1The idea in one paragraph

A habitat is where an organism lives; its niche is what it does there. The niche includes everything that affects its growth, survival and reproduction: the abiotic conditions it tolerates, such as whether it can live with or without oxygen, and its biotic interactions, above all how it gets its food. The big divide is between autotrophs, which make their own carbon compounds, and heterotrophs, which take them from other organisms, with mixotrophs doing both and archaea doing it in more ways than anyone. Each way of feeding comes with its own adaptations: teeth that fit a diet, mouthparts that fit a leaf, plants armed against being eaten, predators and prey locked in a contest, forest plants each with their own route to light. Two species cannot share exactly the same niche for long: competition either removes one or squeezes both into part of what they could occupy.

2The ecological niche

An ecological niche is the role of a species in an ecosystem. It is described by all the interactions that influence the species' growth, survival and reproduction.

• Abiotic interactions: the range of temperature, water, light, salinity, oxygen and so on that the species tolerates and uses.
• Biotic interactions: what it eats and what eats it, what it competes with, what parasites and pathogens attack it, what it depends on, such as pollinators or partners.
• How it obtains food: the most important part of any niche, and most of this page.

A habitat is an address; a niche is an occupation. Two species can share a habitat, a pond or a patch of forest floor, but if they share every part of the niche as well, trouble follows (section 11).

3Living with and without oxygen

Oxygen is an abiotic factor that divides organisms sharply. Figure 1 shows how three types of bacteria grow in tubes of nutrient jelly, where oxygen diffuses in from the top.

The word obligate means "has no choice"; facultative means "can do either". A facultative anaerobe grows best near the top because aerobic respiration yields far more ATP per glucose than anaerobic respiration. The guide limits you to tolerance of oxygen, so you do not need the biochemistry behind why oxygen harms obligate anaerobes.

4Autotrophs, heterotrophs and the modes of nutrition

Figure 2 maps the modes of nutrition covered in this subtopic.

Photosynthesis is the mode of nutrition of plants, of algae (from single-celled ones to seaweeds) and of several groups of photosynthetic prokaryotes, of which cyanobacteria are the best known. Photosynthetic organisms are autotrophs: they make their own carbon compounds from carbon dioxide, using light as the energy source. They are the producers on which almost every food chain rests.

All animals are heterotrophs: they obtain carbon compounds by eating other organisms or their products. Animals do it by holozoic nutrition, shown in Figure 3.

1. Ingestion: taking food into the body, into the gut.
2. Digestion: breaking large food molecules into small, soluble ones, mechanically and with enzymes, inside the body.
3. Absorption: the small molecules cross the gut lining into the blood or body cells.
4. Assimilation: absorbed molecules are used to build the body's own tissues or are respired.

Undigested material leaves by egestion. The key feature is that food is taken in before it is digested.

Saprotrophic nutrition is the other way to be a heterotroph. Many fungi and bacteria feed on dead organic matter: dead leaves, dung, fallen wood, corpses. They secrete digestive enzymes onto the food, so digestion happens outside their cells, and then absorb the small molecules produced. Saprotrophs are decomposers: by breaking down dead matter they release mineral nutrients such as nitrates and phosphates back into the soil, where plants can reuse them. Bracket fungi on a rotting log and the Penicillium mould on old fruit are saprotrophs.

Holozoic: ingest, then digest inside. Saprotrophic: digest outside, then absorb.

5Mixotrophs: both at once

Some protists are mixotrophic: they are both autotrophic and heterotrophic. Euglena is the well-known freshwater example. It is a single cell with chloroplasts and photosynthesises in the light, but it can also take in carbon compounds from the water around it, and in the dark it can live entirely heterotrophically.

Mixotrophy is not a curiosity. Many species in the ocean's plankton are mixotrophs, photosynthesising and eating smaller cells as well, which makes them important in marine food webs.

The guide asks you to distinguish two kinds.

• A facultative mixotroph can live by either mode alone and uses both when it can. Euglena is facultative: kept in the dark with organic food, it survives as a heterotroph.
• An obligate mixotroph needs both modes to grow: photosynthesis alone or feeding alone is not enough.

The advantage is flexibility: a mixotroph can keep growing when light is poor or when food is scarce, conditions that would stop a pure autotroph or pure heterotroph.

6Archaea: the most varied feeders of all

Archaea are one of the three domains of life, alongside Bacteria and Eukarya. They are prokaryotes, but they are as different from bacteria in their chemistry as either is from us. They are also metabolically very diverse. To make ATP, different archaea use one of three energy sources:

• light, captured by pigments in their membranes;
• oxidation of inorganic chemicals, such as hydrogen, hydrogen sulfide, ammonia or iron ions;
• oxidation of carbon compounds, as heterotrophs do.

This diversity lets archaea live in places where little else can, such as hot springs, very salty lakes and deep-sea vents, as well as in ordinary soils and oceans. You are not required to name examples.

7Teeth and diet in the Hominidae

The family Hominidae includes humans and the other great apes, living and extinct. The guide asks you to relate dentition, the type, size and arrangement of teeth, to diet in omnivorous and herbivorous members, using skulls or digital collections. Figure 4 compares upper teeth.

