B4.2 Ecological niches

Every species in an ecosystem has its own ecological niche: its role in that ecosystem. The niche is everything that affects whether the species can survive, grow and reproduce there. It is useful to separate two kinds of factor.

• Abiotic factors are non-living conditions. Each species has a range of tolerance for each one, such as temperature, light intensity, water availability, pH, salinity and oxygen concentration. Together these ranges decide where it can live (its habitat). See B4.1 Adaptation to environment for tolerance limits.
• Biotic factors are interactions with other organisms: the food it eats, the predators, parasites and pathogens that attack it, the species it competes with, and the species it depends on, such as pollinators, seed dispersers or trees that provide nest holes.

How a species obtains food

Food supply is a key part of any niche. A species is either autotrophic (it makes its own carbon compounds) or heterotrophic (it obtains them from other organisms), and within each group there are many specialized ways of doing it. Specializing reduces competition with other species, but each specialized way of feeding needs its own adaptations, such as particular teeth, mouthparts or enzymes. The rest of this page looks at these feeding modes and adaptations.

A niche has many dimensions

Because so many factors are involved, the niche is multidimensional: you can think of it as a region in a space with one axis for each factor. A species can only persist where all of its requirements are met. The idea of a niche also explains why no two species in a community are quite the same (see competitive exclusion, below).

Oxygen concentration is one abiotic dimension of a niche. Plants and animals need oxygen for aerobic respiration, but many microorganisms do not, so they can live in anoxic (oxygen-free) habitats such as waterlogged soils, the mud of swamps and lake beds, deep water and the intestines of animals. Organisms fall into three groups according to their tolerance of oxygen.

Facultative anaerobes grow faster when oxygen is present, because aerobic respiration releases far more energy from each glucose molecule than anaerobic respiration does. This is why, in a tube of broth where oxygen diffuses in only from the top, they grow throughout but most densely near the surface.

Photosynthesis

An autotroph makes all its carbon compounds from simple inorganic substances, using carbon dioxide as its carbon source and an external energy source. In photosynthesis the energy source is light. Three groups of organisms feed in this way:

• plants: mosses, ferns, conifers and flowering plants;
• eukaryotic algae: seaweeds and unicellular algae such as diatoms and Chlorella;
• several groups of prokaryotes, including the cyanobacteria. Many other bacteria are not photosynthetic.

Photosynthesis is therefore found in two of the three domains, Eukarya and Bacteria. (Some bacteria and archaea are autotrophs of a different kind, chemoautotrophs, which use energy from oxidizing inorganic chemicals instead of light.)

Holozoic nutrition

All animals are heterotrophs: they obtain carbon compounds by eating other organisms or their products. Most feed by holozoic nutrition, in which pieces of food are taken into a gut and digested inside the body. There are five stages:

1. Ingestion: taking food into the gut through the mouth.
2. Digestion: enzymes hydrolyse large, insoluble molecules (polysaccharides, proteins, lipids) into small, soluble ones (sugars, amino acids, fatty acids), inside the gut.
3. Absorption: the small molecules pass across the gut wall into the blood (or lymph).
4. Assimilation: body cells use the absorbed molecules to build their own macromolecules, such as proteins, so they become part of the body’s tissues.
5. Egestion: undigested material, such as plant fibre, is expelled from the gut as faeces.

Not every animal is strictly holozoic. Spiders inject digestive enzymes into their prey and suck up the liquefied tissue, so digestion begins outside the body. Tapeworms have no gut at all and absorb food already digested by their host.

Mixotrophic protists

Some protists (unicellular eukaryotes) are neither purely autotrophic nor purely heterotrophic. They are mixotrophs, able to use both modes of nutrition.

• Euglena (Euglena gracilis) is a familiar freshwater example. It has chloroplasts and photosynthesizes when there is enough light, and it can also take in organic compounds from its surroundings and use them heterotrophically. Cells kept in the dark, or that have lost their chloroplasts, can survive as heterotrophs, and in light without organic food they live as autotrophs. Euglena is therefore a facultative mixotroph: it can grow by either mode alone, or both.
• Obligate mixotrophs cannot grow unless they use both modes. This may be because prey supplies a substance they cannot make, or because they have no chloroplasts of their own and instead keep the chloroplasts of algae they eat, which stop working and must be replaced by eating more algae.

