CBSE Class 10 Science Our Environment Notes Set 05

Ecosystem and its components

An ecosystem is a community of living organisms interacting with one another and with the non-living features of their surroundings. Natural ecosystems include forests, grasslands, ponds, lakes and oceans. Gardens, crop fields and aquariums are artificial (man-made) ecosystems.

The abiotic components are the non-living parts such as temperature, rainfall, wind, soil and water. The biotic components are all living things, grouped by their role:

1. Producers: green plants, blue-green algae and phytoplankton make food by photosynthesis.
2. Consumers: animals that depend directly or indirectly on producers. Herbivores are primary consumers, small carnivores are secondary consumers, and large carnivores are tertiary consumers.
3. Decomposers: bacteria and fungi break down dead plants and animals and return nutrients to the soil.

Food chains and food webs

A food chain is a series of organisms in which each one eats the one before it, so food energy passes from one living thing to the next. An example is grass, grasshopper, frog, snake and eagle. In nature most animals eat more than one kind of food, so chains join up into a network called a food web. Each step in a chain is a trophic level, and producers always form the first level.

Flow of energy

Plants capture about 1% of the sunlight falling on them and store it as chemical energy. When one trophic level is eaten by the next, only about 10% of the energy is passed on. The rest is used for the life processes of the organism or lost as heat. This is called the 10 per cent law. Energy flows in one direction only, from the sun to producers and then to consumers, and it cannot be reused. Because so little energy is left after three or four steps, food chains are short, and the number of organisms usually falls at higher levels.

Biomagnification

Harmful chemicals such as pesticides (for example DDT) enter the food chain when they wash into soil and water and are absorbed by plants. These substances are non-biodegradable, so they are not broken down or excreted. Their concentration therefore increases at each higher trophic level, a process called biological magnification. Top carnivores, including humans, carry the highest levels.

Waste and its management

Biodegradable waste, such as vegetable peels, paper and dung, is decomposed by bacteria and fungi using their enzymes. Non-biodegradable waste such as plastics, glass and DDT cannot be broken down this way, so it stays in the environment for a very long time and harms soil, water and animals. Good waste management uses the 3 Rs (reduce, reuse, recycle), segregation of waste, composting of kitchen waste and safe disposal of what remains.

Ozone layer

Ozone (O₃) is formed high in the atmosphere when ultraviolet (UV) rays split oxygen molecules (O₂) into atoms, and these atoms combine with other oxygen molecules. The ozone layer in the stratosphere absorbs harmful UV radiation from the sun, which can cause skin cancer and cataract. Chemicals called chlorofluorocarbons (CFCs), used in refrigerators, air conditioners and aerosol sprays, release chlorine that destroys ozone. This produced the ozone hole over Antarctica, and CFC use is now controlled by international agreement and replaced by safer alternatives.

Worked examples

Example 1. In the chain grass, grasshopper, frog, snake and hawk, the grass holds 10,000 J of energy. How much reaches the hawk?
Each step passes on 10%: grasshopper 1,000 J, frog 100 J, snake 10 J, hawk 1 J. The hawk, at the fifth level, receives just 1 J.

Example 2. Name the trophic level and role of each organism in: wheat, rat, snake, owl.
Wheat is a producer (first level), rat a primary consumer (second), snake a secondary consumer (third) and owl a tertiary consumer (fourth).

Example 3. What may happen if all the snakes in the above chain are removed?
The rat population increases as its predator is gone, so the wheat is eaten faster, and the owl loses one of its food sources. The balance of the ecosystem is disturbed.

Quick Check Questions