IB DP Biology: Adaptation to environment: abiotic factors, tolerance and biomes (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 IB Diploma Programme Biology section B4.1 (Adaptation to environment, B4.1.1 to B4.1.8) from the IB Biology guide, first assessment 2025. The content is the same for SL and HL. Revise first with the study guide and the revision notes. The course hub is IB DP Biology and the printable checklist tracks your progress. Data in the questions are supplied for practice.

Questions

1. Distinguish between the habitat and the ecological niche of a species. [2]

2. A flat leaf of a dune grass loses 7.5 mg of water per hour in still air. The same leaf rolled loses 3.3 mg per hour.

(a) Calculate the percentage reduction in water loss caused by rolling. [2] (b) Explain how a rolled leaf reduces water loss. [2]

3. Explain how pneumatophores and salt glands help a mangrove tree to survive. [3]

4. The table shows the percentage survival of tadpoles of one frog species after two weeks in water of different pH.

pH	4	5	6	7	8	9	10
Survival / %	0	35	88	96	80	20	0

(a) State the optimum pH for survival. [1] (b) State the pH values that mark the outer limits of the range of tolerance. [1] (c) Calculate the percentage fall in survival between the optimum and pH 9. [2] (d) Explain what the survival at pH 5 shows about the range of tolerance. [1]

5. A transect ran from a salt marsh creek inland across six sampling points. Salinity of the soil water and the percentage cover of a plant species were recorded at each point.

Salinity / ppt	10	15	20	25	30	35
Cover / %	3	12	36	52	41	9

(a) Calculate the mean percentage cover. [1] (b) Describe the relationship between salinity and cover. [2] (c) A student concludes that salinity of 25 ppt causes the highest cover. Suggest one reason why this conclusion may not be justified. [1] (d) Suggest two ways to make the data more reliable. [2]

6. (a) State two conditions required for the formation of coral reefs. [2] (b) Explain why water clarity affects reef formation. [2]

7. Seawater pH falls from 8.1 to 7.8. pH is a logarithmic scale. Calculate the factor by which hydrogen ion concentration increases, and state one consequence for reef-building corals. [3]

8. Predict the most likely biome for each location, using the data supplied.

Location	Mean annual temperature / °C	Annual rainfall / mm
A	27	3,000
B	−10	200
C	24	120
D	10	1,100

[4]

9. Kangaroo rats live in North American deserts and jerboas live in African and Asian deserts. Both are small, nocturnal rodents that hop on long hind legs. Explain how convergent evolution accounts for this. [3]

10. In a hot desert, a kangaroo rat produces urine with a concentration of 3,600 units, while human urine is typically 1,200 units (data supplied).

(a) Calculate how many times more concentrated the kangaroo rat urine is. [1] (b) Explain the advantage of this adaptation in a desert. [2] (c) State two other adaptations of a named desert animal, and how each helps. [2] (d) Describe two adaptations of a named desert plant. [2]

11. (a) Explain how buttress roots help a tree in a tropical rainforest. [2] (b) Outline why many orchids and bromeliads grow as epiphytes. [2] (c) State the function of drip tips on leaves. [1]

12. Plan an investigation to test whether soil pH limits the distribution of a woodland plant along a gradient from open ground into woodland in a semi-natural habitat. [9]

Answers

1. The habitat is the place where the species lives, such as its geographical location, physical location and ecosystem type [1]. The niche is the role of the species in the ecosystem, including its feeding and its interactions with other organisms [1]. [2] Examiner insight: A habitat is described by place; any answer that defines it by what the organism does has described a niche and is unlikely to gain the first point.

2. (a) Reduction = 7.5 − 3.3 = 4.2 mg per hour [1]; percentage = 4.2 ÷ 7.5 × 100 = 56.0 % [1] (b) Rolling traps a pocket of humid air next to the stomata [1], so the gradient of water vapour from leaf to air is smaller and less water is lost by diffusion (transpiration) [1]. [4] Examiner insight: On a percentage calculation, divide by the original value, not the new one; explanations should link the feature to diffusion gradient, not just say “less water escapes”.

3. Pneumatophores are roots growing up into the air [1], with pores for gas exchange, supplying oxygen to roots in waterlogged, oxygen-poor mud [1]. Salt glands excrete excess salt from the leaves so tissue fluid does not become too salty for cells and enzymes to work [1]. [3] Examiner insight: Name the abiotic problem (low oxygen, salinity) in each part; a feature with no stated problem is a weaker answer.

4. (a) pH 7 [1] (b) About pH 4 and pH 10, where survival is 0 % [1] (c) Fall = 96 − 20 = 76 [1]; 76 ÷ 96 × 100 = 79.2 % [1] (d) Some tadpoles survive at pH 5 (35 %) but fewer than at the optimum, so it lies in a zone of stress, within the range of tolerance but away from the optimum [1]. [5] Examiner insight: Quote values from the table with units; saying “survival is lower” without a figure is weak when the question says “calculate” or “show”.

