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Biodiversity and Conservation — BIOLOGY STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceBIOLOGYBiodiversity and Conservation4 Marks
Question
Explain loss of biodiversity.
✓

Answer

→ The biological wealth of our planet has been declining rapidly and the accusing finger is clearly pointing to human activities.
→ The colonisation of tropical Pacific Islands by humans is said to have led to the extinction of more than 2,000 species of native birds.
→ The IUCN Red List (2004) documents the extinction of 784 species (including 338 vertebrates, 359 invertebrates and 87 plants) in the last 500 years.
→Some examples of recent extinctions include the
→ Dodo (Mauritius), Quagga (Africa), Thylacine (Australia), Steller's Sea Cow (Russia) and three subspecies (Bali, Javan, Caspian) of tiger.
→ The last twenty years alone have witnessed the disappearance of 27 species.
→ Careful analysis of records shows that extinctions across taxa are not random; some groups like amphibians appear to be more vulnerable to extinction.
→ Adding to the grim scenario of extinctions is the fact that more than 15,500 species world- wide are facing the threat of extinction.
→ Presently, 12 per cent of all bird species, 23 per cent of all mammal species, 32 per cent of all amphibian species and 31per cent of all gymnosperm species in the world face the threat of extinction.
→ From a study of the history of life on earth through fossil records, shows that large-scale loss of species like the one we are currently witnessing have also happened earlier, even before humans appeared on the scene.
→ During long the period (> 3 billion years) since the origin and diversification of life on earth there were five episodes of mass extinction of species.
→ The difference is in the rates; the current species extinction rates are estimated to be 100 to 1,000 times faster than in the pre-human times and our activities are responsible for the faster rates.
→ Ecologists warn that if the present trends. continue, nearly half of all the species on earth might be wiped out within the next 100 years.

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  1. In female genital tract, sperms are made capable of fertilising the egg. This phenomenon of sperm activation is called:
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  2. Cortical reaction.
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  1. Select the correct sequence of various physical and chemical events that take place during fertilisation.
  1. Fusion of cortical granules with plasma membrane of secondary oocyte.
  2. Formation of fertilisation cone to receive sperm.
  3. Release of sperm lysin from acrosome.
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  1. R → Q → P → S
  2. Q → S → R → P
  3. Q → R → S → P
  4. R → P → Q → S
  1. Assertion: Only one sperm can fertilise an ovum.

Reason: The secretion of acrosome help the sperm to enter into cytoplasm of ovum through zona pellucida and plasma membrane.

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  4. Subspecies of tiger.
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