Ecological niches

Biology · B Form and function. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An ecological niche is the role of a species in its ecosystem. It includes where the species lives, how it gets its food and energy, the abiotic conditions it tolerates and how it interacts with other species.

The fundamental niche is the full range of conditions and resources a species could use if it had no competitors. The realized niche is the smaller part it actually uses when other species are present.

Two species cannot share exactly the same niche for long. One will use the resources better and the other will die out or move to a different niche. This is competitive exclusion.

Rules to remember

Obligate aerobes need oxygen. Obligate anaerobes are killed or inhibited by oxygen. Facultative anaerobes use oxygen when it is there but can live without it.
Photosynthetic: makes carbon compounds using light. Holozoic: takes in solid food, then digests it inside the body. Saprotrophic: secretes enzymes onto dead matter and absorbs the products. Mixotrophic: can do both autotrophic and heterotrophic nutrition (Euglena).
Holozoic stages in order: ingestion, digestion, absorption, assimilation, egestion.
Archaea show great variety: some use light, some oxidize inorganic chemicals, some oxidize carbon compounds.
Hominid teeth: large flat molars with thick enamel and a heavy jaw suggest a tough plant diet; smaller teeth and a lighter jaw suggest a mixed diet with softer food or meat.
Plants defend with thorns and toxins; some herbivores evolve ways to detoxify them. Predators have speed, claws or venom; prey use camouflage, warning colours or chemical defence.
Forest plants reach light in different ways: tall trees, lianas that climb, epiphytes that grow on branches, shade-tolerant shrubs and herbs.

Common mistake

Students mix up the two niches. The realized niche is never larger than the fundamental niche: competition can only take part of it away.

Worked example

A bacterium is stabbed deep into a tall tube of nutrient agar. After incubation it has grown only at the bottom of the tube. How is it classified?

Oxygen diffuses in from the air, so there is most oxygen at the top and almost none at the bottom.
The bacterium grows only where oxygen is absent, so oxygen stops its growth.
An organism that cannot grow in the presence of oxygen is an obligate anaerobe.

Answer: It is an obligate anaerobe.

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B4.2 Ecological Niches

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