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.

Species differ in oxygen tolerance: some require oxygen-rich conditions, while others can survive or grow when oxygen is scarce.

Oxygen availability changes cellular respiration and the kinds of metabolism possible. The tolerance range therefore determines which water layers, sediments or hosts a species can occupy.

Compare species by: oxygen requirement; response as oxygen falls; habitat zone; metabolic strategy.

A fish that needs high dissolved oxygen may disappear from a warm, stagnant pond while an anaerobic microbe remains active in the sediment.

Low oxygen is not automatically lethal to every organism; judge it against the species’ physiology.

Photosynthetic organisms use light energy to build organic molecules from inorganic carbon, forming the basis of many food webs.

Pigments absorb photons and drive electron transfer, producing chemical energy that supports carbon fixation. Light availability and carbon dioxide therefore constrain this nutrition mode.

Identify: light capture; carbon source; energy conversion; organic product.

A green plant uses absorbed light to make carbohydrates from carbon dioxide and water, then uses them for growth.

Photosynthesis is not simply ‘feeding on sunlight’: light supplies energy, while carbon is incorporated into organic molecules.

Holozoic nutrition involves ingesting organic food, digesting it into absorbable molecules and assimilating those molecules into the organism.

The organism obtains complex material from another source, breaks it down mechanically or chemically, absorbs products and uses them for energy, growth or repair.

Trace: ingestion; digestion; absorption; assimilation; egestion of indigestible residue.

A mammal eats starch, hydrolyses it to glucose, absorbs the glucose and uses it in respiration or storage.

Ingestion alone is not holozoic nutrition; digestion and assimilation complete the process.

Mixotrophs can combine autotrophic production with heterotrophic feeding, changing the balance according to light, nutrients or prey availability.

Photosynthesis supplies carbon and energy when conditions permit, while ingestion or absorption provides an alternative when light or inorganic nutrients are limiting.

Classify the mode under the stated condition: light available; carbon source; external food; metabolic output.

A photosynthetic protist can make carbohydrates in bright light but ingest bacteria when light is scarce.

Mixotrophy is not simultaneous use of every mode in every condition; the balance depends on environment and species.

Saprotrophic organisms secrete enzymes onto dead organic matter, digest it externally and absorb the soluble products.

External digestion lets fungi and microbes access large or insoluble material. Enzyme action converts polymers into smaller molecules that can cross the organism’s surface.

Trace: dead substrate; extracellular enzymes; hydrolysis; soluble products; absorption.

A fungus secretes enzymes onto leaf litter, releases sugars and amino acids and absorbs them for respiration and growth.

Saprotrophs do not ingest intact food in the animal sense; digestion occurs outside the cells or body.

Archaea include organisms adapted to chemically extreme or oxygen-poor environments, using varied energy sources and carbon pathways.

Membranes, enzymes and metabolic pathways can function under high salt, heat, acidity or anaerobic conditions. Their diversity reflects different environmental constraints rather than one universal archaeal lifestyle.

Describe an archaeal niche by naming: stressor; energy source; electron acceptor or carbon source; adaptation.

A methanogenic archaeon can obtain energy anaerobically in oxygen-free sediments, where methane is produced as a metabolic end product.

Not all archaea are extremophiles and not all produce methane; match the claim to the lineage and metabolism.

Hominid dentition reflects the mechanical demands of diet: tooth shape and arrangement influence cutting, tearing and grinding.

Incisors cut, canines can pierce or tear and molars grind. Jaw mechanics and enamel wear provide clues about food texture, although behavior and food preparation also matter.

Infer diet cautiously by checking: tooth form; wear pattern; jaw movement; likely food texture.

Broad molars with thick enamel are consistent with repeated grinding of tough or abrasive foods.

Dentition does not identify a complete diet by itself; convergent tooth shapes and food processing can mislead.

Herbivores have feeding structures and digestive strategies for plant material, while plants evolve physical or chemical defenses that reduce consumption.

Herbivore teeth, gut microbes and jaw movement affect what can be eaten. Plants counter with spines, tough tissues or toxins, creating reciprocal selection pressures.

Link each trait to the interaction: herbivore feeding problem; plant defense; cost or counter-adaptation.

A ruminant’s microbial gut helps digest cellulose, while a thorny plant makes leaf access more costly for grazers.

A defense reduces herbivory but may carry growth or chemical costs; it does not make a plant invulnerable.

Predators evolve traits that improve detection, capture and handling, while prey evolve traits that reduce detection or increase escape.

Each side changes the selection environment for the other. Speed, camouflage, warning signals, armor or toxins are useful only relative to the opposing strategy.

Evaluate a trait by asking: which interaction step it changes; which opponent responds; what trade-off follows.

Camouflage can lower a prey animal’s detection probability, while a predator may evolve sharper vision or a different search pattern.

A conspicuous color is not always a camouflage failure; it may warn predators of toxicity.

Plant architecture changes leaf area, angle and spacing to capture light while reducing self-shading and water or structural costs.

Leaves compete for photons, so height, branching and canopy arrangement determine which surfaces are illuminated. Form also affects transport distance, support and transpiration.

Explain a light-harvesting form by linking: light distribution; leaf position/angle; carbon gain; trade-off.

A canopy leaf held at a different angle can receive more direct light while reducing shading of leaves below.

More leaf area does not guarantee more photosynthesis if nutrients, water or CO₂ become limiting.

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.

If two species use exactly the same limiting resources in the same way, stable coexistence is unlikely; one may outcompete the other or the niches may shift.

Small differences in resource use, timing or space can reduce competition and permit coexistence. Competitive exclusion is therefore a prediction about overlap and limiting factors, not a universal rule that one species always disappears.

Test the claim by identifying: shared limiting resource; degree of overlap; fitness advantage; any partitioning mechanism.

Two bird species can coexist when one feeds in the canopy and the other on the ground, reducing overlap despite sharing the forest.

Coexistence does not prove zero competition; it may reflect partial partitioning or changing conditions.