B4.2 Ecological niches

An ecological niche is the full role of a species: the conditions it tolerates, resources it uses and interactions that affect its survival and reproduction.

A niche includes both abiotic requirements and biotic relationships such as competition, predation and mutualism. Two species can share a habitat yet occupy different niches.

Describe a niche with: physical limits; food or resource use; timing or location; interactions with other species.

Two birds may nest in the same forest but feed at different heights and times, reducing direct competition.

Niche is not just habitat. Habitat is where a species lives; niche explains how it lives there.

Oxygen tolerance divides organisms into obligate aerobes, obligate anaerobes and facultative anaerobes according to whether oxygen must be present, must be absent or may vary.

Obligate aerobes require oxygen and cannot grow without it. Obligate anaerobes are harmed or killed by oxygen and occupy anoxic environments. Facultative anaerobes grow with or without oxygen, changing metabolic pathway as conditions change.

Examples: Mycobacterium tuberculosis is an obligate aerobe; many strict anaerobes grow only in oxygen-free sediments or tissues; Escherichia coli is facultative and can tolerate both oxic and anoxic conditions.

Across a sediment gradient, an obligate aerobe is restricted to the oxygenated surface, an obligate anaerobe to deeper anoxic layers, and a facultative anaerobe may occur in both.

Anaerobe does not automatically mean killed by oxygen: that describes an obligate anaerobe. Facultative anaerobes tolerate oxygen absence but often grow differently when oxygen is available.

Photosynthetic nutrition uses light energy to build organic molecules from carbon dioxide in plants, algae and several groups of photosynthetic prokaryotes.

Photosynthetic pigments absorb light and drive energy conversion; the resulting chemical energy supports carbon fixation into organic compounds used for growth and respiration.

Plants and algae are major eukaryotic photoautotrophs. Cyanobacteria are photosynthetic prokaryotes whose oxygen-producing photosynthesis contributed to the rise of atmospheric oxygen; detailed prokaryotic pathway types are outside this objective.

A cyanobacterium and a green plant both use light as the energy source and carbon dioxide as a carbon source, even though one is prokaryotic and the other is eukaryotic.

Photosynthesis is not 'feeding on sunlight': light supplies energy, while carbon atoms come from inorganic carbon. Do not add detailed prokaryotic photosynthesis pathways beyond the syllabus.

All animals are heterotrophs, and holozoic nutrition obtains organic matter by ingestion followed by internal digestion, absorption and assimilation.

Food is taken into the body, mechanically and chemically broken into small soluble molecules, moved across the gut wall and incorporated into tissues or used in metabolism.

Sequence: ingestion → internal digestion → absorption → assimilation; indigestible material is egested. Herbivores, carnivores and omnivores differ in food sources but all use this holozoic sequence.

A mammal ingests starch, hydrolyses it to glucose, absorbs glucose into blood and assimilates it by respiration, glycogen synthesis or construction of other molecules.

Ingestion alone is not the complete mode, and egestion is removal of undigested material rather than excretion of metabolic wastes.

A mixotroph combines autotrophic and heterotrophic nutrition. Some mixotrophs require both modes, while facultative mixotrophs can shift their balance with conditions.

Euglena is a freshwater protist with photosynthetic chloroplasts that can also obtain organic carbon heterotrophically. Many oceanic plankton combine photosynthesis with uptake or ingestion of organic material.

Obligate mixotrophy requires contributions from both modes to meet nutritional needs; facultative mixotrophy permits a switch when light, inorganic nutrients or prey availability changes.

A facultative planktonic mixotroph may rely more on photosynthesis in bright, nutrient-rich surface water and increase heterotrophic feeding when light becomes limiting.

Mixotrophic does not mean every mode is used equally or simultaneously. State the carbon and energy source under the particular condition.

Saprotrophic fungi and bacteria are heterotrophic decomposers that digest dead organic matter externally and absorb the soluble products.

They secrete hydrolytic enzymes onto a substrate; polymers are converted into small soluble molecules that cross the fungal hypha or bacterial cell surface.

Sequence: dead material → extracellular enzyme secretion → hydrolysis → absorption → assimilation or respiration. This returns inorganic nutrients to ecosystems while chemical energy is transferred and some is released as heat.

A fungus growing through leaf litter secretes enzymes that release sugars and amino acids, then absorbs them for growth and respiration.

Saprotrophs do not ingest intact pieces like animals, and they are distinct from detritivores that consume dead material and digest it internally.

Archaea are one of the three domains of life and are metabolically diverse: different species use light, inorganic chemicals or carbon compounds to supply energy for ATP production.

