Ecological Niches

Niche = the role a species plays in its ecosystem — the way it interacts with the biotic and abiotic environment.

A niche is defined by how an organism lives, not just where:

• How it obtains resources — its mode of nutrition, food source, light or nutrient requirements.
• Where and when it is active — temperature/pH/oxygen tolerances, time of day, position in a habitat (the spatial habitat is only one part of the niche).
• Its interactions — with predators, prey, competitors, mutualists and parasites.

Habitat vs niche. The habitat is the physical place an organism lives (its "address"). The niche is its full role and set of requirements (its "profession"). Two species can share a habitat but occupy different niches.

Fundamental niche = the full range of conditions and resources a species could theoretically use in the absence of competition or other limiting interactions.

Realised niche = the part of the fundamental niche a species actually occupies once competition, predation and other interactions are taken into account. The realised niche is usually narrower than the fundamental niche.

For example, two barnacle species on a rocky shore each could survive across a broad tidal range (their fundamental niches overlap), but competition restricts each to a narrower band (their realised niches) — a classic Connell barnacle study.

An organism's mode of nutrition is a major part of its niche.

Autotrophs synthesise their own carbon compounds from inorganic sources.

• Photoautotrophs use light energy to fix carbon (e.g. plants, algae, cyanobacteria) — photosynthesis.
• (Chemoautotrophs use chemical energy from inorganic reactions, e.g. some bacteria.)

Heterotrophs obtain carbon compounds from other organisms. Sub-types:

• Holozoic — ingest food, then digest it internally and absorb the products (e.g. most animals).
• Saprotrophs / decomposers — secrete digestive enzymes onto dead organic matter and absorb the soluble products of external digestion (e.g. many fungi and bacteria). They recycle nutrients.
• Detritivores — ingest non-living (dead) organic matter (detritus) and digest it internally (e.g. earthworms, woodlice). Distinct from saprotrophs, which digest externally.
• Parasites — live in or on a living host, obtaining nutrients from it and harming the host (e.g. tapeworms, Plasmodium).

Mixotrophs combine autotrophic and heterotrophic nutrition. They may be:

• Facultative mixotrophs — switch between modes depending on conditions (e.g. Euglena: photosynthesises in light, ingests/absorbs organic matter in darkness).
• Obligate mixotrophs — require both modes simultaneously.

Metabolic niches defined by oxygen. The way an organism uses (or avoids) oxygen is itself a niche dimension:

• Obligate anaerobes — cannot survive in oxygen; respire only anaerobically (e.g. Clostridium tetani).
• Facultative anaerobes — respire aerobically when oxygen is available, but switch to anaerobic respiration/fermentation when it is not (e.g. yeast, E. coli).

These distinctions matter because organisms with different nutritional or metabolic niches can coexist in the same habitat without directly competing.

Competitive exclusion principle. Two species cannot occupy the same niche in the same habitat indefinitely. If their niches are identical, one species will be a slightly better competitor for the limiting resource and will outcompete the other, driving it to local extinction or forcing it to shift its niche.

Gause's experiments. G. F. Gause grew two Paramecium species:

• Grown separately, both P. aurelia and P. caudata thrived.
• Grown together on the same food source, P. aurelia outcompeted P. caudata, which declined to extinction — demonstrating competitive exclusion.

Resource partitioning (niche differentiation). Similar species can coexist if their niches differ enough to reduce direct competition. They partition the resource by using different parts, times or methods.

Examples:

• MacArthur's warblers — five North American warbler species feed in the same spruce trees but each forages in a different zone of the tree (top, middle, inner, outer, lower), partitioning the insect resource so they coexist.
• Galápagos finches — beak size and shape differ between species, so they specialise on different seed sizes, reducing competition (an example linked to natural selection and character displacement).

So competition does not always end in exclusion: it can instead drive divergence, sharpening the differences between species' realised niches.

Adaptations suit an organism to its niche. Adaptations can be:

• Structural — e.g. a finch's beak shape matched to its food.
• Physiological — e.g. enzymes that function at a specific temperature/pH range.
• Behavioural — e.g. nocturnal activity to exploit cooler conditions and avoid competitors.

Together these adaptations define and fit the organism to its particular niche.