Ecological niches

📚The biology

The ecological niche

An ecological niche is the role of a species in an ecosystem. It includes every interaction that influences the species’ growth, survival and reproduction:

• abiotic interactions — the range of temperature, light, water, pH and so on that it tolerates, and the space and time (day or night, season) in which it is active;
• biotic interactions — how it obtains food, what feeds on it, what it competes with, and its parasites, pathogens and mutualistic partners.

A habitat is an address; a niche is a profession.

Tolerance of oxygen

Photosynthesis

Photosynthesis is the mode of nutrition of plants, algae and several groups of photosynthetic prokaryotes, such as cyanobacteria. These organisms are autotrophs: they use light energy to make their own carbon compounds from carbon dioxide and water (C1.3).

Holozoic nutrition

All animals are heterotrophs: they obtain carbon compounds from other organisms. Most animals use holozoic nutrition, in which food is:

1. ingested — taken into the body;
2. digested — broken down internally by enzymes into small soluble molecules;
3. absorbed — taken across the gut wall into the blood;
4. assimilated — used to build the animal’s own tissues and molecules.

Undigested material is then egested as faeces.

Mixotrophic nutrition

Mixotrophs are both autotrophic and heterotrophic. Euglena, a single-celled freshwater protist, has chloroplasts and photosynthesizes in the light, but can also absorb or ingest organic food, and does so when light is limited. Many species in oceanic plankton are mixotrophs too.

• Facultative mixotrophs can live by either mode alone, switching according to conditions — like Euglena, which can survive in darkness by feeding.
• Obligate mixotrophs need both modes to survive: neither photosynthesis nor feeding alone is enough.

Saprotrophic nutrition

Saprotrophs are heterotrophs that feed on dead organic matter. They secrete digestive enzymes onto their food, digesting it externally, then absorb the products. Many fungi (bread mould, bracket fungi, mushrooms) and bacteria are saprotrophs. They are also called decomposers, and they recycle nutrients locked in dead material back into the ecosystem (C4.2).

The diversity of archaea

Archaea are one of the three domains of life (A3.2). They look like bacteria but are metabolically very diverse, and many live in extreme environments. Different archaea obtain energy for ATP production in three different ways:

Dentition and diet in the Hominidae

Observations of living mammals have led to well-supported theories linking the structure of the teeth and skull to diet. Those theories can then be used deductively to infer the diet of extinct species from their fossil skulls.

Skull models or digital collections let you compare these features directly: tooth size and enamel thickness, the shape of the molar surfaces, the size of the canines, and the attachment areas for jaw muscles.

Herbivores and plant defences

Herbivores are adapted to feed on plants:

• Leaf-eating insects have mouthparts suited to their food. Caterpillars and grasshoppers have chewing mouthparts with strong mandibles that bite off and grind leaf tissue. Aphids have piercing and sucking mouthparts: a needle-like stylet inserted into the phloem to feed on sap.
• Grazing mammals have broad grinding teeth, and gut bacteria that digest cellulose.

Plants resist herbivory in two main ways:

• Physical structures — thorns and spines (acacias, roses), stinging or sticky hairs, and tough, silica-rich leaves (grasses).
• Toxic secondary compounds in seeds and leaves — for example alkaloids such as nicotine and caffeine, and compounds in cassava that release cyanide when the tissue is damaged.

Some herbivores have evolved metabolic adaptations to detoxify these compounds. Koalas have liver enzymes and gut bacteria that break down the toxins in eucalyptus leaves, which most animals cannot eat. Monarch butterfly caterpillars tolerate the toxins of milkweed and even store them, making themselves poisonous to predators.

Predators and prey

Predators are adapted to find, catch and kill prey; prey are adapted to avoid being eaten. Both show chemical, physical and behavioural adaptations.

Harvesting light in forests

In a forest, light is the resource most intensely competed for. Plants use different strategies to reach it:

Fundamental and realized niches

Competitive exclusion

When two species compete for the same limited resource, their niches overlap. The principle of competitive exclusion states that two species cannot occupy exactly the same niche in the same place indefinitely. There are two possible outcomes:

• Elimination of one species. The better competitor out-competes the other, which dies out locally. In classic laboratory experiments with two species of Paramecium grown in the same culture, one species consistently died out, although each thrived when grown alone.
• Restriction of both species to parts of their fundamental niches, so that each occupies a different realized niche and competition is reduced. On rocky shores, one barnacle species can live across a wide vertical band but is confined to the upper shore where a larger competing barnacle occupies the lower shore; when the competitor is removed, it spreads downwards.

Because of competitive exclusion, every species in a stable community has a unique niche.