A light-driven archaeon captures light energy without necessarily carrying out plant-like oxygenic photosynthesis. Chemolithotrophic archaea oxidize inorganic substances, while heterotrophic archaea oxidize organic carbon compounds.

These energy strategies allow archaea to occupy saline, hot, acidic, oxygen-poor and ordinary environments. Methanogens are one anaerobic example, but named examples are not required for this objective.

Two archaeal species may share the same domain yet occupy different niches because one uses light-driven energy capture and another obtains ATP by oxidizing an inorganic chemical.

Not all archaea are extremophiles or methanogens, and archaeal light use should not automatically be labelled oxygen-producing photosynthesis.

Hominid tooth and jaw form can be used to infer the mechanical demands of an omnivorous or herbivorous diet, while recognizing that skull evidence is indirect.

Examine physical models or digital skull collections for tooth size and shape, enamel wear, jaw robustness and muscle-attachment areas, then connect each observation to cutting, tearing or grinding.

Large post-canine teeth and a robust jaw in Paranthropus robustus support repeated grinding of resistant plant foods; the more generalized human dentition is consistent with an omnivorous diet.

Dentition constrains plausible diets but cannot reveal every food eaten. Food processing, wear, ancestry and convergent form mean the inference should be stated as evidence, not certainty.

Herbivores have structures and physiology for obtaining plant food, while plants resist herbivory through physical barriers and toxic secondary compounds.

Leaf-eating insects may use chewing mandibles to remove tissue or piercing mouthparts and stylets to reach sap. Vertebrate herbivores may have grinding teeth, long digestive tracts or microbial partners that help process cellulose.

Plants use thorns, spines, stinging hairs and tough tissues as physical defences. Toxic compounds in leaves or seeds reduce feeding; some herbivores counter them with detoxifying enzymes or symbiotic gut microbes.

An aphid stylet bypasses outer tissue to reach phloem sap, whereas cactus spines deter larger grazers; a specialist herbivore may still feed if its metabolism detoxifies the plant compound.

A defence reduces the probability or benefit of feeding rather than making a plant invulnerable. Distinguish an herbivore feeding adaptation from the plant counter-adaptation it addresses.

Predators use chemical, physical and behavioural adaptations to find, catch and kill prey; prey use the same categories to avoid detection, capture or consumption.

Peregrine-falcon speed increases capture success, while a hedgehog's spines reduce handling success. Poison-dart-frog warning colours advertise chemical defence, and harmless coral-snake mimics can deter attack.

A chameleon combines visual detection and a rapidly projected tongue to capture prey; the prey's camouflage changes the detection stage rather than its ability to survive after capture.

A conspicuous signal is not necessarily poor camouflage—it may be aposematic. Classify the trait by the interaction stage and mechanism, not only by appearance.

Forest plants occupy vertical light strata using different forms to reach strong light or tolerate shade beneath the canopy.

Each strategy trades access to photons against costs of support, water supply, nutrient capture and dependence on other plants. Epiphytes use hosts for position, not nutrition.

A liana reaches canopy light by climbing a tree, whereas a shade-tolerant herb remains near the ground and captures low light rather than investing in a tall trunk.

More leaf area or height does not guarantee higher photosynthesis when water, nutrients or carbon dioxide limit growth. Do not label an epiphyte a parasite unless it extracts resources from the host.

A fundamental niche is the full range of conditions a species could use without biotic restrictions; its realized niche is the narrower range it occupies with competitors, predators or mutualists present.

Biotic interactions exclude some otherwise suitable conditions. Removing a competitor can therefore expand where a species is found without changing its abiotic tolerance.

Compare niches by holding abiotic conditions constant, then add or remove the named interaction.

A barnacle may survive across a broad tidal zone alone but occupy only the upper zone when a stronger competitor excludes it from lower levels.

A realized niche is not always smaller in every dimension; mutualists can expand access to resources.

Competitive exclusion occurs when species with very similar niches compete for the same limiting resource strongly enough that stable complete overlap cannot persist.

One possible outcome is elimination of the weaker competitor from the habitat. Another is restriction of both species to different parts of their fundamental niches through resource, spatial or temporal partitioning.

Gause's mixed cultures illustrate the principle: Paramecium aurelia outcompeted P. caudatum under shared conditions, whereas reduced niche overlap can allow coexistence.

If two bird species use the same limiting prey in the same place and time, one may decline; feeding at different heights can reduce overlap and restrict each realized niche rather than eliminate either.

Coexistence does not prove absence of competition, and competitive exclusion requires a limiting shared resource—not merely two species living in the same habitat.