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12th Biology 2012 Set2 Delhi Board Paper Solution

Question 28

(a) Draw a pyramid of numbers of a situation where a large population of insects feed upon a very big tree. The insects in turn, are eaten by small birds which in turn are fed upon by big birds.
(b) Differentiate giving reasons, between the pyramid of biomass of the above situation and the pyramid of numbers that you have drawn.
(a) What are the two types of desirable approaches to conserve biodiversity? Explain with examples bringing out the difference between the two types.
(b) What is the association between the bumble bee and its favourite orchid ophrys? How would extinction or change of one would affect the other?


(a) Pyramid of Number


Pyramid of Number

This is spindle shaped pyramid.

Pyramid of biomass

Pyramid of biomass

This is irregular shaped pyramid.
(b) In case of pyramid of number for first two steps number increases but them it decreases.
Whereas in pyramid of biomass there is no clear trend.
(a) Two types of desirable approaches to conserve biodiversity are:
(i) In-situ conservation: In-situ conservation is the most appropriate method to maintain species of wild animals and plants in their natural habitats. This approach includes protection of total ecosystems through a network of protected areas. These are the biogeographical areas where biological diversity along with natural/cultural resources are protected, maintained and managed.
The common natural habitats (protected areas) that have been set for insitu conservation of wild animals and plants include –
(I) National parks
(II) Wild life sanctuaries
(III) Biosphere reserves
(IV) Several wetlands, mangroves and coral reefs
(V) Sacred grooves and lakes.
(ii) Ex-situ conservation: Ex-situ conservation includes the following:
(I) Sacred plants and home gardens.
(II) Seed banks, field gene banks, cryopreservation.
Botanical gardens, Arborata, Zoological gardens, Aquaria.

All these approaches help to conserve species and population diversity outside the natural habitats.


In-situ Ex-situ
1. It is the most appropriate 
method to maintain species of
wild animals and plants in their
natural habitat.
1. In this approach, threatened 
animals and plants are not
preserved in their natural
habitat and placed in special
setting, where they can be
protected and special care.
2. No such technique can be used
for In-situ conservation.
2. Gametes of threatened species
can be preserved in viable and
fertile condition for long periods
using cryopreservation
3. In this method organism
alongwith its entire habitat is
preserved. Chances of
fertilization and propagation
are higher.
3. Reproduction in captivity often
slows down and may not give
desired results.
4. Eg. National Park, Wild Life
4. Eg. Seed banks.
(b) Association between the bumble bee and its favourite orchid ophrys is mutualism. Orchids show a bewildering diversity of floral patterns many of which have evolved to attract the right pollinator insect (bees and bumblebees) and ensure guaranteed pollination by it. If the female bee’s colour patterns change even
slightly for any reason during evolution, pollination success will be reduced unless the orchid flower co-evolves to maintain the resemblance of its petal to the female bee.