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

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover sub-topic B4.2, Ecological niches (statements B4.2.1 to B4.2.13), from the IB Biology guide, first assessment 2025. All of it is SL and HL; there is no HL-only content. Teaching is in the study guide and condensed recall in the revision notes. Course hub: IB DP Biology. Printable list: checklist.

Questions

1. State what is meant by an ecological niche and name two abiotic factors that form part of a niche. [3]

2. Relative growth of three bacterial cultures was recorded after 24 hours at three oxygen concentrations (data supplied, arbitrary units).

Culture	0% oxygen	5% oxygen	21% oxygen
J	7	0	0
K	0	5	9
L	5	8	9

(a) Identify the oxygen-tolerance group of each culture, with a reason for each. [3] (b) Suggest why culture L grows faster as oxygen increases. [1]

3. (a) Outline holozoic nutrition. [2] (b) A fungus grows on a fallen log. State its mode of nutrition and explain how it obtains nutrients. [3]

4. A marine protist was grown in four conditions. The net change in cell number per day is shown (data supplied).

Condition	Net change per day
Light, with prey	2.8
Light, no prey	-0.4
Dark, with prey	-0.6
Dark, no prey	-0.9

(a) Deduce, with evidence, whether the protist is an obligate or a facultative mixotroph. [3] (b) State how the data for a facultative mixotroph would differ. [1]

5. (a) State the three domains of life. [1] (b) Outline three ways in which archaea can obtain energy for ATP production. [3]

6. Skull measurements of two hominid species are given (data supplied).

Species	Molar occlusal area (mm²)	Incisor area (mm²)
P	260	40
Q	150	70

(a) Calculate the ratio of molar area to incisor area for each species. [2] (b) Deduce which species is likely to have ground more plant material, and which diet suits the other. [2] (c) Explain how theories made from living mammals allow diet to be deduced for extinct species. [1]

7. Seeds on plants with a high concentration of a toxic alkaloid and on plants with a low concentration were counted. Of 200 seeds from each type, 38 high-alkaloid seeds and 120 low-alkaloid seeds had been eaten.

(a) Calculate the percentage of seeds eaten for each type. [2] (b) Explain the difference. [2] (c) Suggest a metabolic adaptation that would let an animal eat the high-alkaloid seeds. [1] (d) Distinguish chewing mouthparts from piercing mouthparts in insects. [2]

8. Clay models of prey were placed on a forest floor. Sixty models had bold stripes copied from a venomous wasp and sixty were plain and unpatterned. Twelve striped and 33 plain models showed predator bite marks.

(a) Calculate the percentage of each type attacked and the difference in percentage points. [2] (b) Explain the result. [2] (c) Suggest one limitation of using clay models. [1] (d) State one behavioural adaptation of prey for resisting predation. [1]

9. Light intensity above a tropical forest canopy is 2000 units. Measurements show 5% reaches the shrub layer and 0.8% reaches the forest floor.

(a) Calculate the light intensity at each level. [2] (b) Explain how a liana and an epiphyte each reach the light. [2] (c) Suggest two features of a herb that survives on the forest floor. [2] (d) Outline how a strangler epiphyte gets established. [2]

10. A stream fish tolerates 12 to 30 °C when alone. In streams with a competing species it is found only at 18 to 26 °C.

(a) Distinguish the fundamental niche from the realized niche. [2] (b) Calculate the width of each temperature range and the realized range as a percentage of the fundamental range. [3] (c) Suggest why the realized niche is smaller. [1]

11. Two species, R and S, were grown alone and together with the same limited food supply. Population sizes (arbitrary units) are given.

Day	R alone	S alone	R together	S together
0	10	10	10	10
10	50	70	55	60
20	170	190	180	120
30	300	270	290	40
40	310	280	310	0

(a) Describe the outcome for species S when grown together with R. [2] (b) Calculate the percentage change in S at day 30 when with R compared with S alone. [2] (c) State the term for this outcome and explain why it happens. [2] (d) State a different possible outcome of competition between two species. [1] (e) Explain what the investigation suggests about the niches of R and S. [2]

12. A drawing of the mandible of a leaf-eating insect is 45 mm long. The actual mandible is 1.5 mm long.

(a) Calculate the magnification. [2] (b) Compare how a grasshopper-type insect and an aphid feed. [3] (c) Discuss how plant defences and predation shape the niche of a leaf-eating insect. [4]

Answers

1. The role of a species in its ecosystem [1], including its biotic interactions such as how it gets food [1]. Two abiotic factors, for example temperature and light (or water, pH, salinity) [1]. [3] Examiner insight: “Where it lives” describes a habitat only; include interactions as well as place.

2. (a) J grows only at 0% and not at 5% or 21%, so it is an obligate anaerobe [1]. K does not grow at 0%, so it is an obligate aerobe [1]. L grows at all concentrations, so it is a facultative anaerobe [1]. (b) Aerobic respiration releases more ATP per glucose than anaerobic respiration, so more energy is available for growth [1]. [4] Examiner insight: Give the data that supports each group; naming the group alone is a weaker answer.

3. (a) Food is ingested and digested inside the body [1], then absorbed and assimilated [1]. (b) Saprotrophic [1]. It secretes digestive enzymes onto the dead wood [1] and absorbs the soluble products [1]. [5] Examiner insight: “Absorbs the food” does not distinguish a saprotroph from an animal; state that the digestion happens outside the body.

