The term ecosystem was first proposed by Tansley in 1935. In the study of ecology, it is regarded as the major starting point and a most important concept. It is generally accepted as the main structural and functional unit of ecological studies. Ecosystems include the biotic (living ) and abiotic (non-living) components interacting with one another, such that there is a flow of energy through the feeding pattern (food levels) of the components, biotic diversity (classes of organisms) and material cycles (nutrient flow) within the system. Both components, not only interact among themselves, but are also joined by a number of physical, chemical and biological processes. The study of these biotic components and their relationships in the environment are therefore very important.
Since a functioning ecosystem includes both biotic and abiotic communities, each influencing the properties of the other and necessary for the maintenance of life as we have it on earth, four major constituents are recognisable. These are:
a. The abiotic or inorganic substances such as air, carbon dioxide, nitrogen, salts, water, soil as well as all other organic compounds of the environment.
b. The primary producers (autotrophs), e.g. green plants (the vegetal world).
c. Consumers (heterotrophs): These are non-producing biotic components of the ecosystem, e.g., Animals.
d. The decomposers (saprophytes): They feed on dead and decaying biotic components. They play a major role on the nutrient cycle, e.g. fungi and bacteria.
A simple plant nutrient cycle
Biotic components of an ecosystem obtain energy and nutrients from their environment in order to remain alive. We can see this through their food relationships or feeding pathways. Energy is transferred from one level of life to another within the system, i.e., it passes from green plants (producers) to herbivores, and from herbivores to carnivores, and finally to the decomposers.
Functioning ecosystems use chemical energy and inorganic nutrients. These flow from one organism to another through food chains and food webs. Organic materials (food) synthesised by producers are eaten and absorbed by consumers. With the aid of decomposers, all the organic materials incorporated into the bodies of the consumers are eventually broken down into simple inorganic materials. These are then rebuilt into organic compounds by the synthetic activities of the producers. Although matter circulates, energy cannot be recycled indefinitely because some of it is always lost as heat. The chemical nutrients in an ecosystem however can be used over and over again. This process is what is known as nutrient cycling. The cycling of these nutrients in nature is brought about by physical, chemical and biological processes.
1. In an ecosystem, the primary producers are the green plants. They are autotrophs. They are eaten by consumers ranging from primary (herbivores) to secondary (carnivores) and tertiary consumers at the top of the food chain. Producers in the aquatic ecosystem are water plants, algae and phytoplankton, while consumers are insect larvae, mites and small fish. In the terrestrial ecosystem, producers are green grasses, crops, shrubs and trees, while consumers are grasshoppers, lizards and hawks.
2. The food relationship in an ecosystem shows a clear dependence of one component on another to stay alive. The food relationship produces a chain of the eater and the eaten, the prey and predator beginning with green plants (producers) through consumers to man in a food chain. A food chain is the linear (in a line) feeding relationship from green –> grass –> grasshopper –> Lizard –> hawk. When the feeding relationships are interwoven and involved several feeding alternatives, it becomes a food web. Feeding breaks the chain into feeding levels (trophic levels) which give rise to food pyramids:
a. Pyramid of biomass: Quantity of materials eaten beginning from producers to consumers.
b. Pyramid of energy: Energy is continually lost from producers to consumers (similar to biomass).
c. Pyramid of numbers: Numbers of individuals reduce from producers to consumers but a single individual producer (like an oak tree) can produce enough leaves to feed a community of earthworms that decompose the leaves to feed the primary producers.
3. The source of energy in nature is sunlight. It is transformed into chemical energy in food substances by photosynthesis. Food substances are eaten and the energy they contain is carried along with losses at all stages of transfer in the form of heat. Energy undergoes transformation from one form to another but cannot be destroyed. Energy transfer is not 100% efficient.
4. Nutrients are recycled in nature by the carbon cycle, water cycle and mineral cycle. This is made possible, because of the action of micro- and macro-decomposers. They bring about decay in nature and return minerals and nutrients to the soil where they are reabsorbed for use by plants (primary producers).