Question

(A) What is a trophic level in an ecosystem? What is 'standing crop' with reference to it?
(B) Explain the role of the 'first trophic level' in an ecosystem.
(C) How is the detritus food chain connected with the grazing food chain in a natural ecosystem?

Answer 1

(a) Organisms occupy a place in the natural surroundings or in a community according to their feeding relationship with other organisms. Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level. Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third. The amount of energy decreases at successive trophic levels. When an organism dies it is converted to detritus or dead biomass that serves as an energy source for decomposers. Organisms at each trophic level depend on those at the lower trophic level for their energy demands. Each trophic level has a certain mass of living material at a particular time called as the standing crop. The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area. The biomass of a species is expressed in terms of fresh or dry weight. Measurement of biomass in terms of dry weight is more accurate.

(b) Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level. Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third. Autotrophs are the primary producers in a food chain, such as plants on land or algae in the water. They occupy the first trophic level. They are capable of manufacturing their own food by photosynthesis or by chemosynthesis. Photosynthesis is the process by which green plants and certain other organisms transform light energy into chemical energy (carbohydrates). Chemosynthesis is a process by which some organisms, such as certain bacteria, use chemical energy to produce carbohydrates. The two major types of autotrophs are chemoautotrophs and photoautotrophs. Photoautotrophs are those organisms that make their food by photosynthesis while chemoautotrophs are those that make their own food by chemosynthesis.

(c) The green plants in the ecosystem that prepare food are called producers. All animals depend on plants (directly or indirectly) for their food needs.

They are hence called consumers and also heterotrophs. If they feed on the producers, the plants, they are called primary consumers, and if the animals eat other animals which in turn eat the plants (or their produce) they are called secondary consumers. The consumers that feed on these herbivores are carnivores, or more

correctly primary carnivores (though secondary consumers). Those animals that depend on the primary carnivores for food are called secondary carnivores. A simple grazing food chain (GFC) is given below:

Grass $\to$ Goat $\to$ Man

(Producer) (Primary Consumer) (Secondary Consumer)

The detritus food chain (DFC) begins with the dead organic matter. It is made up of decomposers which are heterotrophic organisms, mainly fungi, and bacteria. They meet their energy and nutrient requirements by degrading dead organic matter or detritus. These are also known as saprotrophs. Decomposers secrete digestive enzymes that break down dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them.

In an aquatic ecosystem, GFC is the major conduit for energy flow. As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC. Detritus food chain may be connected with the grazing food chain at some levels: some of the organisms of DFC are prey to the GFC animals, and in a natural ecosystem, some animals like cockroaches, crows, etc., are omnivores. These natural interconnections of food chains make it a food web.