Class 12-science NCERT Solutions Biology Chapter 13 - Biodiversity and Conservation

(a) Genetic diversity

(b) Species diversity

(c) Ecosystem diversity

Ecologists make a statistical comparison of the species richness of exhaustively studied groups of insects of the temperate and tropical regions and extrapolate this ratio to other groups of animals and plants to calculate the gross estimate of the total number of species present on the Earth.

Biodiversity is not uniformly distributed throughout the world. Polar regions have very little biodiversity, whereas South America has the greatest biodiversity on the earth. There are many hypotheses for higher biodiversity in tropics:

i. There are no unfavourable seasons in the tropics. Continued favourable environmental conditions have helped tropical organisms to flourish more.

ii. There is more solar energy available in the tropics due to which productivity is higher and this contributes to greater diversity.

iii. The tropical environment is older, thus allowing more time for the evolution of a greater number of plants and animals.

Alexander Von Humboldt observed that within a definite region, species richness increases up to some extent with an increase in area. This relationship between species richness and area is a rectangular hyperbola for a large variety of taxa.

On a logarithmic scale, it is a straight line

log S = log C + Z log A,

where S = Species richness

Z= Slope of the line (regression coefficient)

C= Y-intercept

A= Area

Z is generally 0.1-0.2 regardless of taxonomic group or region, i.e. on analysis of the species-area relationship in small areas, the values of the slopes of regression are remarkably similar regardless of the taxonomic group or region.

On analysis of the species-area relationship in very large areas such as a whole continent, the slope of the regression line would be much steeper.

The major causes of species losses in a geographical region are

i.Habitat loss and fragmentation: The destruction of habitat is the primary cause of loss of diversity. It occurs due to cutting down of trees, filling a wet land, ploughing a grassland or by burning a forest. This causes a change in the natural habitat and kills or forces out many plants, animals and microorganisms. With the fragmentation of a large forest tract, the species occupying the deeper parts of the forests get disappeared.

ii.Disturbance and pollution: Communities are largely affected by natural disturbances such as fire, tree fall and defoliation as well as by man-made disturbances. Man disturbs the community by using a large number of synthetic compounds, releasing of radiation or spilling of oil in the sea. Lead poisoning and eutrophication cause mortality of species and reduce species diversity.

iii.Introduction of exotic species: New species which enter a geographical region are called exotic or alien species. Introduction of such species may cause disappearance of native species through changed biotic interactions. Example: Nile perch, an exotic predatory fish introduced into Lake Victoria, threatens the entire ecosystem of the lake by eliminating several native species of the small cichlid fish species which were endemic to this fresh water aquatic system.

iv.Co-extinction: When a species becomes extinct, the plant and animal species associated with it also become extinct. When a fish species becomes extinct, its assemblage of parasites also meets the same fate.

i.Biodiversity is essential for the maintenance of the gaseous composition of the atmosphere. The Amazon forest produces about 20% of the total oxygen in the earth's atmosphere through photosynthesis.

ii.It ensures pollination, without which plants cannot give fruits and seeds.

iii.It controls climate through forest and oceanic systems, natural pest control, formation and protection of soil, conservation and purification of water and mineral cycling.

iv.Biodiversity has great aesthetic value, such as ecotourism, bird watching, wildlife, pet keeping and gardening. In most Indian villages and towns, plants such as Ocimum sanctum (Tulsi) and Ficus religiosa (Peepal) are considered sacred and worshipped by people. Several birds and snakes are considered sacred.

Sacred groves are sacred forest patches around places of worship. They are held in high esteem by tribal communities. Tribals do not allow cutting of even a single branch of a tree in a sacred grove. These groves are found in several parts of India, e.g. Rajasthan (Aravalli hills), Madhya Pradesh (Sarguja, Bastar), Meghalaya and Maharashtra.

Sacred groves help in the conservation of many endemic species (rare species) flourishing in that area. It helps in maintaining viable populations of all native species as well as conserves the genetic diversity of all the present species.

Plants play an important role in the control of flood and soil erosion. The roots of plants bind the soil particles firmly and do not allow the top soil to be carried away by flowing water or winds. Roots of plants also make the soil porous so that the water may percolate into the soil.

Compared to plants, animals have increased size and genetic variation. Animals also possess a complex nervous system to control and coordinate various body activities. Animals possess receptor organs for receiving various environmental stimuli and are able to respond to them. The ability of locomotion is also a factor for greater diversification of animals.

Plant species are largely affected by seasonal disturbances (natural or manmade), and this leads to lesser diversity than that present in animals.

The destruction of microorganisms harmful to society is justified if they are not essential biotic components of the ecosystem. For example, we are trying to eradicate disease-causing organisms (Polio virus, HIV). The loss of few such harmful microorganisms would not affect the functioning of the ecosystem. Therefore, under these situations, deliberately making a species extinct is justified.