Niche isn't where an organism lives. It's the job it does, the resources it competes for, and the limits on both.
Every sub-topic below feeds at least one of these questions.
What are the advantages of specialized modes of nutrition to living organisms?
How are the adaptations of a species related to its niche in an ecosystem?
The required syllabus content for B4.2, in order. Each card is one lesson-sized checkpoint.
IB Biology DP | B4.2 Ecological Niches | B4.2.1
IB Biology DP | B4.2 Ecological Niches | B4.2.2
IB Biology DP | B4.2 Ecological Niches | B4.2.3
IB Biology DP | B4.2 Ecological Niches | B4.2.4
IB Biology DP | B4.2 Ecological Niches | B4.2.5
IB Biology DP | B4.2 Ecological Niches | B4.2.6 – B4.2.7
Third Domain of Life | Metabolically Diverse
IB Biology DP | B4.2 Ecological Niches | B4.2.8
IB Biology DP | B4.2 Ecological Niches | B4.2.9
IB Biology DP | B4.2 Ecological Niches | B4.2.10
IB Biology DP | B4.2 Ecological Niches | B4.2.11
IB Biology DP | B4.2 Ecological Niches | B4.2.12
IB Biology DP | B4.2 Ecological Niches | B4.2.13
An ecological niche isn't just where a species lives — it's everything about how it makes a living: what it eats, what eats it, what it competes with, what conditions it tolerates.
Competition for food and space; predator-prey relationships; disease and parasitism; mutualisms and symbioses; interactions with other members of the same species (intraspecific) and other species (interspecific).
Temperature range tolerated; light availability; water/moisture; soil pH and nutrients; dissolved oxygen (aquatic).
Oxygen tolerance places organisms in three categories — and determines where they can live.
Cannot survive in oxygen. Lives in waterlogged soils, deep sediments, the human gut. Anaerobic respiration only.
Survives with or without O₂. Uses aerobic respiration when available (more efficient), switches to anaerobic when needed. E. coli is the classic example.
Cannot survive without oxygen. Must live where O₂ is available. Most animals and many bacteria.
Every organism either makes its own organic compounds or gets them from another organism. Five named modes — the IB wants you to recognise each.
| Mode | Auto/hetero | Examples |
|---|---|---|
| Photosynthesis | Autotroph | Plants, algae, cyanobacteria. |
| Holozoic | Heterotroph | All animals. Ingestion → digestion → absorption → assimilation. |
| Mixotrophic | Both | Euglena viridis — photosynthesises and ingests food. Common in oceanic plankton. |
| Saprotrophic | Heterotroph | Fungi and many bacteria. Secrete enzymes onto dead matter; absorb resulting small molecules. Decomposers. |
| Archaea — diverse | Various | Phototrophs (light), chemolithotrophs (inorganic chemicals like H₂S), organotrophs (organic compounds). |
Holozoic nutrition has four steps: ingestion (food into the body) → digestion (broken into small molecules) → absorption (small molecules cross the gut wall into the blood) → assimilation (used by cells for energy, growth, repair).
Studying the dentition of living mammals lets us deduce the diet of extinct species from their fossils — a clean illustration of inference in palaeobiology.
By comparing fossil skull and tooth proportions with those of living species of known diet, palaeontologists infer extinct species' feeding ecology.
Each pressure produces counter-adaptations. The result is co-evolution — every defence eventually meets a counter-strategy.
Piercing or chewing mouthparts to access plant tissues; flat molars for grinding cell walls; long gut to digest cellulose; metabolic detox of plant toxins.
Thorns, spines, prickles. Tough leathery leaves. Toxic alkaloids (nicotine, caffeine). Sticky resin or latex. Mutualisms with predators of herbivores.
Sharp claws/teeth, forward-facing eyes for depth perception, speed. Venom in snakes and spiders. Pack hunting, ambush, nocturnal foraging, mimicry to lure prey.
Camouflage; armour; aposematic warning colouration. Toxins, unpalatable secretions. Group living (vigilance), flight, shifting activity to predator-free hours.
In forests, light is the limiting resource. Different plant types exploit different positions in the light niche.
Reach high into the sunlight; broad leaves minimise self-shading. Dominate the canopy light niche by investing in trunk and crown.
Climb canopy trees rather than building their own trunks. Rapid vertical growth reaches light cheaply.
Orchids and bromeliads grow on branches of canopy trees. Reorient leaves to canopy gaps. Get water from rain.
Start as epiphytes — germinate in the canopy. Send aerial roots down to soil, gradually surrounding the host tree. The host may eventually die.
Shade-tolerant understorey plants take the opposite approach — broad leaves, high chlorophyll concentrations, and complex branching to maximise capture of the dim light filtering through the canopy.
Where a species could live (fundamental niche) is broader than where it actually lives (realized niche) — because competitors squeeze it.
The full set of abiotic and biotic conditions a species could tolerate. Defined by adaptations and tolerance limits alone.
The subset of the fundamental niche the species actually occupies, given competition with other species. Always ≤ fundamental niche.
"No two species can occupy the same niche at the same time." Two outcomes possible:
Classic case: Chthamalus and Balanus barnacles on Scottish rocks — Chthamalus is competitively excluded from the lower rocks by Balanus, even though it would survive there in the absence of competition.
If you can't define one of these in a sentence, that's where to revise next.
“Q1: What are the relative advantages of specificity and versatility in nutrition?”
“Q2: For each form of nutrition, what are the unique inputs, processes and outputs?”