B4.2Theme B — Form and function

Ecological niches — IB Biology SL Revision Notes & Topical Questions

5 min read 12 parts Lesson 10 of 13 2 quizzes

0%read

Lesson summary

B4.2 Ecological niches — IB BIOLOGY SL notes: Explain niches, competitive exclusion, modes of nutrition and the adaptations of herbivores, predators, prey and plants.Niche = the role of a species within its habitatIt includes what the species eats and what feeds on it.It includes…

Habitat compared with niche
Abiotic factors
Unique niche
Paramecium
Early Earth
Facultative anaerobes

Part 1 of 10Niche

Niche

Niche the role of a species within its habitat

It includes what the species eats and what feeds on it.
It includes when the species is active and exactly where it lives and feeds.
It includes the abiotic and biotic factors acting on the species.

Habitat compared with niche

Habitat compared with niche the habitat is where a species lives and the niche is what it does there

Abiotic factors

Abiotic factors temperature, light, pH and water set limits on where a species can live

Biotic factors

Biotic factors predators, prey, competitors and parasites shape the niche

Unique niche

Unique niche no two species can permanently share exactly the same niche
Species that seem to share one differ subtly, for example in feeding time or food source.

Part 2 of 10Fundamental and realised niches

Feature	Fundamental niche	Realised niche
Definition	The full range of conditions and resources a species could use with no competitors	The range actually occupied
Biotic limits	Ignored	Competition and predation included
Size	Larger	Smaller or equal
Barnacle example	Much of the rocky shore	Upper shore only

Barnacles

Chthamalus dalli could live over a wide shore zone,
but competition from Balanus glandula confines it to the upper shore where it is exposed to air longer

Competitive exclusion

Competitive exclusion the elimination of a competing species from its niche

The loser is forced into a different niche or made locally extinct, or both species are restricted to part of their fundamental niches.

Paramecium

P. aurelia and P. caudatum thrive alone, but when grown together P. caudatum dies out
This was in laboratory conditions, and exactly shared niches are rare in nature.

Warblers

Warblers related species feed in different parts of trees, which reduces overlap

Part 3 of 10Oxygen and respiration

Feature	Obligate aerobe	Obligate anaerobe	Facultative anaerobe
Oxygen	Always needed	Never needed and harmful	Used if available
Respiration	Aerobic only	Anaerobic only	Both
Typical habitat	Air and surface water	Deep mud and guts	Soil, guts and food
Example	Humans and Mycobacterium tuberculosis	Clostridium	Yeast and E. coli

Early Earth

Early Earth early bacteria were obligate anaerobes in an atmosphere without oxygen
Photosynthesis added oxygen, so obligate anaerobes now live in oxygen-free places such as deep soil, deep water and inside other organisms.

Facultative anaerobes

Facultative anaerobes they switch to anaerobic respiration without ill effect, and aerobic respiration gives more ATP

Part 4 of 10Autotrophs

Nutrition

Nutrition how an organism gets the organic molecules that fuel respiration

Autotroph

Autotroph makes its own organic molecules from simple inorganic substances
Photoautotrophs use light energy.
Chemoautotrophs use energy from the oxidation of chemicals, such as iron-oxidising bacteria turning Fe²⁺ into Fe³⁺.

Photosynthesis

Photosynthesis pigments such as chlorophyll absorb light to turn carbon dioxide into organic molecules
It transfers light energy to chemical energy and does not create energy.
Producers include plants, algae and cyanobacteria.
It released the oxygen that made aerobic respiration possible.

Hydrothermal vents

Hydrothermal vents chemoautotrophs support food webs around deep-sea vents

Part 5 of 10Heterotrophs

Heterotroph

Heterotroph gets organic molecules from the tissues of other organisms

Holozoic nutrition

Holozoic nutrition ingestion, digestion, absorption, assimilation and egestion, with digestion inside the body

Ingestion is eating and digestion breaks large molecules into small ones.
Absorption moves molecules from the gut into the cells and assimilation builds them into tissues.
Humans and detritivores such as earthworms and woodlice use holozoic nutrition.

Saprotroph

Saprotroph a heterotroph that secretes enzymes onto dead matter and absorbs the products of external digestion

Fungi and many bacteria are saprotrophs and decomposers.
They release mineral ions such as ammonium and phosphate, and some stay in the soil for plant roots.
Houseflies digest externally, so they are not strictly holozoic.

Mixotroph

Mixotroph uses more than one mode of nutrition
Obligate mixotrophs need both modes and facultative mixotrophs survive on one, boosted by the other.
Euglena photosynthesises in light, engulfs bacteria by endocytosis and digests them in lysosomes, and an eyespot orients its chloroplasts to light.
Other mixotrophs are sundews and pitcher plants, corals with symbiotic algae and filter feeding, and many dinoflagellates.

Part 6 of 10Nutrition in archaea

Phototrophic archaea

Halobacteria use bacteriorhodopsin to absorb light and pump H⁺ across a membrane
ATP synthase then makes ATP, but they take their carbon from other organisms.

