IB DP Biology: Adaptation to environment: abiotic factors, tolerance and biomes (DP Biology) – Revision Notes

These revision notes condense IB Diploma Programme Biology section B4.1 (Adaptation to environment, outcomes B4.1.1 to B4.1.8) from the IB Biology guide, first assessment 2025. All of it applies to both SL and HL, because the guide gives B4.1 no extra higher level material. Full explanations and worked examples are in the study guide; test yourself with the practice set. Use the IB DP Biology hub, the checklist and the free diagnostics to plan your revision.

Definitions to know

Term	Meaning
Habitat	Place occupied by a community, species, population or organism: geographical location, physical location, ecosystem type
Abiotic factor	Non-living variable: temperature, light, water, salinity, pH, oxygen, wind, soil minerals
Adaptation	Inherited feature that improves survival and reproduction in a given environment
Range of tolerance	Span of values of an abiotic variable within which a species can survive
Limiting factor	Variable that restricts growth or distribution because it is furthest from the optimum
Biome	Group of ecosystems with similar communities because of similar abiotic conditions and convergent evolution
Convergent evolution	Unrelated species evolving similar features under similar selection pressures

B4.1.1 and B4.1.2: habitat and adaptation

• Describe a habitat with place plus ecosystem: “rocky upper shore, Atlantic coast, intertidal ecosystem”.
• Habitat is where; niche is role. Do not mix them.
• Answer pattern for adaptations: abiotic problem, then feature, then how it helps.

Marram grass on dunes

Problem	Feature	Effect
Dry, windy	Rolled leaves, sunken stomata, hairs	Humid air trapped, less transpiration
Dry, windy	Thick cuticle	Less evaporation
Deep water, loose sand	Long roots, rhizomes	Reaches water, anchors, spreads
Burial by sand	Grows upwards	Survives burial

Mangrove trees

Problem	Feature	Effect
Soft mud	Prop roots	Anchorage and support
Low oxygen in mud	Pneumatophores	Gas exchange for roots
Salt	Salt excluded or excreted	Tissue fluid not too salty
Tidal flooding	Viviparous seeds	Large seedlings establish in mud

B4.1.3 and B4.1.4: tolerance

Variables to quote: plants (light, temperature, soil pH, water, minerals, salinity); animals (temperature, water, dissolved oxygen, salinity, pH).

Sketch to learn (growth against an abiotic variable):

growth
  |         /\
  |        /  \
  |      _/    \_
  |  ___/        \___
  +--------------------- variable
     limit  optimum  limit
     (zones of stress either side)

• Narrow range of tolerance: restricted to stable habitats, often an indicator of conditions.
• Wide range: many habitats.
• Survival range is wider than the range for good growth or breeding.

Transect method in steps

1. Choose a natural or semi-natural habitat and a clear environmental gradient.
2. Lay a line or belt transect.
3. At fixed intervals, record species abundance in quadrats.
4. At the same points, measure the abiotic variable (sensors for temperature, light intensity and pH).
5. Plot a graph or compare tables and describe the correlation.
6. State the limitation: correlation is not causation; repeat transects and control other variables.

Worked reminder: frequency falls from 18 to 5 quadrats out of 20. Percentage change = (5 − 18) ÷ 18 × 100 = −72.2 %.

B4.1.5: coral reefs

Factor	Requirement	Reason
Depth	Shallow	Light for photosynthetic algae in coral tissue
Clarity	Clear water	Light penetration, no smothering
Temperature	Warm, roughly 23 to 29 °C	Narrow tolerance; bleaching if too hot
Salinity	Stable, near open ocean	Coral cells cannot cope with large dilution
pH	Slightly alkaline, about 8.1	Carbonate availability for calcium carbonate skeleton

pH is a logarithmic scale. A fall from pH 8.1 to 7.9 is a 10^0.2 = 1.58-fold rise in hydrogen ion concentration, a 58 % increase.

B4.1.6: biome distribution

• For a given temperature and rainfall, one ecosystem type is likely to develop.
• Graph: temperature on the horizontal axis, rainfall on the vertical axis, regions for each biome.
• Hot and wet: tropical forest. Hot and dry: hot desert. Cold and dry: tundra. Cold with more precipitation: taiga. Moderate temperature: temperate forest (wetter) or grassland (drier).
• Overlap areas: seasonality, soil and disturbance decide.

B4.1.7: biome climates and convergence

Biome	Climate summary
Tropical forest	Warm all year, very high rainfall
Temperate forest	Moderate temperatures, four seasons, moderate to high rainfall
Taiga	Long cold winters, short cool summers
Grassland	Seasonal, moderate rainfall, too dry for forest
Tundra	Very cold, short growing season, low precipitation
Hot desert	Very low rainfall, hot days

Convergent evolution examples: cacti (Americas) and succulent Euphorbia (Africa); kangaroo rats and jerboas.

