B4.2 Ecological niches

Oxygen availability changes which respiration pathways can operate and whether reactive oxygen compounds can be tolerated. That physiology predicts position along an oxygen gradient.

“Anaerobe” does not always mean oxygen is poisonous: that is true of an obligate anaerobe, whereas a facultative anaerobe can grow on either side of the oxygen boundary.

A nutritional mode specifies the source of carbon and energy and how usable molecules enter cells. The same habitat can contain species with very different routes.

Do not classify by energy alone: light is an energy source, while carbon dioxide or organic food supplies the carbon atoms used to build biomass.

Photosynthetic pigments absorb light. The captured energy produces ATP and reducing power, which drive the reduction of carbon dioxide into organic molecules.

Plants and algae are eukaryotic photoautotrophs; cyanobacteria are prokaryotic photoautotrophs whose oxygen-releasing photosynthesis helped change Earth's atmosphere. Light supplies energy—not carbon or newly created energy.

Saprotrophs release inorganic nutrients that producers can reuse, but energy is not recycled: decomposers respire organic molecules and energy ultimately leaves the ecosystem as heat.

A detritivore ingests dead material and digests it internally, so it is holozoic rather than saprotrophic. Egestion removes undigested food; excretion removes metabolic waste.

Archaea form one domain, but different species occupy different metabolic niches. Domain membership does not imply one habitat, carbon source or energy pathway.

Methanogens occupy anoxic niches and use electron donors such as hydrogen while producing methane. Their metabolism shows why not all chemical energy entering food webs originally comes from sunlight.

Dietary inference uses more than one tooth: relative tooth area, cusp shape, enamel wear, jaw robustness and muscle-attachment sites reveal the forces and movements emphasized during feeding.

Form supports a probable processing strategy, not a complete menu. Tools, cooking, ancestry and convergent evolution can weaken a simple tooth-to-diet rule.

Make the inference from the combined pattern, then state its limit. Tooth wear and muscle attachment strengthen a claim; one isolated feature cannot prove exactly which foods were eaten.

A defence lowers feeding success or raises its cost; it rarely makes a plant invulnerable. If heritable counter-adaptations improve feeding, selection can favour them in the herbivore population, which in turn changes selection on the plant.

Nettle hairs deliver irritants, oleander contains cardiac glycosides and cassava releases cyanide precursors unless processed. Always link the named chemical or structure to how it changes herbivore survival or feeding.

Forward-facing eyes increase binocular overlap for depth estimation; side-facing eyes broaden surveillance. Neither arrangement alone labels every species as predator or prey—the mechanism must be interpreted with behaviour and ecology.

Adaptations often work together: rolling into a ball is useful because a tough exterior then faces the predator, and a stealth predator benefits from both still behaviour and camouflage.

Mimicry works only if predators encounter and avoid the defended model often enough. Conspicuous colour is therefore not automatically poor camouflage; its function depends on the receiver's learned or evolved response.

A strangler fig can surround and eventually kill its host, while an ordinary epiphyte uses support without necessarily harming it. Position changes access to light, but also water, minerals and pollinators.

A realized niche is often narrower than the fundamental niche, but beneficial interactions can also make occupancy possible. Absence alone does not reveal which interaction or dispersal barrier is responsible.

In Gause's cultures, Paramecium aurelia and P. caudatum both grew when cultured separately. In mixed culture, P. aurelia increased while P. caudatum declined to exclusion. The difference between separate and mixed cultures identifies interspecific competition.

The principle is conditional, not a claim that competitors never coexist. Field communities can retain competition when niches overlap only partly or environments vary.