FOOD CHAIN AND TROPIC LEVELS

TOPIC:

FOOD CHAIN AND TROPIC LEVELS

OBJECTIVE

The objective of this script is to know the basic concept of food chain and trophic levels in various ecosystems.

Concept

Plants convert solar energy into chemical energy stored in food material. The transfer of food energy from plants through a series of organisms forms the food chain. A phytoplankton may be eaten by a copepod, which is eaten by a small fish, the small fish is ultimately eaten by large fish, and large fish by a swan.

This is a food chain that is generally observed in rivers. In pasture grasses are eaten by grasshopper and grasshopper by a toad. In the two examples the base of the food chain is formed by a plant, which is grazed on by a herbivore, which is predated by a carnivore, which is eaten by another carnivore.

In nature, food chain relationships are very complex, as one organism may form the food source of many organisms. For example, grass may be grazed on by grasshoppers as well as rabbits or cattle and each of these herbivores may be eaten by many carnivores, such as toads, snakes, birds, or hyena. Thus, instead of a simple food chain we find a web-like structure called food web.

When the food is obtained from the same number of steps then the organisms are said to belong to the same trophic level. Organism whose food is obtained from plants are said to belong to the trophic level two or primary consumer or herbivore. The green plants occupy the first trophic level and are called the producers. Examples of herbivores are: plant grazing insects, rabbits, rodents, deer, cattle and so on. Flesh eaters that eat herbivores form the third trophic level or the secondary consumer or carnivore level one. The next trophic level is the tertiary consumer or carnivore two, which eat the fleshs of herbivores and secondary consumers. The trophic level can be expanded taking the food habits of organisms into account.

Types of Food Chains

In ecosystems such as grassland, pond, or lake, a substantial part of the primary production is grazed on by herbivores. Rodents and cattle are the main grazers in grassland, whereas zooplanktons are the main grazers in ponds and lakes. Usually up to 50% of the net primary production is grazed on by these animals in their respective ecosystems and the remaining 50% goes to the decomposer organisms as dead organic matter. Therefore, in these ecosystems, the food chain is herbivore based and herbivores are considered important consumers. This is classified as a grazing food chain.

In a forest ecosystem, the dominant primary consumers, that is, herbivores are the insects, which usually consume less than 10% of the net primary production. The rest 90% is consumed later as dead plant material by the small detritus like protozoa, fungi, actinomycetes, and bacteria. The animals consume the detritus and process it in their gut by reducing it into small pieces and sometimes digesting it partially or fully, thus making organic materials available for bacterial and fungal attack. These macro-organisms also act as food for many soil animals. This food chain is classified as a detritus or decomposer food chain. The detritus food chain exists in every ecosystem and is very important for the circulation of materials. Even in a grazing food chain, the faeces and urine of grazing herbivores ultimately come to the decomposer pathway.

Significance of Food Chains

Food chain studies help understand feeding relationships and the interaction between organisms in any ecosystem. They also help us comprehend the energy flow mechanism and matter circulation in ecosystems, and understand the movement of toxic substances in the ecosystem and the problem of biological magnification. Certain harmful substances, usually ones not found in nature but introduced by man, may get injected into plants and animals. These poisonous substances like DDT may not be broken down in the body or excreted easily and quickly. Instead, they accumulate in the tissues, and as the living organism eats more, the concentration of these substances increases and they pass from one trophic level to the next. As man is an omnivore (eat both herb and flesh) and has access to all trophic levels for food gets large amounts of toxic substances like DDT into the body.