The below mentioned article provides a study-note on the food chain and food web.
Energy Flow:
The interdependence of the flora and fauna of an ecosystem is evident from the fact that some species met their energy (both catabolic and anabolic) need by consuming some other species. The basic source of energy for all living beings on earth is solar energy. Utilizing this energy, the primary producers (autotrophs) synthesize food.
On land these are the various types of vegetation, while in water they are the algae, phytoplankton and various other aquatic plants. Some species (the herbivores) partake of these autotrophs as their food. These herbivores are the primary consumers, which are consumed by the secondary consumers (the carnivores). Some bigger carnivores (the tertiary consumers) live on the smaller ones. This energy flow pattern, from species to species, is schematically shown in Fig. 1.2.
In a simplified way, the sequential dependence of a species of a higher trophic level on another of a lower trophic level for food is termed as the “food chain”. However, the dependence pattern is a complex one and a better term to represent the situation is “food web”, as many species take in, as food, species of different trophic levels.
The efficiency of assimilation of energy is higher at a lower trophic level and lower at a higher trophic level. Because of this, the population and the total mass of a species of lower trophic level are much larger than that of the immediate higher trophic level. The population structure is termed as “population pyramid”, as shown in Fig. 1.3.
The Food Chain:
The food chain is an idealized concept of trophic dependence of one species on another. Classically, it is considered that, the producer, i.e., the species belonging to the plant kingdom (termed as autotroph) synthesize food to meet their nutrition requirement by utilizing carbon dioxide, water and sunlight.
All other species, directly or indirectly, live on the autotrophs. The herbivores (animals belonging to the next higher trophic level) live on autotrophs. Similarly, the carnivores belonging to the third trophic level live on the herbivores.
The Food Web:
In reality, a species of a specific trophic level is dependent on several species belonging to different trophic levels for its nourishment. Because of such food habits, a complex relation exists between the different species as consumer and consumed.
The actual complex food habit of different species in an ecosystem is termed as the food web. As a consequence of food chain /food web, once a pollutant gets absorbed in a species, it is transmitted to the other species of the higher trophic levels.