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

This study guide covers IB Diploma Programme Biology, section B4.1 (Adaptation to environment), as set out in the IB Biology guide, first assessment 2025. It teaches B4.1.1 to B4.1.8 and applies to Standard Level and Higher Level alike, because the guide gives B4.1 no extra higher level material. For condensed notes use the revision notes, and for exam-style questions use the practice set. The wider theme is on the Theme B study guide, the course hub is IB DP Biology, and the printable checklist lets you tick off each outcome. To find weak spots first, try the free diagnostics.

What B4.1 covers

Section	What you must be able to do	Level
B4.1.1	Define habitat and describe the habitat of a species	SL and HL
B4.1.2	Explain adaptations to abiotic conditions, with a dune grass and a mangrove tree	SL and HL
B4.1.3	Give abiotic variables for plants and animals; link adaptations to a range of tolerance	SL and HL
B4.1.4	Interpret the range of tolerance of a limiting factor; use transect data to correlate distribution with an abiotic variable	SL and HL
B4.1.5	State the conditions needed for coral reef formation	SL and HL
B4.1.6	Explain how temperature and rainfall determine biome distribution, using a climate graph	SL and HL
B4.1.7	Explain what a biome is and why similar biomes arise (similar conditions, convergent evolution)	SL and HL
B4.1.8	Give adaptations of named plants and animals in hot deserts and tropical rainforest	SL and HL

B4.1.1 Habitat

A habitat is the place that a whole community, one species, a population or a single organism occupies. A good description can include:

• a geographical location, such as the Atlantic coast of north-west Europe;
• the physical location, such as the upper shore, a rock crevice or the forest canopy;
• the type of ecosystem, such as a salt marsh or a coniferous forest.

A habitat is a place. It is not the same as the organism’s role in the ecosystem, which is its niche (B4.2). “The leaf litter of a temperate woodland floor” is a habitat description; “feeds on dead leaves” is not.

B4.1.2 Adaptations to the abiotic environment

An adaptation is an inherited characteristic that helps organisms survive and breed in their particular surroundings. Abiotic factors are the non-living parts of the environment, such as temperature, water availability, light, salinity and soil conditions.

A grass of sand dunes: marram grass (Ammophila arenaria)

Dunes are dry, windy, salty and low in nutrients, and the sand shifts.

• Leaves roll inwards. This traps a pocket of humid air around the stomata, so the gradient of water vapour from leaf to air is shallower and less water is lost by transpiration.
• Stomata are sunken in grooves and hairs line the groove, which also reduces air movement and water loss.
• A thick waxy cuticle reduces evaporation from the surface.
• Very long roots reach water deep in the sand, and horizontal rhizomes spread the plant and anchor it.
• Growth continues upward when sand buries the plant, so burial does not kill it.

A tree of mangrove swamps: Rhizophora and Avicennia

Mangrove swamps are flooded by tides, salty, and the waterlogged mud has little oxygen.

• Prop (stilt) roots in Rhizophora spread the weight of the tree and anchor it in soft mud.
• Pneumatophores in Avicennia are roots that grow upwards into the air. Pores in them allow gas exchange for roots buried in oxygen-poor mud.
• Salt is excluded at the roots or excreted from salt glands on leaves, which prevents the tissue fluid from becoming too salty for enzymes to work.
• Thick fleshy leaves store water, because salty water is hard to absorb by osmosis.
• Seeds germinate while still attached to the parent tree (vivipary), so the seedling is large enough to survive when it drops into mud.

Notice the pattern in good answers: name the abiotic problem, state the feature, then explain how the feature solves the problem.

B4.1.3 Abiotic variables and distribution

Species are found only where abiotic conditions suit them. Examples of abiotic variables:

Organisms	Variables
Plants	light intensity, temperature, soil pH, water availability, soil mineral content, salinity, wind
Animals	temperature, water availability, dissolved oxygen (aquatic), salinity, pH, oxygen concentration at altitude

Each species’ adaptations give it a range of tolerance: the range of values of an abiotic variable within which it can survive. Outside that range it dies or cannot reproduce.

B4.1.4 Tolerance range of a limiting factor

A limiting factor is the variable in shortest supply or furthest from the optimum, so it restricts distribution or growth. The range of tolerance for one variable has recognisable zones:

• Optimum: best growth, survival and reproduction.
• Zones of stress: the organism survives but grows or reproduces poorly.
• Limits of tolerance: the lowest and highest values at which the organism survives. Beyond them it is absent.

Worked example: tolerance data

A fish was reared for a month at eight temperatures (data supplied).

Temperature / °C	4	8	12	16	20	24	28	32
Growth / mm per day	0	1.1	2.6	3.9	3.1	1.8	0.4	0

1. The optimum is about 16 °C, where growth is greatest (3.9 mm per day).
2. Growth is zero at 4 °C and 32 °C. These are the approximate limits of the range for growth, a width of 28 °C.
3. Growth at 24 °C is (3.9 − 1.8) ÷ 3.9 × 100 = 53.8 % lower than at the optimum, which shows a zone of stress.
4. The curve is not symmetrical: it rises gradually and falls more steeply. A species with a narrow range is restricted to stable habitats; one with a wide range can occupy many.

Skill: using transect data

You should collect your own data from a natural or semi-natural habitat (influenced by humans but dominated by wild species), then correlate distribution with an abiotic variable. A belt or line transect is laid across an environmental gradient, such as from open dune towards woodland. At regular intervals you count or estimate species abundance (quadrats) and measure the abiotic variable. Sensors can measure temperature, light intensity and soil pH.

