Class 7 Science Chapter 12 Notes Forests Our Lifeline
→ Canopy : The uppermost branches and leaves of tall trees which act as a roof over the forest ground is called canopy. It is the highest layer of vegetation in the forest.
→ Crown : The branch part of a tree above the stem is known as the crown of the tree.
→ Decomposers : The organisms which derive their food from the dead and decaying animals or plants are called decomposers.
→ Deforestation : The cutting down of forests trees for personal as well as for commercial purpose is called deforestation.
→ Humus : Microorganisms feed upon the dead plants and animals tissues and convert them into a dark coloured substances called humus.
→ Regeneration : It is the process of renewal, restoration and growth of new organisms or the existing organisms in an area.
→ Seed dispersal : The transportation of seeds from the place of origin to far away places through various agents is called seed dispersal.
→ Soil erosion : The phenomenon of washing away of the top fertile layer of soil by water or wind is called soil erosion.
→ Understorey : It is an underlying layer of vegetation, specifically, the . vegetative layer an especially the trees and shrubs between the forest canopy and the ground cover.
→ A forest can be defined as an area having many trees, shrubs, herbs, grasses, creepers and climbers.
→ Due to the presence of vegetation of different heights, several horizontal layers are created in a forest.
- The tallest trees form the emergent layer.
- The branches of tall trees form a green roof over the other plants, called as canopy.
- The vegetation that grows under the shade of the canopy is known as understorey.
→ Importance of forests are as following :
- They prevent floods and maintain the water-table.
- They also prevent soil erosion.
- They maintain the supply of oxygen and absorb carbon dioxide in the atmosphere.
- They cool the air, increase the rate of rainfall and regulate water cycle.
- They provide food and shelter for wildlife.
- They provide fuel and raw materials for making paper and furniture.
- They provide medicinal plants such as neem, eucalyptus, amla, cassava. Cinchona trees provide the medicine called Quinine which is used for treating malaria disease.
- They provide materials used for making essential oils, latex, resins etc. Varieties of grasses like khus khus, lemon grass, etc., are sources of several kinds of essential oils and latex. Resins obtained from forests are used to make varnish and paint, and rubber.
→ Plants are also dependent on animals in the following manner :
- Animals like honey-bee, butterfly, moth, etc., help during pollination in flowers. They also help in seed dispersal.
- Animal excreta and dead animals add nutrients to the soil that acts as manure for plant growth.
→ Organisms that can make their own food are called producers. They are also called as autotrophs.
→ Organisms that depend on plants both directly or indirectly are called heterotrophs or consumers.
→ Consumers are of different types:
- Herbivores are animals that depend directly on plants for obtaining food. They are also called as primary consumers. Examples: cow, deer, goat, etc.
- Carnivores are animals that obtain their food indirectly from plants. They are also called as secondary consumers. Examples: tiger, lion, wolf, etc.
- Omnivores are animals that consume both plants and animals. Examples: Human, dog, cat, crow, bear, etc.
- Scavengers are animals that consume dead animals and help in keeping the environment clean. Examples: Crow, jackal, hyena, vulture, etc.
- Decomposers are also consumers that feed on dead remains of plants and animals. They act as recyclers and serve a very useful purpose of maintaining the balance of nutrients in the soil.
→ Recycling of nutrients is one of the major role of forests.
- Nutrients apsorbed by the producers from the soil are passed to consumers.
- When a producer or a consumer dies, decomposers act on them and decompose them into the nutrients they are made of.
- These nutrients return to the soil and recycle the nutrients that have been taken from the soil.
→ Mushrooms and earthworms are also decomposers but they are not microscopic in size.
→ Food chain is a simple representation of the flow of energy from producers to consumers, and to decomposers. It tells who eats what in the environment.
→ The position that an organism occupies in a food chain is called trophic level.
→ Producers (autotrophs) or green plants constitute the first trophic level of a food chain. Producers convert the solar energy or sunlight energy into chemical energy of food by the process of photosynthesis. Hence, sun is the ultimate source of energy in a food chain.
→ Primary consumers or herbivores which feed on plants or plant products, occupy the second trophic level, e.g., grasshoppers, rabbit, deer, elephant etc.
→ Secondary consumers or carnivores e.g., fox, jackal, etc., constitute the next trophic level in a food chain
→ The larger carnivores which feed upon smaller carnivores (secondary consumers) are called tertiary consumers. Tertiary consumers occupy the next higher trophic level.
→ The animals which come at the end of a food chain are called as top carnivores or top consumers. These are the animals which are not preyed upon by any other animal e.g., a lion, an alligator, a hawk etc.
→ The arrow in a food chain indicates energy flow.
→ The role of decomposers comes when plants and animals die. Decomposers break down the dead bodies of plants and animals into simple inorganic nutrients, which are released into the soil. Decomposers operate at all levels of food chain.
→ Food web represents different inter-connected food chains in a habitat.
→ Deforestation is the large-scale cutting down of trees.
→ Desertification is caused due to deforestation. The Sahara desert once used to be a lush green forest. became a desert as a result of extensive deforestation.
→ In the absence of forests, the level of carbon dioxide in the atmosphere will increase. This results in the increase in temperature of the earth due to greenhouse effect. This is called global warming.
→ Greenhouse effect is the trapping of solar radiations which enter into the earth’s surface by greenhouse gases such as carbon dioxide, methane, carbon monoxide, sulphur dioxide, etc.
→ Afforestation is the large-scale planting of trees in deforested areas.
→ Seed dispersal is the process by which seeds get scattered to different areas.
→ Reforestation refers to the re-planting of trees that were once covered with forests and are recently destroyed due to excessive falling of trees, forest fires, etc.