted to a part of their fundamental niche.

A

I and II only

B

I only

C

III only

D

I, II and III

Exam points

  • Explain how niche overlap and competition for a shared limiting resource can result in competitive exclusion.
  • Describe Gause’s Paramecium experiments as evidence that P. aurelia can outcompete P. caudatum in mixed culture.

Question 1

[Maximum number: 1]

Paramecium aurelia and Paramecium caudatum are two species of paramecium that grow well individually. Scientists grew these two species of paramecium together, and the result is shown in the graph.

Figure for Question 1 — IB Biology SL

What could be deduced from this data?

A

Predation results in P. caudatum consuming P. aurelia.

B

Allelopathy results in P. caudatum outcompeting P. aurelia.

C

Interspecific competition results in P. aurelia outcompeting P. caudatum.

D

Intraspecific competition results in P. aurelia eliminating P. caudatum.