5. (a) Total = 3 + 12 + 36 + 52 + 41 + 9 = 153; 153 ÷ 6 = 25.5 % [1] (b) Cover rises as salinity increases from 10 to 25 ppt (3 % to 52 %) [1], then falls at higher salinity to 9 % at 35 ppt, so cover peaks at an intermediate salinity [1]. (c) Correlation does not show causation: other abiotic factors, such as waterlogging, or competition, may change along the transect [1]. (d) Any two: repeat the transect; use more quadrats at each point; use quadrats of the same size and place them at random within each interval; measure salinity several times and take means [2]. [6] Examiner insight: A description of a data set should quote values and name the trend; “salinity affects cover” is an unsupported claim rather than a description.

6. (a) Any two: shallow water (light for algae); clear water; warm temperature (about 23 to 29 °C); stable salinity near open ocean; slightly alkaline pH (about 8.1) [2] (b) Photosynthetic algae inside coral polyps need light to photosynthesise and supply organic compounds to the coral [1]. Cloudy water scatters and absorbs light, and sediment can smother polyps, so reef growth is reduced [1]. [4] Examiner insight: A condition must be stated with its direction (shallow, clear, warm); listing only the variable names, such as “temperature”, is unlikely to be enough.

7. Change in pH = 0.3 [1]; factor = 10^0.3 = 2.0 times (about double) [1]. Higher hydrogen ion concentration reduces carbonate ions, making it harder for corals to build calcium carbonate skeletons [1]. [3] Examiner insight: Show the power of ten and the answer; a bare “2” with no working would leave the method unclear.

8. A: tropical forest [1]. B: tundra [1]. C: hot desert [1]. D: temperate forest [1]. [4] Examiner insight: Support the choice using both variables; a biome name alone cannot be checked against temperature and rainfall.

9. Convergent evolution is the evolution of similar features in unrelated species [1]. Both live in hot, dry deserts with sparse cover, so similar selection pressures act on them [1]. Individuals with nocturnal habits and hopping to escape predators and avoid heat survived and reproduced more, passing on these alleles, so similar features arose independently [1]. [3] Examiner insight: State that the species are not closely related and that the same selection pressure acted; claiming they “evolved to match” does not explain the mechanism.

10. (a) 3,600 ÷ 1,200 = 3.0 times [1] (b) Concentrated urine means water is reabsorbed and less water is lost [1], which reduces the need to drink where fresh water is scarce [1]. (c) Fennec fox: large ears increase surface area to lose heat [1]; nocturnal behaviour avoids midday heat and reduces water loss [1] . (d) Saguaro cactus: any two of pleated stem that swells to store water; shallow, widely spread roots; spines; stomata open at night [2]. [7] Examiner insight: Give a named species or recognised common name, such as saguaro cactus, rather than “a desert plant”, and state a benefit for each adaptation.

11. (a) Rainforest soils are shallow and nutrient poor, and the trees are very tall [1]. Buttress roots widen the base and provide support against toppling [1]. (b) The canopy receives more light than the dark forest floor [1]; epiphytes reach this light without building a tall trunk [1]. (c) Drip tips shed water quickly [1]. [5] Examiner insight: Link each feature to the abiotic factor, which is light for epiphytes and heavy rain for drip tips, to show understanding and not memorised lists.

12. Variable of interest: soil pH, measured at each sampling point [1] using a pH sensor or probe or soil test kit [1]. Lay a line transect from open ground into the woodland, which crosses the gradient [1], and sample at regular intervals using a quadrat of fixed size to record abundance (percentage cover or counts) of the plant [1]. Control or record other abiotic variables, such as light intensity, soil moisture and temperature, because they also change along the gradient [1]. Repeat with parallel transects and calculate means for reliability [1]. Plot abundance against pH and state expected correlation (for example higher abundance at the pH where the plant grows best) [1]. Note that correlation does not prove causation [1], so grow the plant in pots of soil of different pH under otherwise identical conditions to test pH as the limiting factor [1]. [9] Examiner insight: Plans that name the variable, how it is measured, the controls and the repeats are easier to credit than descriptions of a general survey; a final comment on causation shows evaluation.

Where marks are usually lost

• Describing a niche when asked for a habitat.
• Stating a feature without the problem or the benefit.
• Dividing by the new value in a percentage change.
• Giving no values when asked to describe a trend in data.
• Concluding causation from a single transect.
• Saying the optimum is the limit of tolerance.
• Omitting the algae and light link in coral reef answers.
• Calling any hot environment “desert” and forgetting rainfall.
• Using an unnamed organism when asked for a named species.

Next steps

• Review the revision notes and study guide.
• Visit the IB DP Biology hub and tick off outcomes on the checklist.
• Try all free 10-minute diagnostics to find other weak topics.
• Book a free trial class.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Section B4.1, Adaptation to environment.