B4.2 Ecological nichesIB Biology SL: Revision notes

An ecological niche is the role of a species in its ecosystem: all the biotic and abiotic interactions that affect its growth, survival and reproduction, including how it obtains food.

Oxygen is one abiotic factor that limits where organisms can live:

• Obligate aerobes need oxygen for respiration and cannot survive without it.
• Facultative anaerobes respire aerobically when oxygen is present and anaerobically when it is absent; they grow best with oxygen because aerobic respiration yields more ATP.
• Obligate anaerobes can live only where oxygen is absent; oxygen is toxic to them.

Each group has a tolerance limit for the presence or absence of oxygen, which restricts it to particular habitats, for example waterlogged soils or sediments for obligate anaerobes.

Autotrophs make their own carbon compounds. Plants, algae and several groups of photosynthetic prokaryotes use photosynthesis.

All animals are heterotrophic and use holozoic nutrition: food is ingested, digested internally, absorbed and assimilated into the body's tissues.

Some protists are mixotrophic: they photosynthesise and also feed heterotrophically. Euglena is a freshwater example, and many oceanic plankton are mixotrophs. Facultative mixotrophs can survive using either mode alone; obligate mixotrophs need both.

Saprotrophs (many fungi and bacteria) secrete enzymes onto dead organic matter, digest it externally and absorb the products. They are decomposers.

Archaea, one of the three domains of life, are metabolically very diverse: different species obtain energy for ATP production from light, the oxidation of inorganic chemicals or the oxidation of carbon compounds.

Teeth reveal diet. Herbivores that eat tough, abrasive plants have large, flat molars and premolars with thick enamel for grinding; omnivores have smaller, less specialised teeth.

• Paranthropus robustus: huge flat molars, thick enamel, massive jaw and a sagittal crest for large chewing muscles, indicating a diet of hard, tough plant material.
• Homo sapiens: small molars and small canines, suited to a varied omnivorous diet (much of it processed or cooked).
• Homo floresiensis: small teeth, found with stone tools and cut-marked bones, suggesting an omnivorous diet.

NOS — deductions from theories: observing living mammals led to theories linking dentition to herbivorous or carnivorous diets. These theories allow the diet of extinct species to be deduced from fossil skulls.

Herbivore adaptations include the chewing mouthparts of leaf-eating insects such as caterpillars and beetles, and the piercing mouthparts of sap-feeding insects such as aphids.

Plants resist herbivory with physical structures such as thorns and spines, and with toxic secondary compounds in seeds and leaves. Some animals have metabolic adaptations that detoxify these compounds, allowing them to eat plants others cannot.

Predators have chemical, physical and behavioural adaptations for finding, catching and killing prey, such as keen senses, speed, claws or venom. Prey resist predation using chemical defences (toxins, often with warning colours), physical defences (camouflage, spines, shells) and behaviour (hiding, freezing, living in groups).

In forests, plants use different strategies to reach light:

• Canopy trees grow tall trunks to hold leaves in full sunlight.
• Lianas root in the soil and climb other trees, reaching the canopy without building a self-supporting trunk.
• Epiphytes grow on branches high in the canopy, reaching light without soil roots.
• Strangler epiphytes (strangler figs) germinate on branches, send roots to the ground and eventually surround and replace the host tree.
• Shade-tolerant shrubs and herbs survive in the low light of the forest floor, often with broad, thin leaves.

The fundamental niche is the potential niche of a species, set by its adaptations and tolerance limits. The realized niche is the actual extent of its niche when it is in competition with other species; it is usually smaller.

The competitive exclusion principle states that two species cannot occupy the same niche in the same place indefinitely. Competition has two possible outcomes:

• elimination of one species from the area;
• restriction of both species to part of their fundamental niche.

So each ecological niche is unique. Barnacles on rocky shores are a classic example: Chthamalus can live in the lower zone when its competitor is removed, but is normally confined to the upper zone.