Revise Ecological niches for IB Biology (B4.2)

The ecological niche of a species is the role it plays in its community — the way it uses resources together with the way it interacts with everything else. It is easy to confuse the niche with the habitat, but they are not the same. The habitat is the physical place a species lives, its 'address', and it is largely a matter of abiotic conditions such as temperature, pH, salinity and light. The niche is the species' 'profession': it includes the habitat, but it also captures how the species obtains energy and nutrients, when and where it is active, the range of abiotic conditions it can tolerate, and its biotic interactions — what it eats, what eats it, what it competes with and what it depends on. Two species can share a habitat while occupying quite different niches; what they cannot do is share an identical niche.

Because feeding is the heart of a niche, organisms are first classified by how they obtain organic carbon compounds and energy. AUTOTROPHS synthesise their own organic compounds from inorganic raw materials such as carbon dioxide; a PHOTOautotroph does this using light energy, and plants, algae and cyanobacteria are the photoautotrophs that act as the producers of most communities. HETEROTROPHS cannot make their own organic compounds and must obtain them ready-made from other organisms. A few organisms are MIXOTROPHS, able to use both strategies — Euglena, for example, photosynthesises in the light but ingests organic particles when light is scarce; facultative mixotrophs switch between the modes as conditions change.

Heterotrophs are subdivided by what they feed on and how. CONSUMERS feed on other living organisms and typically show HOLOZOIC nutrition: food is ingested, digested internally, absorbed, and the indigestible remainder egested — the pattern in most animals. SAPROTROPHS (mainly fungi and bacteria) feed on dead or decaying organic matter by SAPROTROPHIC nutrition: they secrete digestive enzymes onto the material and absorb the soluble products, so digestion is external — these are the decomposers that recycle nutrients back to producers. DETRITIVORES, such as earthworms and woodlice, also feed on dead organic matter (detritus) but INGEST it and digest it internally, so they are holozoic feeders that happen to eat detritus. Finally, in PARASITIC nutrition a heterotroph obtains nutrients from a living host, harming it — parasites may live on the host (ectoparasites such as ticks) or inside it (endoparasites such as tapeworms).

No species lives in isolation, and the presence of competitors changes what part of its potential niche a species can actually occupy. The FUNDAMENTAL niche is the full range of conditions and resources under which a species could survive and reproduce in the ABSENCE of interspecific competition — its potential, defined by its own tolerances and requirements. The REALISED niche is the part of that range the species actually occupies once competitors (and other limiting species) are present. Because competition tends to restrict a species to where it competes best, the realised niche is a subset of, or at most equal to, the fundamental niche — in practice, usually smaller. The clearest way to demonstrate the difference is experimental: remove the competitor, and the species expands to fill more of its fundamental niche.

Whenever a resource is limited and more than one organism needs it, there is competition. INTRAspecific competition is between members of the SAME species. Because individuals of one species have essentially identical niches, they demand exactly the same resources, so intraspecific competition is the most intense form and a powerful agent of natural selection. INTERspecific competition is between members of DIFFERENT species whose niches overlap; the more the niches overlap, the more the two species compete. It is interspecific competition — not intraspecific — that can lead to one species being driven out, and it is what restricts a species from its fundamental to its realised niche.

The competitive exclusion principle states that two species cannot occupy the exact same niche in the same place indefinitely. If two species compete for an identical set of limiting resources, one of them will inevitably be the more effective competitor — even by a small margin — and over time it will secure enough of the shared resource to survive and reproduce while the other declines to local extinction. The principle is often summarised as 'complete competitors cannot coexist'. It was demonstrated in classic experiments in which two closely related species reared together in the same conditions always ended with one persisting and the other dying out, even though each could thrive alone. The principle does not say that similar species can never live together — only that they cannot do so if their niches are truly identical.

$\text{Niche}(\text{Species A}) = \text{Niche}(\text{Species B}) \implies \text{coexistence is impossible (one is excluded)}$

The escape from competitive exclusion is for similar species to use the shared resource DIFFERENTLY — resource partitioning. Species may divide a resource in space (feeding in different places, such as insect-eating birds foraging at different heights and on different parts of the same tree), in time (being active at different times of day or in different seasons — temporal partitioning), or by form (taking different sizes or types of the same food, such as seed-eaters specialising on different seed sizes). Partitioning makes the species' realised niches differ, which reduces the intensity of interspecific competition and allows the species to coexist. This is how a community can support many species that would otherwise exclude one another.

Put together, these ideas explain how a community is organised. Because competitive exclusion forbids identical niches, the species that live together must each occupy a distinct realised niche — they partition the available resources. The community therefore ends up structured by feeding role and resource use: photoautotrophic producers capture energy; consumers, saprotrophs, detritivores and parasites occupy different heterotrophic roles; and even species with broadly similar diets are separated by microhabitat, timing or food size. A species' niche thus determines where it fits in the community and how many similar species can be supported alongside it. The more finely resources are partitioned, the more species a community can hold — so the niche concept links the behaviour of individual species to the diversity and structure of the community as a whole.

B4.2 explanation questions are marked analytically: each distinct, valid biological point is worth one mark, up to the total available. Method-style points (M) credit correct reasoning, answer points (A) credit a correct conclusion, and error-carried-forward (ECF) means one weak line does not sink the marks that follow, provided the rest of your reasoning stands. The mark scheme accepts equivalent wording, so the key is to make each idea DISTINCT — repeating the same point in two forms scores once. Study how each mark below is tied to a specific named idea.

The niche concept is the backbone of community ecology. Modes of nutrition set up food chains and food webs and the energy flow through them; competition and partitioning explain species richness and how communities assemble; and the fundamental-versus-realised distinction underlies why species' ranges shift when competitors or climate change. Master the habit of describing a species by its full niche — how it feeds, the conditions it tolerates and how it interacts — and you have a framework that carries straight into the study of populations, energy flow and biodiversity.