It is quite obvious that the different ecosystems are populated by entirely different kinds of organisms, with the possible exceptions of a few kinds of bacteria that may be able to live permanently in many or possibly all ecosystems.
Yet the same basic ecological components are present (with the few exceptions discussed below) and function in more or less the same manner in all types of ecosystems.
According to Odum, from the trophic (food) point of view, an ecosystem has two components:
(a) Autotrophic component (autotrophic = self nourishing), in which the fixation of light energy, the use of simple inorganic substances, and the manufacture of complex materials predominates.
(b) Heterotrophic component {heterotrophic = other nourishing, which utilizes, rearranges and decomposes the complex materials synthesized by the autotrophs.
These functional components of an ecosystem are arranged in overlapping layers with the greatest autotrophic metabolism occurring in the upper stratum where light energy is available. The most intense heterotrophic activity takes place where the organic matter accumulates in the soils and sediments.
Every ecosystem has the following four structural components:
Producers:
Producers or the autotrophic organisms are those living members of the ecosystem that utilize sunlight as their energy-source and simple inorganic materials like water, carbon dioxide and salts to produce their own food. Producers are largely photosynthetic plants and their kind varies with the kind of ecosystem For example, in a dense forest the trees are the most important producers.
In lakes and ponds, the producers are rooted or large floating plants and microscopic plants (phytoplankton), usually the algae. In the ocean, the rooted and floating algae are the producers. However, whether large or small, rooted in the soil or floating in the water, all producers support an array of consumers and decomposers.
Consumers:
Consumers are those living organisms that ingest other organisms and are, therefore, called heterotrophs. They derive their food directly or indirectly from the producers or the green plants. The food is then digested, i.e., broken down to simple substances which are metabolized in the consumer’s body and the waste products of their metabolism are released to the environment. The vast majority of consumers are the animals.
The primary consumers in an ecosystem are herbivores, which feed directly on the producers (green plants). However, primary consumers also vary with the kind of the ecosystem. For example, a deer or a giraffe is a primary consumer in a forest ecosystem, while a cow or a goat is a primary consumer in a grassland or crop ecosystem.
Microscopic animals, such as the protozoans and certain crustaceans, which feed on the floating algae of ponds and seas, are the primary consumers in their habitats. The secondary consumers (carnivores), such as the predaceous insects and game fish in a pond, feed on the primary consumers. There are, in most ecosystems, some organisms that eat other carnivores like snake eats a frog, or a bird eats all types of fishes including carnivores.
These are called the tertiary consumers. Some ecosystems may have the top carnivores like the lion and vulture, which are not killed or rarely killed and eaten by other animals. Another important type of consumers is the bottom-living detritivores, which subsist on the rain of organic detritus from autotrophic layers above.
The consumer animals ingest plant products or other organic materials as food, break them in their digestion process and produce the new types of organic materials like the proteins and animals tissues. Therefore, animals are also busy in the process of organic production. Based on this concept, the green plants are called primary producers and the heterotrophic organisms as the secondary producers.
Decomposers:
Decomposers or the micro-consumers are also called the saprobes or saprophytes. They are also the living components, chiefly the bacteria and fungi, that break down the complex compounds of dead protoplasm of producers and consumers, absorb some of the decomposition products, and release simple substances usable by the producers. Molds and mushrooms (Agaricus) of the forest floor are the largest of the decomposers that are visible to a naked eye. In a pond, the aquatic bacteria, flagellates and fungi are distributed throughout, but they are especially abundant in the bottom, where dead bodies of plants and animals accumulate.
The role of decomposers is very important in an ecosystem, for without their activity the entire cycle of minerals will get blocked. Decomposers are sometimes also called detritivores, particularly when they break down animal and plant material called detritus.
Abiotic Substances or Components:
The abiotic component consists of two things: the materials and the energy. The materials are like water, minerals atmospheric gases, and salts. They also include some organic matter, such as the amino acids and other products of decay of living things. The quantity of abiotic materials like the minerals present at any given time in an ecosystem is termed as the standing state similar to the standing crop, which refers to the quantity of biotic components.
A small portion of these nutrient materials circulates through other living components of the ecosystem and through their death and decay returning to the soil and the environment. The whole process is called mineral circulation or the bio-geochemical cycle. A much larger portion of the materials is held in reserve in the environment as well as in the organisms themselves.
The energy is in the form of light, heat, and stored energy in chemical bonds. While the materials circulate, the solar energy is continuously trapped by the green plants on the one hand, and lost in space through respiration and loss of heat by all types of organisms, on the ether. The energy flows through an ecosystem on a one-way traffic and for the proper maintenance of any ecosystem the energy must be consistently added and transferred from organism to organism.
Thus, in an ecosystem, the first three of the four structural components are composed of living things. Green plants are the predominant producers; animals, the consumers; and fungi and bacteria, the decomposers. However, some ecosystems may lack one or another of the components. For example in the deep oceans producers are absent. But their place is taken by the rain of dead decaying protoplasm that sinks from the surface. Other aquatic communities are sometimes devoid of consumers for short periods, and decomposers will attack the producers directly, although usually only when the plants are dead.
The four components collectively function as a whole system to maintain the stability of an ecosystem. If the balance is disturbed, the whole system breaks down. As mentioned above, for the proper functioning of an ecosystem, there must be a continuous flow of energy and cycling of minerals among the organisms of the ecosystem.