4. (a) The protist grows only with light and prey together (2.8) [1]. In light alone (-0.4) and in the dark with prey (-0.6) numbers fall [1]. So it needs both autotrophy and heterotrophy: it is an obligate mixotroph [1]. (b) A facultative mixotroph would grow with light alone, or with prey alone in the dark, as well as with both [1]. [4] Examiner insight: Quote values from at least two rows.

5. (a) Archaea, Bacteria and Eukarya [1]. (b) From light [1], from oxidation of inorganic chemicals [1], and from oxidation of carbon compounds [1]. [4] Examiner insight: Three different energy sources are needed; rewording one source does not give a second point.

6. (a) P = 260 ÷ 40 = 6.5 [1]. Q = 150 ÷ 70 = 2.14 [1]. (b) P has the larger relative molar area, so it probably ground more plant material [1]. Q has a smaller ratio, consistent with an omnivorous diet [1]. (c) Observed diets of living mammals led to theories linking tooth form to diet, and these can be applied to fossil teeth [1]. [5] Examiner insight: Use cautious wording such as “suggests”; teeth alone do not prove a diet.

7. (a) High alkaloid 38 ÷ 200 × 100 = 19% [1]. Low alkaloid 120 ÷ 200 × 100 = 60% [1]. (b) The alkaloid is a toxic secondary compound that deters or harms herbivores [1], so fewer high-alkaloid seeds are eaten [1]. (c) An enzyme that detoxifies the alkaloid, for example in the liver or gut [1]. (d) Chewing mouthparts cut and grind pieces of tissue [1]; piercing mouthparts are inserted into a vessel to suck up sap [1]. [7] Examiner insight: A comparison needs both sides stated; describing only chewing mouthparts will not show the distinction.

8. (a) Striped 12 ÷ 60 × 100 = 20% and plain 33 ÷ 60 × 100 = 55% [1]. Difference = 35 percentage points [1]. (b) Striped models were attacked less [1] because the pattern acts as warning colouration that predators have learned to avoid, advertising a chemical defence [1]. (c) Models do not taste, smell or move like real prey, so the result may not apply to living animals [1]. (d) For example herding, alarm calls, freezing or fleeing [1]. [6] Examiner insight: State which group is higher and by how much; raw counts alone do not show the comparison.

9. (a) Shrub layer 2000 × 0.05 = 100 units [1]. Floor 2000 × 0.008 = 16 units [1]. (b) A liana roots in the soil and climbs on other plants, so it reaches the canopy with little trunk [1]. An epiphyte grows on the branches of trees, high in the light, without roots in the soil [1]. (c) Any two: large thin leaves [1]; efficient photosynthesis at low light [1]; a high chlorophyll content. (d) It starts to grow on a branch [1], then sends roots to the ground and may engulf and kill the host [1]. [8] Examiner insight: Give a separate statement for each plant when asked about both.

10. (a) Fundamental niche: the potential niche set by adaptations and tolerances without competitors [1]. Realized niche: the actual niche occupied when competing species are present [1]. (b) Fundamental width 30 - 12 = 18 °C [1]. Realized width 26 - 18 = 8 °C [1]. 8 ÷ 18 × 100 = 44.4% [1]. (c) Competition for the same limiting resources excludes the fish from part of its tolerable range [1]. [6] Examiner insight: Show each width subtraction before the percentage so your working can be followed.

11. (a) S rises to 120 by day 20 [1], then declines and reaches 0 by day 40 [1]. (b) (40 - 270) ÷ 270 × 100 = -85.2% [1], a fall of 85.2% [1]. (c) Competitive exclusion [1]. R and S use the same limited food, and R is the better competitor so S is eliminated [1]. (d) Both species persist, each restricted to part of its fundamental niche [1]. (e) Their niches overlap too much for both to persist under these conditions [1], so each species needs a different niche to coexist [1]. [9] Examiner insight: Support each point with values from the table.

12. (a) Magnification = image ÷ actual [1] = 45 ÷ 1.5 = ×30 [1]. (b) Grasshopper-type insect has chewing mouthparts and bites off leaf tissue [1]. An aphid has piercing mouthparts that enter a phloem vessel [1], taking sugar-rich sap without chewing [1]. (c) Plant defences such as thorns, tough leaves and toxins limit which plants it can eat [1]. Detoxifying enzymes would widen the range [1]. Predators restrict where and when it feeds, and defences such as camouflage or warning colours reduce predation [1]. The niche is the sum of food, competitors and enemies, so these interactions together define its role [1]. [9] Examiner insight: A “discuss” question needs more than a list; link each defence or predator to an effect on feeding or survival.

Where marks are usually lost

• Defining a niche as the place where a species lives.
• Naming the oxygen group without linking it to the data.
• Saying saprotrophs ingest food.
• Calling a protist facultative or obligate without quoting growth values.
• Forgetting that dentition is an inference, not proof of diet.
• Calculating percentage change without the direction.

Next steps

• Re-read the study guide for any gaps.
• Condense with the revision notes.
• Go to the IB DP Biology hub or the checklist.
• Take the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

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