Life is maintained by organisms particularly plants that use sunlight to convert carbon dioxide and water into energy for all life. This process of fixing carbon dioxide by green plants is called photosynthesis. There are purple bacteria that assimilate carbon dioxide by using hydrogen sulfide (H2S) instead of water (H2O) as the reducing agent. Some other bacteria use organic compounds for reducing carbon dioxide. Green plants and various bacteria are known as the producers in an ecosystem.
Higher plants and green algae perform most of the carbon fixation in the world, and chemosynthetic bacteria are of more significance on moving certain materials such as sulfur, through the sediments for ecosystem.
The producers fix their own food supply and are called Autotrophs (self-feeding); on the other hand Heterotrophs depends upon on other organisms for food. Organisms that feed on other organisms are called consumers. Those that feed on plants are called Primary consumers or Herbivores. Carnivores that feed only on those animals that are secondary consumers is a tertiary consumers.
If there were only producers and consumers the system would not work very well, because the flow of materials would be in only one direction. That is it would be from lower to higher order organism. There needs to be the medium that return these compound to move elemental forms so that they can cycle and be used over and over again in the food chain. This process of recycling requires a group of organisms known as
. Bacteria and fungi play the role of decomposers. Bacteria act on animal tissue and fungi on plant tissue. In this process plant and animal material is degraded enzymatically and released as basic elements into the environment, where the elements are again available to the producers for reuse.
Energy on the other hand is not recycled. It moves undirectionally through an ecosystem, being consumed at each step of the food chain.
Food Chains: An example
A field rat derives its energy by eating grass, then the rat is consumed by weasels, which is then eaten by hawks and owls.
In the aquatic food chain, the phytoplankton, the primary producers are eaten by zooplankton, these are in turn eaten by fish which is then eaten by porpoises, these porpoises are then eaten by killer whales.
Sometimes these animals may escape their predators and die of old age. These dead animals are colonized by blowflies that lay their eggs in the carcasses that are eaten by carrion beetles, and are consumed by bacteria. Food chains does not exist in isolation but interact to form food webs.
The food web is the network within an ecosystem through which energy flows to the entire community of plant and animals. Most food webs have a great deal of stability. They will react when disturbed unless the disturbance is too extreme. If the disturbance is too great the species are wiped out or the entire breeding populations are destroyed then recovery to its previous forms may never occur.
Examples of decimation of ecosystems:
(a) In areas of human activity the natural ecosystems have been replaced by urban ecosystems.
(b) Most of modern Europe has had its original natural ecosystems replaced by cultivated or managed ecosystems.
(c) The Great Lakes fisheries have been damaged by human activities beyond the recovery to their original form.