Mixotrophs are very common in oceanic plankton, among dinoflagellates, ciliates and other groups. The ciliate Mesodinium rubrum, for example, keeps working chloroplasts from the algae it eats but must keep feeding to maintain them.

Saprotrophic nutrition

Saprotrophs are heterotrophs that feed on dead organic matter, such as dead leaves, wood, dead animals and faeces. Many fungi (for example moulds and the bracket fungi on dead trees) and many bacteria feed this way. They have cell walls, so they cannot ingest food. Instead they:

1. secrete digestive enzymes into the dead matter around them (proteases for proteins, cellulases for cellulose, and so on);
2. digest it externally, producing small soluble molecules;
3. absorb the products across their plasma membrane. The long, thin hyphae of fungi give a large surface area for this.

Saprotrophs are decomposers. By breaking down dead matter they release carbon dioxide and mineral nutrients such as nitrates and phosphates back into the environment, where producers can use them again (see C4.2 Transfers of energy and matter). Not all fungi and bacteria are saprotrophs: some are parasites and some live in mutualistic partnerships.

Living organisms are classified into three domains: Archaea, Bacteria and Eukarya. Archaea are unicellular and have no nucleus, like bacteria, but differ from bacteria in their membrane lipids, cell walls and many of their genes; in some respects they are more similar to eukaryotes. Some archaea live in extreme environments such as hot springs and very salty lakes, but they are also abundant in soils, the oceans and animal guts. Many cannot yet be grown in the laboratory, so they are less well understood than bacteria.

What stands out is how varied archaea are in the way they obtain energy for ATP production:

Methanogens, which produce methane, are archaea; they are obligate anaerobes and live in swamps, rice paddies and the guts of cattle and termites. No archaea are known to carry out photosynthesis using chlorophyll.

The family Hominidae (the great apes) contains orangutans (Pongo), gorillas (Gorilla), chimpanzees and bonobos (Pan) and humans (Homo), together with extinct genera such as Paranthropus. Some hominids are mainly herbivorous; others are omnivorous, eating animal food as well as plants.

From living mammals to fossils

Observing living mammals with known diets has produced theories linking teeth and skulls to diet:

• Cheek teeth (premolars and molars): large, broad, flat teeth grind tough plant food; tall shearing crests slice leaves; small teeth with low, rounded cusps suit a mixed diet.
• Enamel: thick enamel resists wear from hard or gritty foods.
• Front teeth (incisors and canines): used for biting, stripping and display. Large canines alone do not mean meat-eating.
• Chewing muscles: the temporalis muscle attaches to the side of the skull; a sagittal crest along the top and wide zygomatic arches (cheekbones) indicate very large chewing muscles.
• Jaw: a deep, thick mandible withstands strong, repeated chewing forces.

The named examples

Nature of science: deduction and indirect evidence

These diets are deduced from a theory based on living species, and nobody can watch an extinct species eat. The inferences are indirect and could be wrong, because similar teeth can suit different foods. They become more convincing when independent evidence agrees, such as microscopic wear on tooth surfaces, chemical isotopes in tooth enamel, and tools or butchered bones found with the fossils.

Herbivores feed on plants, and plants have evolved ways of resisting them. Each side’s adaptations have been shaped by natural selection acting on the other.

Herbivore adaptations

Insect mouthparts are very diverse, but they are homologous: all have evolved from the same ancestral set of mouthparts.

• Chewing mouthparts: leaf beetles, grasshoppers and caterpillars (the larvae of butterflies and moths) have hard, toothed mandibles that bite off and chew pieces of leaf.
• Piercing and sucking mouthparts: aphids have a long, thin stylet that they push through the leaf or stem tissues into a phloem sieve tube, then feed on the sugary phloem sap.

Mammalian herbivores show other adaptations, such as broad grinding teeth and digestive systems that house microorganisms able to digest cellulose.

Plant defences

• Physical defences: thorns and spines (sharp, pointed modified stems or leaves, as on hawthorn and cacti), stinging hairs that inject irritant chemicals (nettles), and tough leaves strengthened with fibres or silica.
• Chemical defences: toxic secondary compounds, chemicals that are not part of the plant’s basic metabolism. Examples include alkaloids such as nicotine and caffeine, and cyanogenic glycosides, which release cyanide when the tissue is damaged. They are often concentrated in seeds, which are rich in protein, starch or oil and so are especially valuable food.