Homo sapiens (omnivore). Modern humans eat a mixed diet of plants and animals, much of it cooked or processed. The dentition is unspecialised: small incisors for biting, small canines that do not project beyond the other teeth, and moderate premolars and molars with low cusps for grinding. The jaws are relatively small and the arch of the teeth is rounded.

Paranthropus robustus (herbivore). This extinct hominin, known from South African cave sites, lived roughly 2 to 1 million years ago. It had very large, flat premolars and molars with thick enamel, small incisors and canines, a massive lower jaw, and a ridge along the top of the skull, the sagittal crest, where very large chewing muscles attached. These features fit a diet of tough, hard plant foods, such as seeds, nuts, roots and tubers, which needed long, powerful grinding.

Homo floresiensis, a small hominin from the island of Flores, is another species whose diet can be investigated from its teeth and jaws in the same way.

The nature of science: deduction from theory. Nobody has watched Paranthropus eat. Observations of living mammals led to theories linking dentition to diet: grazers and seed-eaters have broad grinding teeth and big chewing muscles; carnivores have sharp shearing teeth and large canines. Those theories let us deduce the diet of an extinct species from its skull. The deduction is only as good as the theory, which is why other evidence, such as wear marks on teeth, is used to test it.

8Herbivores and plants: an arms race

Herbivores eat plants, and plants cannot run away. Each side has adaptations.

Herbivore adaptations. Leaf-eating insects show the two main mouthpart types.

• Chewing mouthparts: caterpillars and locusts have hard, toothed jaws (mandibles) that bite off and grind pieces of leaf.
• Piercing mouthparts: aphids have a long, needle-like stylet that pierces the leaf or stem and reaches the phloem, from which they suck sugary sap.

Some animals have metabolic adaptations for detoxifying plant toxins. The koala eats almost nothing but eucalyptus leaves, which are loaded with toxic compounds; enzymes in its liver break these down, so it can live on a food few other mammals can use. Detoxification opens a niche: a toxic food with little competition.

Plant adaptations against herbivory.

• Physical structures: thorns and spines on stems and leaves (acacias, holly), stinging hairs (nettles), and tough, fibrous or silica-rich leaves (many grasses) that wear down teeth.
• Toxic secondary compounds: chemicals not needed for the plant's basic metabolism but made to deter being eaten, stored in seeds and leaves. Nicotine in tobacco leaves, the cyanide-releasing compounds in cassava leaves and bitter almond seeds, and the toxins in the seeds of many legumes are examples.

9Predators and prey

A predator kills and eats other animals, its prey. The guide asks for chemical, physical and behavioural adaptations on both sides.

Each adaptation on one side selects for counter-adaptations on the other, so predator and prey evolve together.

10Harvesting light in a forest

In a forest the resource plants compete for most fiercely is light. The guide asks for the different strategies forest plants use, shown in Figure 5.

• Canopy trees grow tall trunks and spread a crown of leaves in the upper layer, the canopy, where light is plentiful. Emergents push even higher, above the canopy.
• Lianas are woody climbing plants rooted in the soil. Instead of investing in a thick trunk, they climb other trees to reach the canopy, using the tree as support.
• Epiphytes, such as many orchids and bromeliads, grow on the branches of trees, high up where there is light. Their roots never reach the soil; they get water from rain and humid air and mineral nutrients from debris that collects around them.
• Strangler epiphytes, such as strangler figs, start as epiphytes when a seed germinates on a high branch. They send roots down the trunk to the soil, then thicken and fuse around the host, which may eventually die, leaving the fig standing in its place.
• Shade-tolerant shrubs and herbs live on the dim forest floor. They typically have large, thin leaves with plenty of chlorophyll, and they can photosynthesise enough in low light to grow.

Each is a different niche: the same resource, light, reached in a different way.

11Fundamental and realized niches, and competitive exclusion

A species' fundamental niche is the potential niche it could occupy, given its adaptations and tolerance limits, with no competitors. Its realized niche is the part it actually occupies when competing with other species. The realized niche is usually smaller.

The classic evidence is Joseph Connell's study of two barnacle species on a rocky shore in Scotland, published in 1961. Figure 6 shows the pattern.

Chthamalus grew only in the upper part of the shore; Balanus (now called Semibalanus) grew lower down. When Connell removed Balanus from rocks lower down, Chthamalus larvae settled there and survived. So Chthamalus could live lower on the shore: its fundamental niche is larger than its realized niche, and competition from the faster-growing Balanus, which crowded it out, restricted it to the upper band. Balanus was limited from above by an abiotic factor, drying out when the tide was out, rather than by competition.

Competitive exclusion. Two species with exactly the same niche cannot coexist indefinitely in the same place: one will be better at using the shared resources and will outcompete the other. This is the competitive exclusion principle, and it is why every species' niche is unique. There are two possible outcomes when two species compete, as Figure 7 shows for an experiment of the kind first done with Paramecium by Georgii Gause in the 1930s.

1. Elimination: one species outcompetes the other, which dies out in that place. In Gause's experiment, two Paramecium species each grew well alone, but grown together on the same food one survived and the other died out.
2. Restriction: both species survive, but each is restricted to part of its fundamental niche, as with Connell's barnacles, or as when two birds feeding on the same trees take insects from different parts of the branches.