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Biology · 2025 · 5 marks
CBSE 2025 · Region 7 · Set 1 · Q33
(a) (i) How does alien species invasion cause a decline in biodiversity? Explain. How have the following contributed to biodiversity loss? I. Nile…
(i)
How does alien species invasion cause a decline in biodiversity? Explain. How have the following contributed to biodiversity loss? I. Nile Perch II. Lantana and Eichhornia III. Clarias gariepinus
(ii)
Why have certain regions been declared as biodiversity hotspots by environmentalists of the world ? Name any two such regions in India.
(i)
Write an equation for Verhulst-Pearl Logistic Growth Curve where: N = Population density at time 't' r = Intrinsic rate of natural increase
(ii)
Draw a graph for a population whose population density has reached carrying capacity.
(iii)
Draw a growth curve where resources are non-limiting to growth of population.
(iv)
Which growth curve is considered more realistic and why ? Explain.
Marking-scheme solution
(i)
Alien species invasion – when alien species are introduced
unintentionally or deliberately some of them turn invasive and cause decline or extinction of indigenous species.
I.Nile Perch – In Lake Victoria in East Africa Nile Perch caused extinction of
$\displaystyle 200$ species of cichlid fish.
II. Lantana and Eichhornia – Caused environmental damage and threat to native species.
III.Clarias gariepinus – African catfish introduced for aquaculture purposes poses a threat to indigenous catfishes in our rivers.
(ii)
Regions with high levels of species richness, high degree of endemism
have been declared as biodiversity hotspots to conserve biodiversity.
In India – Indo-Burma , Himalaya ,Western Ghats and Sri Lanka
(i)
dN/dt = rN[(K – N)/K]
(ii)
Sigmoid growth curve
(iii)
J-shaped growth curve
(iv)
-Logistic Growth curve / Sigmoid growth curve is more realistic
-A given habitat in nature has enough resources to support a maximum
possible number beyond which further growth is not possible/ resources become limiting factor
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CBSE Class 12 Biology past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.