Biology Today — January 2018

(ii) Overgrazing : The livestock graze in forest trampling

seedlings and cause soil compaction. This reduces water

storing capacity and increases run off.

(iii) Requirement of wood for timber and paper industry

results in clearing of forest land which leads to soil

erosion resulting in the desert formation.

15. The main causes of global warming are:
    (i) Deforestation
    (ii) Increase in the use of CFCs
    (iii) Burning of fossil fuels
    (iv) Increase in the use of nitrogen fertilisers
    Global warming is a warning to mankind because:
    (i) Rise in temperature is leading to increased melting
    of polar ice caps as well as of other places like the
    Himalayan snow caps. This will result in a rise in sea
    level that can submerge many coastal areas.
    (ii) Changes in the environment results in odd weather
    and climate changes, e.g., El Nino effect.
    (iii) Many tree species and others which are sensitive to
    temperature will die out resulting in conversion of
    forests into scrub vegetation.
    (iv) Small temperature rise may increase crop productivity
    in temperate areas but higher temperature rise will be
    detrimental.
    OR

There are a number of reasons to conserve biodiversity
which can be grouped as:
(a) Narrow utilitarian : Humans derive a major part of their
requirement from organisms. Their direct benefits are
countless like (i) food, cereals, pulses, fruits, vegetables,
milk, egg, meat comes from plants and animals,
(ii) fats and oils are obtained from plants and animal,
(iii) firewood as a source of energy for cooking and
heating, (iv) fibres, e.g., cotton, flax, silk, wool.
(v) industrial products like tannins, lubricant dyes,
resins, and perfumes and (vi) drugs: Nearly 25% of
drugs being used by us are directly coming from plants.
(b) Broadly utilitarian : Biodiversity is fundamental
to ecosystem services of nature. For example,
(i) Oxygen: Through their photosynthetic activity
plants are replenishing oxygen of the atmosphere.
Amazon rainforest is estimated to contribute 20%
of it. (ii) Pollination: Bees, bumble bees, butterflies,
moths, beetles, birds and bats are engaged in
pollination of plants which is essential for formation
of fruits and seeds. (iii) Climate regulation: Forest and
oceanic systems regulate global climate. (iv) Aquifers:
Plant cover is essential for retention of rainwater, its
percolation and storage in aquifers and reservoirs.
(v) Flood and erosion control: Plant cover protects the
soil from wind and water erosion. Run off of rainwater

is reduced so that flood water is rarely formed. (vi)

Nutrient cycling : It is essential for continued availability

of nutrients to plants without which there would be no

photosynthetic activity.

16. (a) Growth curve A represents the J-shaped or exponential
    growth while growth curve B represents S-shaped or
    logistic growth.
    (b) For curve A, population growth is not limited by the
    resources whereas for curve B resources limit the
    population growth.
    (c) Equation for curve B is dN
    dt

rNK – N

K

=

17. Differences between xerarch and hydrarch succession are as
    follows:
    Xerarch succession Hydrarch succession
    (i) It begins with lichens
    or blue green algae.

It begins with

phytoplanktons.

(ii) Initial succession is a

slow process.

Initial succession is quite

fast.

(iii) Succession is seen all

over the area.

Succession is observed in

area where water is not

very deep.

(iv) The whole of the area

is involved in formation

of climax community.

Climax community

develops on the edge

only.

(v) Succession converts

xeric environment into

mesic environment.

It converts aquatic

environment into mesic

environment.

(vi) It reduces bare land

area and converts it

into fertile forested

area.

It fills up water body and

changes it into forested

land.

18.

Tertiary consumers Top carnivores

Primary carnivores

Pyramid of Energy

Producers

Herbivores

10 J

100 J

1000 J

Producers 10,000 Joules

Primary consumers

Secondary consumers

19. (a) Two types of nutrient cycle existing in nature are
    gaseous and sedimentary. Atmosphere and lithosphere
    are reservoirs for gaseous and sedimentary cycle
    respectively. The function of reservoir is to meet
    deficiency of nutrients which occurs due to difference
    in rate of influx and efflux.
    (b) The two ways by which carbon is returned to the
    atmosphere are:
    (i) By respiration of all living organisms: During respiration,
    all living organisms release carbon dioxide which
    returns to atmosphere thereby replenishing its amount
    in the atmosphere.