B4.2 Ecological Niches - IB Biology 2025

B4.2 Ecological Niches

SL and HL Combined Content

IB Guiding Questions

• What are the advantages of specialized modes of nutrition to living organisms?
• How are the adaptations of a species related to its niche in an ecosystem?

B4.2.1: Ecological Niche

• Ecological niche is the role of a species in an ecosystem. Include the biotic and abiotic interactions that influence growth, survival and reproduction, including how a species obtains food. An ecological niche describes the role of an organism in an ecosystem.
• Every species of organism is adapted to a unique niche in an ecosystem.
• The niche of an organism includes biotic and abiotic factors that affects:
  • Growth of individuals
  • Survival of individuals
  • Ability to reproduce
  • Mode of nutrition
  • Interactions with other species
  • Interactions with its own species

B4.2.2: Anaerobes and Aerobes

• Differences between organisms that are obligate anaerobes, facultative anaerobes and obligate aerobes.
• Limit to the tolerance of these groups of organisms to the presence or absence of oxygen gas in their environment.
• Oxygen is an abiotic factor that determines where an organism can live.
• Obligate anaerobes: Cannot survive in the presence of oxygen, and must live in oxygen-free environments such as waterlogged soil. They carry out anaerobic respiration.
• Facultative anaerobes: Can survive in the presence or absence of oxygen. Facultative anaerobes carry out anaerobic respiration in the absence of oxygen, and aerobic respiration in the presence of oxygen.
• Obligate aerobes: Cannot survive in the absence of oxygen. They must live in environments where oxygen is available, and carry out aerobic respiration.

B4.2.3: Photosynthesis

• Photosynthesis as the mode of nutrition in plants, algae and several groups of photosynthetic prokaryotes.
• Details of different types of photosynthesis in prokaryotes are not required.
• Photosynthesis is the production of organic compounds in cells using light energy.
• Photosynthetic organisms convert carbon dioxide and water using light energy into organic compounds, such as glucose, and oxygen gas.

B4.2.4: Holozoic Nutrition

• Holozoic nutrition in animals.
• Students should understand that all animals are heterotrophic. In holozoic nutrition food is ingested, digested internally, absorbed and assimilated.
• Photosynthetic organisms are autotrophs as they produce their own food.
• Heterotrophs cannot make their own food, and they must take their food from other organisms. All animals are heterotrophic.
• Holozoic nutrition is a form of heterotrophic nutrition where an organism:
  • Ingests food
  • Internally digests food
  • Absorbs and assimilates the nutrients from digested food.

B4.2.5: Mixotrophic Nutrition

• Mixotrophic nutrition in some protists.
• Euglena is a well-known freshwater example of a protist that is both autotrophic and heterotrophic, but many other mixotrophic species are part of oceanic plankton.
• Students should understand that some mixotrophs are obligate and others are facultative.
• Mixotrophs can behave as an autotroph by carrying out photosynthesis, and act as heterotrophs by feeding on other organisms.
• Mixotrophs may be:
  • Obligate mixotrophs, which require both autotrophic and heterotrophic modes of nutrition to survive.
  • Facultative mixotrophs, which can switch between the two modes of nutrition based on available resources. They can obtain all of their required nutrients using either autotrophic or heterotrophic modes of nutrition.

B4.2.6: Saprotrophic Nutrition

• Saprotrophic nutrition in some fungi and bacteria.
• Fungi and bacteria with this mode of heterotrophic nutrition can be referred to as decomposers.
• Saprotrophs are heterotrophs that obtain nutrients by external digestion of food.
• Saprotrophic bacteria and fungi are referred to as decomposers.
• Saprotrophs release enzymes, which are used to externally digest food.
• The nutrients are absorbed by the saprotroph for assimilation after digestion.

