Whatever the situation, an ecosystem consists of three nutritional groups:
1. Producers (autotrophs organisms, mainly green plants) so-called because they are responsible for synthesising organic food.
2. Consumers (heterotrophs, mainly animals). These are non-producing, organisms. They depend directly or indirectly on green plants.
3. Decomposers (saprophytic organisms, mainly bacteria and fungi).
The food chain represents a detailed, straight line analysis of energy and nutrient; (food) transfer from one organism to the other in an ecosystem. It starts from a primary producer, (green plant) being eaten by herbivores (primary consumers), which are then eaten by carnivores (secondary consumers). There may be several carnivores in the series. The first being preyed upon by a second and so on, as shown in images below.
The resulting nutritional or feeding sequence is known as food chain. Energy in a food chain flows from the lowest level (green plant) to the highest level (carnivores). Each of the different levels in the food chain is termed trophic level (feeding level). In most ecosystems, five different feeding levels can generally be distinguished in a food chain as follows: