Wednesday 16 January 2013

CBSE Class 7 - Science - Ch 17 - Forests: Our Lifeline

Forests: Our Lifeline

NCERT Chapter Solution

Q1: Explain how animals dwelling in the forest help it grow and regenerate.

Answer: Various types of animals (herbivores, carnivores, omnivores) live in forests and contribute to food chain. Following animal activities show how animals help forest grow and regenerate.
  1. Animals help in dispersal of seeds and pollination of flowers.
  2. Animals excreta and their dead bodies when mixed win soil act as manure for the plants.
  3. Micro organisms convert dead plants and leaves into humus which is rich in nutrients. Humus in soil provide necessary minerals for plant growth.
Q2:  Explain how forests prevent floods.

Answer: Forest acts as a natural absorber of rainwater and allows it to seep. When rainwater falls on leaves of trees and plants, it does not fall directly on the ground. It drips slowly on the forest ground and does not stagnate. Thus trees prevent flooding. Water is able to seep through soil and recharge water table.

Q3: What are decomposers? Name any two of them. What do they do in the forest? 

Answer: Micro-organisms feed mainly on dead remains of plants and animals and convert them into nutrient rich humus. These micro-organisms are called decomposers. Bacteria and fungi are examples of decomposers.
Decomposers recycle and convert the dead matter into humus which mixes with forest soil and provide necessary nutrients to plants. Thus decomposers help in maintaining the necessary nutrient balance in the soil.

Q4: Explain the role of forest in maintaining the balance between oxygen and carbon dioxide in the atmosphere.

Answer: Forests play an important role in maintaining the balance between oxygen and carbon dioxide in the atmosphere. Forests are called lungs. Animals during respiration take in oxygen and release carbon dioxide. Plants take in carbon dioxide and release oxygen through the process of photosynthesis. The plants help to provide oxygen for animal respiration.

Q5: Explain why there is no waste in a forest.

Answer: Forests serve as green lungs and water purifying systems in nature. They are excellent, perfect and natural recycling factories. All animals, whether herbivores or carnivores, depend ultimately on plants for food. Herbivores eat plants and their products. Carnivores eat herbivores. Dead remains of animals and plants are decomposed by micro-organisms into humus. Forests maintain a network of food chains called food web and hence nothing goes waste in a forest.

Food Web

Q6: List five products we get from forests?

  1. Fruits and Vegetables
  2. Timber and wood
  3. Turpentine, latex (used in manufacturing rubber)
  4. Spices, resin, gum
  5. Medicines (Tulsi, Neem, aloe vera etc.)

Q7: Fill in the blank:

(a) The insects, butterflies, honeybees and birds help flowering plants in pollination.

(b) A forest is a purifier of air and water.

(c) Herbs form the lowest layer in the forest.

(d) The decaying leaves and animal droppings in a forest enrich the soil.

(e) Decomposers convert dead plants and animals into _humus_.

(f) Tall trees which look like roof over other plants in the forest is called canopy.

(g) A food _web_ is made up of several connected food chains.

Q8: Why should we worry about the conditions and issues related to forests far from us?

Answer: Forests are important renewable resource. We should be careful and concerned about the issues related to forest due to following reasons:

  1. If there are no forests, there would be more floods and more soil erosion.
  2. No forests means less oxygen in air and it will lead to global warming. 
  3. Forests are the dwelling of many animals. Deforestation will will endanger our life and environment.
  4. In the absence of trees and plants, the animals will not get food and shelter.
  5. Forests provide us large number of useful products including wood, fruits and medicines. These products would not be available in the absence of trees and plants.

Q9:  Explain why there is a need of variety of animals and plants in a forest.

Answer: Variety of animals and plants living in forests build a rich biodiversity. This biodiversity make forests more productive, stable and resilient. All animals (herbivores and carnivores) depend on plants or their products directly or indirectly. If there is no grass, all herbivores would die. If there are no carnivores, all herbivores would eat up all the plants and there is shortage of food. If there are no decomposers, the dead remains of plants and animals would pollute the environment. Due to such biodiversity a balance in nature is maintained.

Q10: In Fig., the artist has forgotten to put the labels and directions on the arrows. Mark the directions on the arrows and label the diagram using the following labels:

clouds, rain, atmosphere, carbon dioxide, oxygen, plants, animals, soil, roots, water table.


Q11(MCQ): Which of the following is not a forest product?
(i) Gum
(ii) Plywood
(iii) Sealing wax
(iv) Kerosene

Answer: (iv) Kerosene

Q12(MCQ): Which of the following statements is not correct? 

(i) Forests protect the soil from erosion.
(ii) Plants and animals in a forest are not dependent on one another.
(iii) Forests influence the climate and water cycle.
(iv) Soil helps forests to grow and regenerate.

Answer: (ii) Plants and animals in a forest are not dependent on one another.

Q13: Micro-organisms act upon the dead plants to produce
(i) sand
(ii) mushrooms
(iii) humus
(iv) wood

Answer: (iii) humus


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