Habitat doesn't shape an organism by hand. Selection does — generation by generation. Same forces, very different fits.
Every sub-topic below feeds at least one of these questions.
How are the adaptations and habitats of species related?
Think about: What features help an organism survive in its specific environment? Why can't most organisms live everywhere?
The required syllabus content for B4.1, in order. Each card is one lesson-sized checkpoint.
IB Biology · B4.1 Adaptation to Environment · SL & HL
IB Biology · B4.1 Adaptation to Environment · SL & HL
Abiotic Variables & Range of Tolerance
IB Biology · B4.1 Adaptation to Environment · SL & HL
IB Biology · B4.1 Adaptation to Environment · SL & HL
Biome Distribution & Characteristics
IB Biology · B4.1 Adaptation to Environment · SL & HL
IB Biology · B4.1 Adaptation to Environment · SL & HL
Habitat = the specific place where an organism is found, with all the conditions that allow it to survive and reproduce.
The ecological hierarchy at a glance:
A complete habitat description has three parts: geographical location (where in the world), physical location (landforms, microhabitats), and ecosystem type. Example: the Sumatran orangutan lives in the tropical lowlands of Sumatra, in the warm humid canopy of tropical rainforest.
Marram grass on sand dunes; mangroves in saline anoxic mud. Each plant's adaptations directly counter the abiotic challenges of its habitat.
Challenge: limited water on sand dunes.
Rolled leaves — stomata on the inside; trap moist air, reduce transpiration.
Sunken stomata with hairs — trap humid air outside pores.
Thick waxy cuticle — reduces evaporative water loss.
Deep tap roots — reach groundwater; surface roots retain moisture in sand.
Challenge: waterlogged anoxic salty soil.
Prop/stilt roots — structural support in unstable sediment.
Pneumatophores — vertical aerial roots absorb O₂ from air.
Salt exclusion — roots filter most salt out.
Salt secretion — some species excrete excess salt through leaf glands.
Vivipary — seeds germinate while attached to parent, drop ready to root.
Distribution of species is determined by abiotic variables. Each species has an optimum range for each variable, with zones of stress on either side, and zones of intolerance beyond.
Light intensity (photosynthesis), soil pH (nutrient availability), soil water content (turgor, root oxygen), temperature (enzymes), mineral ions (growth).
Temperature (especially for ectotherms), salinity (osmotic balance), oxygen concentration (especially aquatic), humidity (desiccation), food availability.
Plot population performance against an abiotic factor. You get a bell-like curve:
Lay a tape across a habitat. At regular intervals, place a quadrat and record species presence/abundance, plus relevant abiotic variables (light, pH, temperature, moisture). Plot species abundance vs. abiotic factor — calculate Spearman's rank to test for correlation.
Coral reefs only form where five abiotic conditions all sit within tight tolerances. Outside any one, the reef doesn't develop.
Zooxanthellae need warmth. Above 30 °C → coral bleaching.
Light must reach symbiotic algae for photosynthesis.
Alkaline seawater for CaCO₃ skeleton building. Acidification threatens this.
Normal marine; freshwater inputs kill corals.
Also: low turbidity — sediment blocks light and smothers polyps. Coral reefs depend on a symbiotic partnership between coral polyps and photosynthetic zooxanthellae algae living in their tissues.
For any combination of mean annual temperature and rainfall, one type of natural ecosystem tends to develop. That's the biome.
| Biome | Temperature | Rainfall | Vegetation |
|---|---|---|---|
| Tropical rainforest | >25 °C year-round | >2000 mm/yr | Dense canopy, high biodiversity |
| Temperate forest | 4–20 °C seasonal | 750–2000 mm/yr | Deciduous trees |
| Taiga (boreal forest) | −10 to +10 °C | 400–900 mm/yr | Coniferous trees |
| Grassland | −5 to 25 °C | 250–750 mm/yr | Grasses dominate |
| Tundra | <−5 °C | <250 mm/yr | Mosses, lichens, sedges |
| Hot desert | >30 °C days | <250 mm/yr | Drought-adapted, sparse |
Different lineages, same biome → similar adaptations evolve independently. North American saguaro cactus and African euphorbia both have succulent stems, spines, and CAM photosynthesis — but they're unrelated. Both arctic fox and Antarctic seabirds evolved thick insulation and white camouflage. Same selective pressure → same engineering solution.
The IB names specific exemplars for each extreme biome. Memorise each species and its key adaptations.
Succulent stem stores water (hundreds of litres). Accordion-like ribs expand when full.
Spines (modified leaves) — near-zero transpiration; create shade; defend.
Thick waxy cuticle reduces evaporation.
CAM photosynthesis — stomata open only at night.
Roots — deep tap root + shallow wide spread (30 m radius).
Fat hump — energy + metabolic water when oxidised.
Concentrated urine + dry faeces minimise water loss.
Body temperature 34–41 °C — tolerates wide range without sweating.
Long legs keep body above hot sand; broad feet for sand.
Nasal passages reabsorb water from exhaled air.
Buttress roots — support in shallow nutrient-poor soils.
Drip tips on leaves — shed water rapidly.
Large broad leaves in canopy — maximise light capture.
Epiphytes grow on branches — share canopy space.
Lianas climb the trunk to reach light without growing their own.
Long arms (2 m span) for brachiation between branches.
Opposable thumbs and big toes — powerful gripping of all four limbs.
Trichromatic colour vision — identifies ripe fruit and detects predators in dense canopy.
Arboreal lifestyle — builds nightly nests; avoids ground predators.
Tool use + intelligence — extracts insects with sticks, uses leaves as umbrellas.
If you can't define one of these in a sentence, that's where to revise next.
“What are the properties of the components of biological systems?”
“Hint: Consider: How do protein structure and membrane composition relate to adaptations? How do cells of xerophytes differ from mesophytes? How do structural properties emerge from molecular components?”
“Is light essential for life?”