B4.2.7: Nutrition in Archaea

• Diversity of nutrition in archaea.
• Students should understand that archaea are one of the three domains of life and appreciate that they are metabolically very diverse.
• Archaea species use either light, oxidation of inorganic chemicals or oxidation of carbon compounds to provide energy for ATP production.
• Students are not required to name examples.
• Many archaea live in extreme environments such as hot springs.
• Archaea are one of the three domains of life.
• Archaea are prokaryotes that are found in a wide variety of environments, and they are metabolically diverse.
• The metabolism of archaea is diverse, but they all produce ATP. Methods of producing ATP in archaea include:
  • Phototrophs: Some archaea use light energy to produce ATP. This is a different process than photosynthesis and no oxygen is produced.
  • Chemolithotrophs: Some archaea oxidize inorganic compounds to produce ATP. Different species of archaea use different inorganic compounds such as ammonia, nitrites and sulfur compounds.
  • Organotrophs: Some archaea oxidize organic compounds, such as sugars and fatty acids, to produce ATP.

B4.2.8: Dentition

• Relationship between dentition and the diet of omnivorous and herbivorous representative members of the family Hominidae.
• Application of skills: Students should examine models or digital collections of skulls to infer diet from the anatomical features. Examples may include Homo sapiens (humans), Homo floresiensis and Paranthropus robustus.
• Nature of Science: Deductions can be made from theories. In this example, observation of living mammals led to theories relating dentition to herbivorous or carnivorous diets. These theories allowed the diet of extinct organisms to be deduced.
• Family Hominidae is the family of the great apes, which are all tailless primates. Members of the family Hominidae are often referred to as hominids.
• The family Hominidae includes modern humans (Homo sapiens) and all of our recent ancestors including Homo floresiensis and Paranthropus robustus.
• Some members of the family Hominidae are herbivores and others are omnivores.
• Herbivores eat predominantly plants. Herbivorous hominids have large flat teeth for grinding seeds, along with strong jaws.
• Omnivores eat plants and animals. Omnivorous hominids have a mixture of sharp incisors for ripping through meat, and flat molars.
• Homo sapiens are omnivores. This is evident from examining the teeth as there are sharp incisors, and small flat molars.
• Homo floresiensis were probably herbivores, as they had large relatively flat molars.
• Paranthropus robustus were probably herbivores, as they had very large flat molars.
• Nature of Science: Observations of living animals allow scientists to develop theories regarding dentition of extinct species. Scientists can make predictions about the diet of extinct species, including hominids, by examining the anatomy of fossils, and comparing them to the anatomy of living descendants.

B4.2.9: Herbivore Adaptations

• Adaptations of herbivores for feeding on plants and of plants for resisting herbivory.
• For herbivore adaptations, include piercing and chewing mouthparts of leaf-eating insects.
• Plants resist herbivory using thorns and other physical structures. Plants also produce toxic secondary compounds in seeds and leaves. Some animals have metabolic adaptations for detoxifying these toxins.
• Plants have evolved many adaptations to prevent herbivores from consuming them.
• Physical Adaptations of plants against insect herbivores include:
  • Physical structures such as thorns on blackberry bushes
  • Sharp trichomes on stinging nettles also contain irritating chemicals
  • Plants like grass have tough fibrous leaves
• Plants produce a range of chemicals to deter herbivores from eating them including:
  • Chilli plants produce capsaicin in fruits and seeds, which causes a burning sensation in the digestive system of many herbivores. However some birds and humans are not deterred by capsaicin.
  • Tobacco plants produce the toxin nicotine, which acts as a deterrent to herbivores consuming the tobacco plant. However, the caterpillars of the tobacco hawk moth (known as tobacco hornworm) have evolved to detoxify nicotine and can feed on the tobacco plant.
• Grass cells have tough cell walls which contain silica, which makes them difficult to chew for animals. However, grazing mammals have the following adaptations so that they can eat grass:
  • Flat molars for grinding grass
  • Teeth continue to grow throughout the life of the animal, so they do not become worn down.
• Many insects have evolved chewing mouthparts to consume plants
  • Grasshoppers have sharp mandibles adapted for eating grass
  • They have strong sharp mandibles with serrated edges that are able to cut through the cell walls in the leaves of plants.
  • The insects have strong muscles which manipulate the mandibles to bite and grind leaves.
  • Some insects have evolved piercing mouthparts, which allows them to feed on plants.
  • Aphids have stylets which they insert directly into the phloem of plants, to obtain the nutrients in the phloem of plants.
  • Stylets are hollow slender tubes which aphids can guide directly into the phloem of plants.

