https://www.le.ac.uk/se/centres/sci/selfstudy/eco3.htm

The main species typical of a particular habitat are called indicator species. Each habitat has its own diverse and characteristic assemblage of interacting living organisms known collectively as a community, and all the individuals of one species at a given time make up a population. Each organism has its own functional position in the community, referred to as its ecological niche. This is often likened to a role or occupation within a human community context. A niche is everything that affects and is affected by an organism during its lifetime. Understanding the niche of a species within a community requires information about where it spends its time and when, feeding relationships (food and predators), shelter (protection, camouflage, etc.), and how it reproduces. No two species in a community have an identical niche, although they may overlap. In terms of feeding, organisms in different habitats may have similar ecological niches (e.g. rabbits grazing on a school field; canada geese grazing on grass near lakes and rivers).

Feeding relationships between organisms in a community are represented by food chains, which show the sequence in which organisms feed on each other. The chain generally begins with an autotroph such as a green plant (e.g. stinging nettle ® small tortoiseshell butterfly caterpillar ® house sparrow ® sparrowhawk), or with dead organic matter (detritus) (e.g. leaf litter ® earthworm ® young blackbird ® fox). The arrows indicate the passage of food, and therefore energy flow, from one organism to another. Our human ancestors were part of natural food chains, eating fruit, deer, etc., until we started farming our own food. Food chains need not always include whole animals (e.g. grass ® cow (milk) ® human), and sometimes they include parasites.

Each organism in a food chain occupies a different feeding level, or trophic level. The organisms in a community can be classified according to which trophic level they are on. The autotrophs, such as green plants, are crucial to the ecosystem because they trap solar energy, and release oxygen as a by-product of photosynthesis which all animals need for respiration. Autotrophs are called producers (producing their own food). Heterotrophs are called consumers. Primary consumers (herbivores) eat the producers, secondary consumers (carnivores) eat primary consumers, and tertiary consumers eat secondary consumers. Consumers are not necessarily large mammals; they may be birds, or even small invertebrates. The animal at the end of a food chain is called the top carnivore (e.g. a pike in a lake, or a fox in a wood). All living things eventually die and every ecosystem has scavengers which eat the dead remains (e.g. magpies, woodlice, maggots, earthworms, shore crabs).

Decomposers also play a very important role in the recycling of nutrients within an ecosystem. These are mainly fungi and bacteria which break down any remaining dead and waste organic matter into simple inorganic molecules (i.e. ‘fertiliser’), which are in turn taken up by the producers.

Feeding relationships in most ecosystems are more complex than suggested by these linear food chains. There are many other organisms in the community, often with several sources of food, and the food chains are interconnected to form food webs which are more accurate representations of community feeding relationships. If the population of one organism in the food web is altered, by natural events or by human activities such as environmental pollution, other organisms are also affected. The complexity of the interactions can lead to unexpected outcomes. In the 1950s, rabbits in Britain were almost completely wiped out by the myxomatosis virus. This was expected to have two beneficial effects for poultry and sheep farmers: the lack of competition from rabbits would create more grazing for sheep; and fewer rabbits would reduce numbers of predatory foxes (which also attack sheep and poultry). However, the absence of rabbits enabled tree seedlings to grow to maturity and large areas of grassland turned into woodland. This provided better cover for foxes and good supplies of voles, invertebrates and blackberries. As a result, some fox populations increased and attacked more lambs and poultry.

Food chains and webs do not indicate the numbers of individuals at each trophic level, and this can be represented by a pyramid of numbers, where the producers are placed at the base of the pyramid and the top carnivores at the apex. Generally the number of individuals decreases at each trophic level resulting in a pyramid shape. However, this is not always the case, and distorted or even inverted pyramids can occur. For instance, one oak tree will support numerous primary consumers (e.g. aphids); and a single secondary consumer (e.g. a hedgehog) will support numerous parasites. For this reason, ecologists prefer to make biomass and energy measurements, although these are destructive and inappropriate for school fieldwork.

A pyramid of biomass is constructed by taking a representative sample of organisms from each trophic level at any one time and calculating their dry weight (biomass). There are obvious drawbacks with this, for example, a deciduous tree will have much greater biomass in summer than winter. Pyramids of energy are the most accurate representation of feeding relationships in a community, providing information about the amount of energy flowing through each trophic level over a set period of time. These are always pyramid-shaped since energy is lost at each level through respiration (especially heat), excretion, etc.

Photosynthesis is not particularly efficient at trapping solar radiation, and only a small amount of the light reaching a green plant is converted into plant biomass. Herbivores only convert about 10% of the food they eat into biomass; carnivores may reach up to 20% efficiency as proteins are more easily digested than carbohydrates. It follows that shorter food chains lose less energy, and the nearer a consumer is to the beginning of the food chain the greater the amount of available energy.

People are an integral part of food chains, albeit modified ones, and as omnivores we can operate at several trophic levels. We may, for instance, feed directly on crop plants, or we may choose to feed the crops to cattle, and eat their meat or their products. This is considerably less efficient due to the energy being lost from the longer food chain. It follows that a vegetarian diet, which reduces the links in the food chain, can sustain far more people, and this has important implications for global food supplies.

The term predator generally refers to an organism which consumes (and usually kills) another, the prey. Interactions between organisms are not simply based on predator-prey relationships (i.e. what eats what), population size is also limited by many other biotic and abiotic factors. These limiting factors might include: disease; variation in climate; competition for resources (e.g. sunlight for green plants, space for barnacles, shelter for woodlice, a mate for male birds); and of course the direct or indirect result of human activities (e.g. hunting, culling, accidental poisoning, conservation management, or biological control by introducing natural predators into an ecosystem).