Worked example: interpreting a transect

Distance / m	0	10	20	30	40	50
Soil pH	8.2	7.8	7.3	6.9	6.5	6.1
Quadrats (of 20) containing species A	18	15	9	5	2	0

• Species A is most frequent where pH is highest and absent at pH 6.1: a negative correlation with distance and a positive correlation with pH.
• Frequency falls from 18 to 5 between 0 and 30 m, a change of (5 − 18) ÷ 18 × 100 = −72.2 %.
• Correlation does not prove causation. Light, moisture and competition may also change along the transect, so you should say pH is a possible limiting factor and suggest repeating the transect or testing A’s growth at different pH values in controlled conditions.
• To improve reliability, use more quadrats per interval, repeat along parallel transects and standardise quadrat size.

B4.1.5 Conditions for coral reef formation

Reef-building corals are animals whose polyps contain photosynthetic algae (zooxanthellae). The algae supply organic compounds, and the coral builds a calcium carbonate skeleton. A reef develops where conditions suit both partners:

• Depth: shallow water, because enough light must reach the algae. Light intensity drops quickly with depth.
• Clarity: clear water with little suspended sediment or plankton, which would block light and smother polyps.
• Temperature: warm water, typically about 23 to 29 °C. A rise of only a degree or two above the normal summer maximum can cause bleaching, when corals expel their algae.
• Salinity: fairly high and stable, around that of open ocean. Heavy freshwater runoff lowers it and damages corals.
• pH: slightly alkaline seawater (about 8.1). Lower pH reduces carbonate ion availability, making skeleton formation harder.

Worked reasoning: if light intensity halves every 5 m (data supplied), 20 m is four halvings, so (1/2)⁴ = 1/16 = 6.25 % of surface light remains. This explains why reef growth declines rapidly with depth.

B4.1.6 Abiotic factors and biome distribution

For a given pattern of temperature and rainfall, one type of natural ecosystem is likely to develop. A biome distribution graph (a Whittaker-type diagram) plots mean annual temperature on the horizontal axis and annual rainfall on the vertical axis, with regions marked for each biome.

How to read it:

• Moving right (warmer) at high rainfall gives tropical forest.
• Moving left (colder) gives taiga, then tundra.
• Low rainfall at any temperature gives desert (hot) or tundra (cold); intermediate rainfall gives grassland.
• Where regions overlap, other factors such as seasonality, soil and fire decide which biome occurs.

B4.1.7 Biomes and convergent evolution

A biome is a group of ecosystems in different places that have similar communities because their abiotic conditions are similar. Approximate climates (typical values, not exact boundaries):

Biome	Climate
Tropical forest	warm all year, very high rainfall
Temperate forest	moderate temperatures, distinct seasons, moderate to high rainfall
Taiga	long cold winters, short cool summers, low to moderate precipitation
Grassland	seasonal rainfall too low for forest, too high for desert
Tundra	very cold, short growing season, low precipitation
Hot desert	very low rainfall, hot days, often cold nights

Similar communities arise because similar selection pressures act on unrelated species and favour similar solutions, which is convergent evolution. Examples: American cacti and certain African Euphorbia species both have succulent, spiny, water-storing stems; kangaroo rats in North America and jerboas in Africa and Asia are both small, nocturnal, hopping desert rodents. Their common features did not come from a recent shared ancestor with those features.

B4.1.8 Adaptations in hot deserts and tropical rainforest

Hot desert (low water, high temperatures)

Organism	Adaptation	How it helps
Saguaro cactus (Carnegiea gigantea)	Pleated stem that swells, shallow wide-spreading roots, spines, stomata open at night	Stores water after rain, absorbs it fast, reduces transpiration and grazing
Kangaroo rat (Dipodomys)	Very concentrated urine, nocturnal, lives in burrows	Conserves water, avoids midday heat
Fennec fox (Vulpes zerda)	Large ears, pale fur, nocturnal	Ears lose heat, pale fur reflects sunlight
Dromedary camel (Camelus dromedarius)	Fat stored in the hump, tolerates large water loss and body temperature change	Fat concentrated in one place leaves less insulation elsewhere, so heat escapes; tolerating water loss reduces the need to drink

Tropical rainforest (high rainfall, warm, dense competition for light)

Organism	Adaptation	How it helps
Kapok tree (Ceiba pentandra)	Buttress roots	Supports a tall tree in shallow, nutrient-poor soil
Many canopy plants	Leaves with drip tips and glossy surface	Shed water quickly, discouraging fungal growth
Bromeliads and orchids	Grow as epiphytes on branches	Reach more light without growing a tall trunk
Lianas	Climb on trees	Reach light cheaply
Three-toed sloth (Bradypus)	Slow metabolism, algae in fur	Low energy need, camouflage in canopy

Common errors

• Describing a habitat as what an organism eats or does. That is its niche.
• Writing “adapted to the desert” without naming the abiotic factor and the mechanism.
• Saying marram “stores water” rather than “reduces water loss by rolling its leaves”.
• Saying a camel’s hump stores water. It stores fat.
• Treating a transect correlation as proof of causation.
• Reading the optimum as the limit of tolerance. Survival is possible well beyond the optimum.
• Explaining convergent evolution as organisms “copying” each other or inheriting features from a common ancestor.

Official syllabus

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