B4.2.10: Predator/Prey Adaptations

• Adaptations of predators for finding, catching and killing prey and of prey animals for resisting predation.
• Students should be aware of chemical, physical and behavioural adaptations in predators and prey.
• Predators hunt and kill prey for food.
• Predators have evolved adaptations to hunt and kill prey.
• Prey have evolved adaptations to avoid being killed by predators.
• Physical adaptations play a role in a predator’s ability to catch its prey. Physical adaptations include:
  • Sharp claws and talons: Sharp claws and talons allow predators to grip and hold on to prey.
  • Powerful jaws and teeth: Sharp teeth aid predators to capture and kill prey.
  • Speed and agility: Predators have evolved to use speed to chase and catch their prey.
  • Camouflage: Many predators use camouflage to blend in with the environment. Camouflage either allows predators to stealthily approach prey undetected, or to wait in ambush for prey.
• Some predators have evolved chemical adaptations to kill prey, which include the production of venom in glands.
  • Komodo dragons have a venom gland in their lower jaw. The venom contains toxins that prevent blood clotting.
  • The predator is adapted to inject the venom into the prey animal.
  • The venom contains toxins that immobilize or kill the prey animal.
  • Many snakes kill their prey using venom. Many insect predators such as scorpions inject their prey with venom.
• Predators have evolved behaviours that improve their ability to capture prey. Behavioural adaptations include:
  • Hunting in a pack: Some predators hunt in groups, which allow predators to overwhelm large prey.
  • Ambush tactics: Some predators lie in wait for prey to come within striking distance. Many predators that ambush prey use camouflage to remain hidden.
• Physical adaptations play a role in a prey animal’s ability to avoid being captured by a predator. Physical adaptations include:
  • Armour or protective coverings: Protective coverings make it more difficult for the predator to kill the prey animal. Protective coverings include spikes on hedgehogs, shells of snails or turtles and hard exoskeletons of insects and crabs.
  • Swift and agile: Many prey animals have evolved to be swift and agile so that they can outrun and avoid predators.
  • Camouflage: Some prey animals have evolved colours and patterns that allow them to blend into the environment, so that they can avoid detection by predators. Some prey animals use mimicry for camouflage.
  • Mimicry: Some prey species have evolved to mimic their environment, to avoid detection from predators. Some prey animals mimic animals which predators recognize as poisonous or unpalatable, so the predator avoids capturing the mimic.
• Some prey animals have evolved chemical adaptations to discourage predators from eating them.
  • Many prey animals produce toxic or unpalatable chemicals which discourages predators from eating them. Most of these animals have evolved physical adaptations of bright colours and patterns to warn predators that they are toxic.
• Prey animals have evolved behaviours that improve their ability to avoid capture by predators. Behavioural adaptations include:
  • Travelling in groups: Many animals travel in groups. The group provides protection for individual animals against predators.
  • Swarms: Animals moving in a swarm can confuse predators, making it difficult for them to identify and attack individuals.
  • Alarm calls: Many prey animals have evolved vocalizations that alert others to the presence of a predator in the vicinity. This allows time for the animals to take evasive action.
  • Nocturnality or diurnality: Prey animals may adapt when they are active to avoid the times when predators are most active.

