(1) A food web is formed by interlinked of food chains and also so it is not straight.
(2) A food web provides alternative pathways of the availability of food.
(3) A food web helps in checking the over-population.
(4) A food web helps in the development of an ecosystem..

(i) Primary Consumer: The primary consumers obtain their food directly from the producers i.e., plants. The primary consumers are the first order consumer which includes the animals eating plants or plant products. They are called herbivores. Examples - In terrestrial ecosystem, it includes cow, deer, goat, camel, rabbit, hare, horse, ox, elephant etc., and the common herbivores in the aquatic ecosystems include snails, mosquito, larvae, tadpoles etc. (ii) Secondary Consumer: The secondary consumers are the second order consumers which include animals feeding on only the flesh of other animals usually the  herbivores. These are called primary carnivores or secondary consumer. Examples - In terrestrial ecosystem it includes frog, kingfisher, lizard, fox, etc., and in the aquatic ecosystems, it includes water bugs, water beetles, small fish etc.
(iii) Tertiary Consumer: The tertiary consumers are the third order consumers which include large animals feeding on the secondary consumers. These are called secondary carnivores. Examples - In terrestrial ecosystem, it includes wolves, snakes, etc., and in the aquatic ecosystems include water birds, large fishes etc.
(iv) Quaternary Consumer: The quaternary consumers are the fourth order consumers which feed on the tertiary consumers. These are called tertiary carnivores. The quaternary consumers are not eaten by other animals and so are called top carnivores. Examples - In the terrestrial ecosystems, it includes tiger, lion, eagles, hawks etc., and in the aquatic ecosystems, it includes crocodiles, sharks and other big fishes etc.

(a) Environmental pollution is an undesirable change in the physical, chemical or biological characteristics of the natural environment, brought about by man's activities. This pollution may affect the soil, rivers, seas or the atmosphere. 

(c) Biodegradable pollutants are sewage and agricultural waste.

Decomposers are the microorganisms that convert the dead remains of plant and animals to humus hence, help in cleaning the environment.
The two consequences of their absence in an ecosystem are
1. Accumulation of dead plants and animals.
2. Replenishment of soil nutrients will not take place.

The flow of energy in an ecosystem happens in following sequence:
Producer\(\rightarrow \) Primary Consumer\(\rightarrow \) Secondary Consumer\(\rightarrow \) Tertiary Consumer
The flow of energy is unidirectional in the food chain due to two reasons. A predator always eats its prey but other way round is not possible. Moreover, out of the total energy consumed by an organism at a particular trophic level; 90% is utilised for its own need and just 10% is left for the organism of next trophic level. Thus, the availability of energy gets reduced at subsequent trophic level. Due to this, the flow of energy is unidirectional in a food chain.

(1) To trap the solar energy and convert it into chemical energy.
(2) To prepare food from inorganic raw materials.
(3) To maintain carbon dioxide and oxygen gases in the atmosphere.
(4) To initiate food chains.

The harmful by products are gases such as SO₂ and NO. They cause extensive air pollution and are responsible for acid rain.

(a) Walking and cycling for commute.
(b) Using jute/cloth shopping bag.
(c) using public transport.
(d) Switching off light/fans in unoccupied rooms.