Ecosystem and Food Chains: Components, Levels and Pyramids

An ecosystem is a unit of nature in which living organisms interact with one another and with the non-living surroundings, exchanging energy and materials. The term was coined by the British botanist A. G. Tansley in 1935. A pond, a forest, a grassland, a desert and even a crop field or an aquarium is an ecosystem; the largest of all is the biosphere, the thin zone of the Earth in which life exists. This chapter is asked in almost every general awareness paper, usually through the food chain and the ten per cent law.

Components of an ecosystem

Every ecosystem has two sets of components. The abiotic or non-living components are sunlight, temperature, water, air, soil and the minerals and organic matter within them. The biotic or living components are grouped by the way they obtain food.

• Producers or autotrophs: green plants, algae and some bacteria, which make their own food by photosynthesis and form the base of every food chain.
• Consumers or heterotrophs: animals, which depend on others for food. Herbivores that feed on plants are primary consumers, the carnivores that eat them are secondary consumers, and those that feed on the secondary consumers are tertiary consumers. Animals that eat both plants and animals are omnivores.
• Decomposers or saprotrophs: bacteria and fungi, which break down dead bodies and waste, release the nutrients back into the soil and water, and so keep the cycle of matter going.

Food chains, food webs and trophic levels

A food chain is the sequence in which one organism is eaten by another, and through which energy passes from one level to the next. Each step of the chain is a trophic level: producers occupy the first, herbivores the second, and so on. Two kinds of food chains run in nature at the same time.

In nature an animal rarely eats just one kind of food, so many food chains cross one another and form a food web. A food web makes an ecosystem more stable, because if one species disappears, the animals that fed on it can turn to another.

Energy flow and the ten per cent law

The Sun is the ultimate source of energy for almost every ecosystem, and green plants capture only a small fraction of the sunlight that falls on them. The total energy fixed by producers is the gross primary productivity; what is left after the plants use some of it in respiration is the net primary productivity, which is the food actually available to the consumers. Energy flow through an ecosystem is unidirectional: it enters as sunlight, passes from producers to consumers to decomposers, and is lost as heat at every step. It never returns to the Sun, which is why nutrients cycle but energy does not.

Only about ten per cent of the energy at one trophic level passes to the next, the rest being spent in respiration and lost as heat. This is the ten per cent law, stated by Raymond Lindeman in 1942. It explains why food chains are short, usually of four or five links, and why the number of top carnivores in any ecosystem is small.

Ecological pyramids

An ecological pyramid is a graphic way of showing the number, the biomass or the energy at each trophic level, with producers at the base.

• Pyramid of numbers: generally upright in a grassland, but inverted in a single tree ecosystem, where one tree supports thousands of insects and those insects support many birds.
• Pyramid of biomass: upright in a forest, but inverted in a pond or sea, because the tiny phytoplankton have a small standing biomass at any moment yet reproduce fast enough to feed a much larger mass of fish.
• Pyramid of energy: always upright, because energy is lost as heat at every transfer and a level can never hold more energy than the one below it.

Biomagnification and succession

Some harmful chemicals, such as the pesticide DDT and mercury compounds, are not broken down by the body and are not excreted. They pass from one trophic level to the next and become more concentrated at every step, so the top carnivores, including fish-eating birds and human beings, receive the heaviest dose. This increase along the food chain is called biomagnification, and in birds it famously caused eggshells to thin and break. Ecological succession, meanwhile, is the orderly change in the community of an area over time: primary succession begins on bare rock or a new pond where no life existed, secondary succession on land cleared by fire or flood where soil remains, and both end in a stable community called the climax community.