Chemoautotrophic archaea

Chemoautotrophic archaea use hydrogen, ammonia, methane or hydrogen sulfide as an energy source and make their own carbon compounds

Chemoheterotrophic archaea

Chemoheterotrophic archaea get ATP from inorganic chemicals but carbon from other organisms

Heterotrophic archaea

Heterotrophic archaea absorb and break down carbon compounds, for example from dead plant material

Methanogens

Methanogens release methane in anoxic habitats

Part 7 of 10Hominid diet and teeth

Hominidae

Hominidae humans, chimpanzees, gorillas, orangutans and gibbons, mostly omnivores
Chimpanzees are mainly frugivores with small jaw muscles and long canines.
Gorillas are mainly herbivores with strong jaw muscles and large grinding premolars and molars.

Tooth roles

Tooth roles incisors cut, canines grip and tear, and premolars and molars grind

Extinct hominids

Extinct hominids skulls and teeth give clues to diet
Paranthropus robustus had a skull crest for large jaw muscles, big molars and thick enamel for tough plants.
Homo floresiensis had large premolars, small canines and fibrous wear, with tools suggesting some meat.

Caution

Caution teeth do not always show diet
Humans eat meat but have teeth like plant-eaters.
Gorillas and orangutans have pointed canines but eat no meat.
Male chimpanzees have longer canines for display or mate competition.
Microwear patterns add evidence.

Nature of science

Nature of science deductions made from theory must be tested against new evidence

Part 8 of 10Herbivores and plant defences

Insect herbivores

Insect herbivores aphid stylets pierce the phloem, and caterpillar and beetle mandibles cut leaves

Mammal herbivores

Grazers have flat grinding teeth.
Ruminants have a multi-chambered stomach, re-chew the cud and rely on gut microbes that make cellulase.

Toxin handling

Toxin handling some deer have salivary proteins that bind tannins, proboscis monkeys have gut bacteria that neutralise toxins and mammals sample new plants cautiously

Mechanical defences

Mechanical defences cactus spines, stinging nettle hairs, thick bark and dense leaf hairs

Chemical defences

Digitalis in foxglove.
Atropine in deadly nightshade.
Bitter tannins that also reduce digestion.
Alkaloids such as nicotine and caffeine.

Arms race

Arms race plants and herbivores show defences and counter-adaptations

Part 9 of 10Predator and prey adaptations

Type	Predator	Prey
Chemical	Haemotoxic or neurotoxic venom, bolas spider moth pheromone mimicry, scent camouflage	Toxic frog skin, skunk spray, distasteful tiger moths, filefish smelling of coral
Physical	Forward-facing eyes, snake Jacobson's organ, bat echolocation, cheetah limbs, streamlined swordfish	Side-facing eyes, camouflage, eyespot mimicry, warning colours, shells and spines
Behavioural	Ambush, pack hunting, pursuit	Nocturnal activity, flocking, mobbing, alarm calls, playing dead, bluffing

Predator and prey adaptations

Predator behaviour

Ambushpuff adders and crocodiles.
Packwolves, orcas and lions.
Pursuitcheetahs and painted dogs.

Aposematism

Aposematism bright warning colours advertise chemical defences, as in poison dart frogs

Mimicry

Mimicry a king snake copies the pattern of a venomous coral snake, and owl butterflies have wing patterns like owl eyes

Group living

Group living groups reduce the chance of capture and confuse predators

Bluffing

Bluffing frill-necked lizards use a neck frill to look larger

Part 10 of 10Plant adaptations for light

Plant adaptations for light

Canopy and emergent trees

Canopy and emergent trees they invest in woody height to get full sun

Lianas

Lianas woody vines that germinate on the forest floor and climb tree trunks to the canopy
They compete with trees for light, nutrients and moisture.

Epiphytes

Epiphytes grow on branches, so they gain height without growing upward
Moss gets water from rain running over bark.
Bromeliads collect rainwater among their leaves.
Some orchids have aerial roots that take moisture from the air.

Strangler fig

Strangler fig begins in the canopy and sends roots down to the soil, and may kill the host tree

Shade-tolerant shrubs and herbs

Shade-tolerant shrubs and herbs grow on the forest floor
They have large leaves and pigments suited to filtered light.
Their flowers are bright or strongly scented to attract pollinators.

Light partitioning

Light partitioning different strategies let species occupy different niches

Key terms

Term	Meaning
Niche	The role of a species within its habitat, including its biotic and abiotic interactions.
Fundamental niche	The full range of conditions and resources in which a species could survive and reproduce.
Realised niche	The range actually occupied, restricted by biotic interactions such as competition.
Competitive exclusion	The elimination of one species from a niche by a better competitor.
Saprotroph	A heterotroph that digests dead matter externally and absorbs the products.
Mixotroph	An organism that uses both autotrophic and heterotrophic nutrition.
Aposematism	The use of bright warning colours to advertise chemical defences.
Epiphyte	A plant that grows on another plant to reach light.

Avoid these mistakes

Using habitat and niche as the same — a habitat is where an organism lives and a niche is its role

Saying decomposers and detritivores feed the same way — decomposers digest externally and detritivores internally

Saying the realised niche is larger than the fundamental niche — competition makes it smaller

Saying energy is produced by photosynthesis — light energy is transferred to chemical energy

Last updated: 18 October 2026