Detoxification by herbivores

Some herbivores have metabolic adaptations that detoxify these compounds. Koalas, for example, eat almost nothing but eucalyptus leaves, which most mammals cannot tolerate; their liver enzymes break down the toxic compounds in the leaves. Because detoxification is specialized, a toxic plant is usually eaten by only a few herbivore species adapted to it.

Predators are adapted to find, catch and kill their prey; prey are adapted to avoid being eaten. Adaptations of both can be chemical, physical (structural) or behavioural.

These adaptations are linked. If prey evolve better defences, predators with better ways of overcoming them are favoured by natural selection, and vice versa, so the adaptations of predators and prey evolve together.

In a dense forest, light is often the limiting factor for photosynthesis. The leaves of the canopy absorb most of the sunlight, so in a tropical rainforest the forest floor may receive only about 1–2 % of full sunlight. Plants have evolved very different forms for obtaining light.

• Canopy trees grow a tall, thick trunk with a dominant leading shoot, so their crowns reach the full sunlight of the canopy and other trees do not shade them. A tall trunk takes a lot of wood (xylem) to support it.
• Lianas are woody climbing plants rooted in the soil. They climb other trees, using them for support, and spread their leaves in the canopy. Because they are supported, they need much less wood than a free-standing tree of the same height.
• Epiphytes, such as many orchids, bromeliads and ferns, grow on the branches and trunks of trees, high above the dark forest floor. They are not parasites: their roots do not enter the soil or the tree, so they obtain water from rain and humid air and minerals from debris that collects around them.
• Strangler epiphytes, such as strangler figs, germinate on a branch from seeds left by birds or bats. They grow like epiphytes at first, then send long roots down the trunk into the soil. The roots thicken and join to encircle the trunk, and the fig’s crown grows over the host’s crown and shades it. The host tree often dies, leaving the fig standing.
• Shade-tolerant shrubs and herbs on the forest floor have large, thin leaves with plenty of chlorophyll and low rates of respiration, so they can photosynthesize faster than they respire even in dim light.

The fundamental niche of a species is its potential niche: the full range of conditions and resources it could use, set by its adaptations and tolerance limits. If there were no competitors, the species would occupy all of it. In real ecosystems, other species compete for the same resources and may exclude it from parts of that range. The part of the fundamental niche a species actually occupies is its realized niche.

Barnacles on a rocky shore

The classic evidence comes from Joseph Connell’s experiments on the Isle of Cumbrae in Scotland, published in 1961. Two barnacles live on the rocky shore:

• Chthamalus stellatus adults are found only on the upper shore, although its larvae settle lower down as well.
• Semibalanus balanoides lives on the middle and lower shore.

When Connell removed Semibalanus from patches of rock, Chthamalus survived and grew in the lower zone. On untouched rock, the faster-growing Semibalanus smothered, undercut or crushed young Chthamalus. So competition, not its tolerance of the conditions, restricts Chthamalus to a realized niche narrower than its fundamental niche. Semibalanus, by contrast, did not spread upwards when Chthamalus was removed: it cannot withstand drying out (desiccation) during the long periods the upper shore is exposed to air at low tide, an abiotic limit.

When the fundamental niches of two species overlap, they compete for the shared resources. The competitive exclusion principle states that two species with identical niches cannot coexist indefinitely in the same place. There are two possible outcomes:

1. Elimination: one species outcompetes the other everywhere in the shared part of their niches, and the weaker competitor dies out in that ecosystem.
2. Restriction: each species is the better competitor in part of the range, so both survive, each restricted to part of its fundamental niche.

Either way, every species that persists ends up with a realized niche that is different from that of every other species: niches are unique.

Evidence

• Gause (1934) grew species of Paramecium in tubes of culture medium. Alone, P. aurelia and P. caudatum each grew to a stable population, but when grown together, both feeding on bacteria, P. caudatum died out. P. caudatum and P. bursaria did coexist, because P. bursaria fed mainly on yeast cells at the bottom of the tube: their niches differed.
• In flour beetles (Tribolium), which species wins depends on conditions: T. castaneum usually excluded T. confusum in hot, humid flour, and T. confusum won in cool, dry flour.
• Five species of warbler coexist in the same spruce forests of North America by feeding in different parts of the trees, so their realized niches differ.

The model shows the key idea. When each species limits its own growth more than it limits the other’s, both persist at lower numbers than alone. With very unequal carrying capacities, even weak competition can exclude one species. When one species competes strongly with the other, the weaker one is excluded, however fast it grows.