Ecological niche

• Ecological niche = the role of a species in an ecosystem.

• Includes biotic interactions and abiotic interactions that affect growth, survival and reproduction.

• A niche includes how a species obtains food and how it uses resources in its environment.

• Exam focus: link a species’ adaptations directly to its niche.

Nutrition and ecological niches

• Obligate anaerobes can live only where oxygen is absent.

• Obligate aerobes require oxygen to live.

• Facultative anaerobes can tolerate both presence and absence of oxygen.

• Photosynthesis is the mode of nutrition in plants, algae and some photosynthetic prokaryotes.

• Holozoic nutrition occurs in animals: food is ingested, digested internally, absorbed and assimilated.

• Mixotrophic nutrition occurs in some protists such as Euglena: they can be both autotrophic and heterotrophic.

• Some mixotrophs are obligate and others are facultative.

• Saprotrophic nutrition occurs in some fungi and bacteria; these are often called decomposers.

• Archaea are metabolically diverse: they may use light, oxidation of inorganic chemicals, or oxidation of carbon compounds to produce ATP.

This labeled Euglena diagram helps explain mixotrophic nutrition. Euglena has chloroplasts for photosynthesis and can also obtain organic food heterotrophically. It is useful for linking structure to mode of nutrition. Source

Feeding adaptations and diet

• Dentition can be used to infer diet in members of the Hominidae.

• Students may be asked to examine skulls or images of skulls to infer whether a species was more omnivorous or herbivorous.

• Examples named in the syllabus include Homo sapiens, Homo floresiensis and Paranthropus robustus.

• In exam questions, relate tooth shape, grinding surfaces and overall skull features to likely diet.

• Herbivores may show feeding adaptations such as piercing mouthparts and chewing mouthparts in leaf-eating insects.

• Plants resist herbivory with thorns and other physical structures.

• Plants also produce toxic secondary compounds in seeds and leaves.

• Some herbivores have metabolic adaptations for detoxifying toxins.

This comparison links dentition to diet, showing how tooth shape differs in herbivores, omnivores and carnivores. It is useful for exam questions that ask you to infer feeding strategy from skull features. Focus on the contrast between cutting teeth and grinding teeth. Source

Predators, prey and defensive adaptations

• Predators have adaptations for finding, catching and killing prey.

• Prey animals have adaptations for resisting predation.

• Adaptations can be chemical, physical or behavioural.

• Exam tip: always state how the adaptation improves success in survival or feeding.

Plant form and harvesting light

• Plant form is part of a species’ ecological niche because it determines how it accesses light.

• In forests, different plants exploit different light environments.

• Canopy trees grow tall to reach the strongest light.

• Lianas climb other plants to access light without investing heavily in supportive tissue.

• Epiphytes grow on branches of trees to reach light.

• Strangler epiphytes begin on trees and later grow around them.

• Shade-tolerant shrubs and forest-floor herbs are adapted to low-light conditions.

• Exam focus: relate plant form to light availability and position in the forest.

This page helps visualize how plant forms are adapted for harvesting light in forests. It is useful when revising the light gradient from forest floor to canopy and why different growth forms occupy different layers. Link this to niche differentiation. Source

Fundamental and realized niches

• Fundamental niche = the potential niche of a species based on its adaptations and tolerance limits.

• It shows where a species could live if there were no restricting biotic interactions such as competition.

• Realized niche = the actual niche occupied by a species when competition with other species occurs.

• The realized niche is often smaller than the fundamental niche.

• Exam tip: if competition is introduced, expect the realized niche to become more restricted.

This diagram shows that a fundamental niche is the full range of conditions and resources a species could use, while the realized niche is reduced by competition and other biotic interactions. It is one of the most important diagrams for exam questions on niche definitions. Use it to explain potential vs actual distribution. Source

Competitive exclusion and uniqueness of niches

• Competitive exclusion occurs when two species compete for the same niche and the same limiting resources.

• One possible outcome is elimination of one competing species.

• Another possible outcome is that both species are restricted to only part of their fundamental niche.

• This supports the idea that ecological niches are unique.

• Exam tip: if two species coexist, explain this by niche separation or restriction to different parts of available resources/conditions.

These graphs show competitive exclusion using Paramecium aurelia and P. caudatum. Each species grows well alone, but together P. aurelia outcompetes P. caudatum. This is the classic visual example for explaining why two species cannot occupy the same niche indefinitely. Source

Exam connections and common mistakes

• Do not confuse habitat with niche: habitat = where an organism lives; niche = its role and interactions.

• Do not define realized niche as just “where it lives”; mention competition.

• When comparing nutrition types, always include the source of energy/food.

• When identifying adaptations, explain the advantage rather than just listing the feature.

• In data or scenario questions, connect adaptation → niche → survival/reproduction.

Key takeaways

• A niche combines a species’ role, resource use and interactions.

• Different modes of nutrition help species occupy different niches.

• Adaptations for feeding, defence and light capture help reduce competition and increase survival.

• Competition restricts species from their fundamental niche to their realized niche.

• Competitive exclusion explains why no two species can occupy exactly the same ecological niche over time.