B4.1.8: named adaptations

Habitat	Species	Adaptation
Hot desert	Saguaro cactus	Water-storing pleated stem, shallow roots, spines, night stomata opening
Hot desert	Kangaroo rat	Concentrated urine, nocturnal, burrows
Hot desert	Fennec fox	Large ears, pale fur, nocturnal
Rainforest	Kapok tree	Buttress roots in shallow soil
Rainforest	Bromeliad or orchid	Epiphyte, reaches light
Rainforest	Many canopy plants	Drip tips shed water
Rainforest	Three-toed sloth	Slow metabolism, algae-green fur camouflage

Worked reminders

Reading a tolerance table

A fish grows 0, 1.1, 2.6, 3.9, 3.1, 1.8, 0.4 and 0 mm per day at 4, 8, 12, 16, 20, 24, 28 and 32 °C (data supplied).

• Optimum: 16 °C.
• Limits for growth: 4 °C and 32 °C, a range of 28 °C.
• Drop from optimum to 24 °C: (3.9 − 1.8) ÷ 3.9 × 100 = 53.8 %.
• Shape: gradual rise, steeper fall, so high temperatures are the greater danger.

Water loss from a rolled leaf

A flat leaf loses 12.0 mg of water per hour and a rolled one loses 4.5 mg per hour in the same air (data supplied). Reduction = (12.0 − 4.5) ÷ 12.0 × 100 = 62.5 %. Explanation: the rolled leaf holds a pocket of humid air, so the diffusion gradient from leaf to air is shallower.

A model explanation

Question: explain how a named plant is adapted to a hot desert.

Answer: the saguaro cactus has a pleated stem (feature) that swells when it takes up rain (stores water for dry months) [1]. Its shallow, widely spread roots absorb surface water quickly after rain [1]. Spines reduce air movement and protect against grazing, and stomata open at night when it is cooler, so less water leaves by transpiration [1].

Reading a biome graph

If a place has a mean annual temperature of 26 °C and 2,800 mm of rain a year (data supplied), it lies in the hot and very wet corner, so tropical forest is the likely biome. If it has 2 °C and 400 mm, it falls in the cold, low-precipitation area, which suggests taiga, and tundra is possible if the growing season is very short. State the two values, name the region and add that other factors can alter the outcome where regions overlap.

Command words in this topic

Command word	What to do
State	Give a short answer: a variable, a feature or a biome
Describe	Say what the data show: trends, values, units, comparisons
Explain	Give a feature and then the reason it helps, using abiotic factor language
Suggest	Apply principles to unfamiliar organisms or data
Compare	Give similarities and differences for both, such as two biomes
Evaluate	Weigh strengths and limits, such as the reliability of a transect

Must-know distinctions

• Habitat versus niche.
• Optimum versus limit of tolerance.
• Correlation versus causation.
• Convergent evolution versus common ancestry.
• Biome (a type, worldwide) versus ecosystem (a particular place).

Quick self-test

1. Define habitat.
2. Give two ways marram grass reduces water loss.
3. Why do mangrove roots need pneumatophores?
4. A species grows at 6 to 30 °C, best at 18 °C. State its limits and optimum.
5. Why is a transect better than random sampling for studying a gradient?
6. List the five coral reef conditions in B4.1.5.
7. Light halves every 5 m. What percentage remains at 15 m?
8. Which two climate variables define the biome graph axes?
9. Give two named desert animals and an adaptation of each.
10. How do cacti and succulent Euphorbia show convergent evolution?

Answers

1. The place occupied by a community, species, population or single organism.
2. Rolled leaves trapping humid air; sunken stomata with hairs; thick waxy cuticle (any two).
3. Waterlogged mud has little oxygen, so aerial roots allow gas exchange.
4. Limits 6 °C and 30 °C; optimum 18 °C.
5. It samples along the gradient at regular intervals, so change in species abundance can be matched to change in the abiotic variable.
6. Depth, pH, salinity, clarity, temperature.
7. Three halvings: 100 × (1/2)³ = 12.5 %.
8. Temperature (horizontal axis) and rainfall (vertical axis).
9. Kangaroo rat: concentrated urine; fennec fox: large ears to lose heat.
10. Unrelated plants in different continents evolved similar succulent, spiny, water-storing stems under similar dry conditions.

Where marks are usually lost

• Naming a feature without saying how it helps.
• Failing to name the abiotic factor that the adaptation deals with.
• Confusing habitat with niche.
• Calling the optimum the limit of tolerance.
• Concluding that pH causes a distribution change from a single transect.
• Forgetting to describe the method details, such as regular intervals and measuring the abiotic variable at each point.
• Leaving out clarity or salinity when listing coral reef conditions.
• Describing convergence as shared ancestry.
• Using unnamed organisms when the question asks for named species.

Official syllabus

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