Class 12 Biology


NCERT Exercise

Question 1: Name the three important components of biodiversity.

Answer: Following are the three important component of biodiversity:

Question 2: How do ecologists estimate the total number of species present in the world?

Answer: Ecologists make a statistical comparison of the temperate-tropical species richness of an exhaustively studied group of insects. This ratio of extrapolated to other groups of animals and plants to estimate the total number of species on earth.

Question 3: Give three hypotheses for explaining why tropics show greatest levels of species richness.

Answer: Three hypotheses for explaining why tropics show greatest levels of richness are as follows:

Speciation is generally a function of time. Temperate regions had been subjected to frequent glaciations in the past. But tropical latitudes have remained relatively undisturbed for millions of years. This allowed a long evolutionary time for species diversification in the tropics.

Tropical environments are less seasonal, relatively more constant and predictable. A constant environment promotes niche specialization, and leads to a greater species diversity.

More solar energy is available in the tropics. It contributes to higher productivity. It may have indirectly contributed to greater diversity in the tropics.

Question 4: What is the significance of the slope of regression in a species – area relationship?

Answer: Ecologists have discovered that the value of Z lies in the range of 0.1 to 0.2, regardless of the taxonomic group or the region. But when this slope is calculated for a larger area, the slope becomes much steeper. This means that number of species shall be larger in a larger area.

Question 5: What are the major causes of species losses in a geographical region?

Answer: Following are the major causes of species losses in a geographical region:

Question 6: How is biodiversity important for ecosystem functioning?

Answer: Each species in an ecosystem has a definite role to play in an ecosystem. Some of the species may have more important role to play than others. Extinction of critical species may play immediate havoc with the ecosystem. This can be understood by rivet popper hypothesis.

Rivet Popper Hypothesis: Stanford based ecologist Paul Ehrlich explained the possible implications of reduction in species diversity by using this analogy. He compared the ecosystem with an airplane and compared the species with rivets. We know that thousands of rivets are used in an airplane to join various parts. If every passenger starts popping a rivet to take home a memento, it may not affect flight safety initially. But as more and more rivets are removed the plane becomes dangerously weak in the long run. Some of the rivets may be more critical than others. For example; loss of rivets on the wings can be more dangerous, compared to loss of rivets on the seats. Similarly, extinction of a few species may not be harmful for the ecosystem. But extinction of a critical species may play havoc with the ecosystem.

Question 7: What are sacred groves? What is their role in conservation?

Answer: Tribal people and some rural people mark some places in the forest as sacred groves. These are the places where these people worship their deities. They also worship various trees in the sacred grove. This is a way to pay gratitude to the nature. This also helps in conservation of natural vegetation and wildlife.

Question 8: Among the ecosystem services are control of floods and soil erosion. How is this achieved by the biotic components of the ecosystem?

Answer: We know that trees hold the topsoil in place. Trees also create a natural barrier to the flow of water. This helps in reducing the chances of flash flood and helps in recharge of groundwater. It also prevents topsoil from being washed away by water, or being blown away by wind. Thus, biotic components of the ecosystem help in controlling flood and soil erosion.

Question 9: The species diversity of plants (22 per cent) is much less than that of animals (72 per cent). What could be the explanations to how animals achieved greater diversification?

Answer: Animals need to move from one place to another in search of food, shelter and for protection from predators. Plants remain fixed at one place. Ability to be mobile has enabled animals to develop greater number of adaptations so that they can survive in changing situation. Due to this, animals have achieved greater diversification than what plants could do.

Question 10: Can you think of a situation where we deliberately want to make a species extinct? How would you justify it?

Answer: There are many organisms which are highly harmful to us. For example; malaria parasite and Anopheles mosquito are harmful for human beings. Similarly, there are many pathogens and parasites which cause dangerous diseases. We need to deliberately make these species extinct.