Substances are classified as biodegradable and non-biodegradable because some substances can be decomposed by microorganisms and some cannot.
Substances that are broken down into simple soluble forms are called biodegradable substances and the substances that are not decomposed by microorganisms into harmless substances are called non biodegradable substances.

Biodegradable substances affect the environment by:
→ The biodegradable substances such as tree leaves, plant parts, and kitchen wastes can be used as humus after composting. This will enhance the soil fertility.
→ The biodegradable substances mainly contain carbon. These substances after decomposition release that carbon back into the atmosphere.

Non-biodegradable substances affect the environment by:
→ They contaminate soil and water resources as they cannot be decomposed by micro-organisms.
→ These substances, when accidentally eaten by stray animals, can harm them and can even cause their death.

The various links or steps in a food chain at which the transfer of food and energy takes place are called trophic levels.
The producers form the first trophic level as they manufacture food. The primary consumers form the second trophic level, the secondary consumers form the third, and the tertiary consumers form the fourth trophic level.

food-chain
In the above fig. Grass is the producer which is eaten by the grasshooper known as primary consumer. Then the grasshooper is eaten by shrew, secondary consumer and lastly shrew eaten up by owl, tertiary conusmer.

Various role played by decomposers in the ecosystem are:
→ They clean the environment.
→ They decompose biodegradable substances into useful substances.
→ They release nutrients into soil by decomposing dead and decaying matter, thus making the soil fertile.
→ They maintain the nutrient pool by returning back the nutrients in the pool.