Ecological niches — IB Diploma Biology SL
Niches, oxygen tolerance, modes of nutrition, hominid dentition, herbivory, predation, light harvesting and competitive exclusion. IB Biology B4.2.
Revision notes
7 short notes, in the order of the specification. Each one in short:
An ecological niche is the role of a species in an ecosystem, including all the biotic and abiotic interactions that influence its growth, survival and reproduction and how it obtains food. Tolerance of oxygen is part of a niche: obligate anaerobes are poisoned by oxygen, facultative anaerobes grow with or without it, and obligate aerobes need it.
Photosynthesis is the mode of nutrition in plants, algae and several groups of photosynthetic prokaryotes, which make their own carbon compounds from carbon dioxide using light energy. Mixotrophic protists such as Euglena are both autotrophic and heterotrophic. Some mixotrophs are obligate, needing both modes to grow, while facultative mixotrophs can survive using either mode alone.
All animals are heterotrophic, and holozoic nutrition means food is ingested, digested internally, absorbed and assimilated. Saprotrophic fungi and bacteria secrete enzymes onto dead organic matter, digest it externally and absorb the products, so they are decomposers. Archaea, one of the three domains of life, are very varied in how they obtain energy for ATP production.
Dentition and diet in the Hominidae
PracticalDentition shows diet in the Hominidae. Omnivorous Homo sapiens has small incisors and canines and moderate molars. Homo floresiensis had small canines and molars and is also inferred to be omnivorous. Herbivorous Paranthropus robustus had very large molars and premolars with thick enamel and a sagittal crest for large jaw muscles, suited to grinding tough plant food.
Herbivores have adaptations for feeding on plants, such as the chewing or piercing mouthparts of leaf-eating insects, and plants resist herbivory with thorns and toxic secondary compounds, which some animals can detoxify. Predators have chemical, physical and behavioural adaptations for finding, catching and killing prey, and prey animals have matching adaptations for resisting predation.
Plants in forests compete for light and use different strategies to reach it. Canopy trees grow tall trunks, lianas climb other trees, epiphytes grow on high branches, strangler epiphytes grow down from branches and enclose their host, and shade-tolerant shrubs and herbs photosynthesise efficiently in the dim light of the forest floor.
The fundamental niche is the potential of a species based on its adaptations and tolerance limits, and the realized niche is the actual extent of its niche when in competition with other species. By the competitive exclusion principle, two species cannot share the same niche indefinitely: one is eliminated, or both are restricted to part of their fundamental niche.
Exam questions
8 exam-style questions (25 marks), each with its mark scheme.
Answer the questionsFlashcards
22 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of ecosystems (form and function) on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
Define ecological niche.
Show answer
The role of a species in an ecosystem, including the biotic and abiotic interactions that influence its growth, survival and reproduction.
What is a facultative anaerobe?
Show answer
An organism that can live with or without oxygen, respiring aerobically when oxygen is present and anaerobically when it is absent.
List the four stages of holozoic nutrition in order.
Show answer
Ingestion, digestion, absorption, assimilation.
Name an example of a mixotrophic protist.
Show answer
Euglena.
State two possible outcomes of competition between two species.
Show answer
One species is eliminated, or both are restricted to part of their fundamental niche.
Frequently asked questions
What is the difference between a fundamental and a realized niche?
The fundamental niche is the potential niche of a species, based on its adaptations and tolerance limits. The realized niche is the actual extent of its niche when it is in competition with other species, so it is usually smaller. Removing a competitor often lets a species expand towards its fundamental niche.
What is the competitive exclusion principle?
The competitive exclusion principle states that two species with the same niche cannot coexist indefinitely in the same place. Competition leads either to the elimination of one species or to both being restricted to part of their fundamental niche. This is why each ecological niche in an ecosystem is unique.
What is the difference between holozoic and saprotrophic nutrition?
Holozoic nutrition, used by all animals, involves ingesting food, digesting it internally, then absorbing and assimilating the products. Saprotrophic nutrition, used by some fungi and bacteria, involves secreting enzymes onto dead organic matter, digesting it outside the cells and absorbing the products. Saprotrophs are therefore decomposers.
Why is Euglena called a mixotroph?
Euglena is called a mixotroph because it is both autotrophic and heterotrophic. It has chloroplasts and photosynthesises in the light, but it can also take in carbon compounds from its surroundings. It is a facultative mixotroph, because it can survive heterotrophically in the dark when photosynthesis is not possible.
How do teeth show what Paranthropus robustus ate?
Paranthropus robustus had very large, flat molars and premolars with thick enamel, small front teeth, a heavy jaw and a sagittal crest that anchored large chewing muscles. These features suit grinding tough, hard plant material, so its diet is inferred to have been herbivorous, unlike omnivorous Homo sapiens.
Written and checked against the IB Biology SL specification · Updated October 2026