B4.2.11: Plant Adaptations

• Adaptations of plant form for harvesting light.
• Include examples from forest ecosystems to illustrate how plants in forests use different strategies to reach light sources, including trees that reach the canopy, lianas, epiphytes growing on branches of trees, strangler epiphytes, shade-tolerant shrubs and herbs growing on the forest floor.
• Plants are autotrophs which require light for photosynthesis.
• Plants compete for light, and plants have evolved different strategies to obtain sufficient light. Light harvesting strategies include:
  • Trees that reach the canopy
  • Lianas
  • Epiphytes growing on branches of trees
  • Strangler epiphytes
  • Shade-tolerant shrubs and herbs growing on the forest floor.
• Adaptations of canopy trees for harvesting light include:
  • Height and crown structure: These trees have tall trunks to reach the sunlight, and broad crowns to maximize the absorption of light.
  • Broad leaves: Broad flat leaves to maximize the absorption of light. The leaves grow in arrangements that minimize self shading.
• Lianas are woody vines with the following adaptations for harvesting light:
  • Climbing mechanisms: Lianas climb the trunks of canopy trees to reach the light. Different species have evolved different mechanisms for climbing such as twisting around trunks, using adhesive pads and using hook like structures to latch onto vegetation.
  • Rapid growth: Rapid growth of stems and leaves allows lianas to quickly reach the sunlight.
  • Flexible and thin stems: Flexible stems allow lianas to grow around obstacles as they grow towards the light.
  • Large broad leaves: Lianas produce large broad leaves when they reach the light, to maximize light absorption for photosynthesis.
• An epiphyte is a plant that grows on another plant. Most epiphytes do not harm the host plant. Adaptations of epiphytes include:
  • Growing on canopy trees.: Epiphytes grow on the branches of canopy trees, allowing them to harvest light that filters through the canopy.
  • Broad flat leaves: Epiphytes have broad flat leaves that maximize the surface area for light absorption.
  • Flexible growth: Epiphytes adjust their growth towards the light. They can reorient their leaves to gaps in the canopy.
• Strangler epiphytes germinate in the branches of canopy trees, and then send aerial roots downwards to the forest floor.
• Eventually the strangler epiphyte surrounds the trunk of the host tree, and outcompetes it for light. The strangler epiphyte gains access to the canopy by strangling the host tree.
• Shade tolerant plants live on the forest floor, and have adapted to cope with low levels of light. Adaptations of shade tolerant plants include:
  • Branching: Branching allows shrubs and herbs to increase their surface area for absorbing light
  • Broad leaves: Broad leaves allow shade tolerant plants to maximize absorption of available light.
  • High concentration of chlorophyll: Higher concentrations of chlorophyll allow shade tolerant plants to capture more of the available light.

B4.2.12: Fundamental vs Realized Niches

• Fundamental and realized niches.
• Students should appreciate that fundamental niche is the potential of a species based on adaptations and tolerance limits and that realized niche is the actual extent of a species niche when in competition with other species.
• A fundamental niche is the niche that an organism could potentially occupy in the absence of competition from other species.
• The fundamental niche of an organism is determined by their adaptations to the environment, and their zones of tolerance.
• A realized niche is the niche that an organism does occupy due to competition from other species.

B4.2.13: Competitive Exclusion

• Competitive exclusion and the uniqueness of ecological niches.
• Include elimination of one of the competing species or the restriction of both to a part of their fundamental niche as possible outcomes of competition between two species.
• Organisms compete for resources. Two species competing for the same resources leads to competitive exclusion.
• Competitive exclusion states that no two species can occupy the same niche.
• Two species cannot exist in the same niche, as one will be better adapted for the niche, and will outcompete and exclude the other species from the niche.
• The less well adapted species will either be eliminated from the niche, or restricted to a part of the niche.
• Competitive exclusion leads to species occupying their realized niche.

Key Terms

• Ecological Niche
• Biotic
• Abiotic
• Obligate Anaerobes
• Facultative Anaerobes
• Obligate Aerobes
• Photosynthesis
• Prokaryote
• Autotrophic Nutrition
• Holozoic Nutrition
• Heterotrophic Nutrition
• Mixotrophic Nutrition
• Obligate Mixotroph
• Facultative Mixotroph
• Saprotrophic Nutrition
• Decomposer
• Archaea
• Phototrophs
• Chemolithotrophs
• Organotrophs
• Omnivore
• Herbivore
• Family Hominidae
• Predator
• Prey
• Camouflage
• Venom
• Mimicry
• Nocturnal
• Diurnal
• Lianas
• Epiphytes
• Strangler Epiphytes
• Shade Tolerant Plants
• Fundamental Niche
• Realized Niche
• Competitive Exclusion