Q.In a given habitat we have 20 plant species and 20 animal species. Should we call this as 'diversity' or 'biodiversity'? Justify your answer.

Step 1 — Checking Statement I

Ecologists have long recognised that species diversity shows a clear latitudinal gradient: it is highest in the tropics and progressively decreases as one moves towards the poles. For example, Colombia (near the equator) has nearly 1,400 bird species, compared with about 650 species in temperate New York State (USA) of similar area, and India (with a large tropical extent) has more plant/animal species than most European countries. So the claim that tropical latitudes have less biodiversity than temperate regions is false.

Step 2 — Checking Statement II …

Step 1 — The context.

The Earth's biological history records five episodes of mass extinction — natural events, spread over long geological time. Ecologists warn that the 'Sixth Extinction' is presently in progress, and its distinguishing feature is that it is caused by human activities rather than by a natural catastrophe.

Step 2 — The rate.

NCERT states the current species-loss rate is 100 to 1000 times faster than in the pre-human era, and if this rate continues, nearly half of all species on Earth could be wiped out within the next 100 years.

Step 3 — Why the number is so large.

The magnitude comes from the four anthropogenic causes NCERT calls 'The Evil Quartet':

1. Habitat loss and fragmentation — the single biggest cause (e.g. the tropical rainforests, once ~14 % of land, now ~6 %; the Amazon being cleared for soyabean/grasslands). …

Step 1 — Fit the sentence together.

The blanks read: 'Species diversity ____, as we move away from the ____ towards ____.'

The grammar of 'away from … towards …' forces the pair to be (equator → poles) — one cannot move 'away from the poles towards the equator' and have the second blank be the poles. So the last two blanks are Equator and Poles, and the only real question is the first blank.

Step 2 — Recall the pattern (the latitudinal gradient).

This is one of the best-known ecological patterns: species diversity decreases as we move away from the equator towards the poles. With very few exceptions, tropics harbour more species than temperate or polar areas. Colombia (near the equator) has ~1,400 bird species; New York at $41^\circ$N has 105; Greenland at $71^\circ$N only 56. India, largely tropical, has >1,200 bird species.

Step 3 — Why the tropics are so rich (the reason, not just the fact).

1. Time. Temperate regions were repeatedly glaciated in the past; tropical latitudes have remained relatively undisturbed for millions of years — a long, uninterrupted evolutionary time for species diversification. …

1. What a biodiversity hotspot is

The term (Norman Myers) identifies regions that satisfy two criteria simultaneously:

1. Very high species richness, with high endemism — species found nowhere else on Earth; and
2. Serious threat of habitat loss — a large fraction of the original vegetation already destroyed.

The number of recognised hotspots was initially 25 and was later raised to 34. Three of them lie (wholly or partly) in India: the Western Ghats–Sri Lanka, the Indo-Burma region, and the Himalaya.

2. Why hotspots give such a large return for such a small area

This is the whole logic of the hotspot idea — conservation triage. Although hotspots occupy a very small percentage of the Earth's land surface, they harbour a hugely disproportionate share of its endemic species. Because those species exist nowhere else, protecting that small area saves a very large number of species from extinction.

The standard quantitative statement of this pay-off is: …

1. The concept being tested

The IUCN Red List documents that in the last 500 years a large number of species have been driven to extinction, overwhelmingly by human activity — the sixth (Anthropocene) mass extinction. A short standard list of recent, human-caused extinctions is quoted in every syllabus, and each species is tied to the place it was wiped out. The question asks us to find the INCORRECT pairing, so we must verify each one.

2. Checking each pair

• (A) Dodo — Mauritius ✓ A large flightless pigeon endemic to the island of Mauritius in the Indian Ocean; extinct by the late 17th century after sailors and their introduced animals arrived. Correctly matched.

• (B) Quagga — Africa ✓ A partially-striped subspecies of plains zebra from South Africa; hunted out, the last individual dying in 1883. Correctly matched.

• (C) Thylacine — Australia ✓ The Tasmanian wolf / Tasmanian tiger, a carnivorous marsupial — and marsupials are the hallmark of the Australian region. The last known animal died in 1936. Correctly matched. …

1. The pattern to be explained: latitudinal gradient in diversity

Species diversity is not uniform over the globe: it decreases as we move away from the equator towards the poles. Tropical rainforests such as the Amazon are the richest habitats on Earth, whereas temperate and polar regions are species-poor. Environmental biologists offer several hypotheses for this gradient; this question tests one of them.

2. The hypothesis in the question — environmental constancy

The argument runs:

1. Temperate and polar regions have repeatedly suffered glaciations and endure severe seasonal swings. The environment there is unpredictable.
2. Tropical latitudes have remained relatively undisturbed for millions of years and are constant / predictable — no long winter, little seasonality.
3. A species can only afford to become a specialist if the conditions it specialises on are reliably present. In a constant environment, specialisation is a viable strategy; in a fluctuating one, a species must stay a generalist to survive the bad season.
4. Therefore a constant tropical environment promotes niche specialisation — each species uses a narrower slice of the available resources.

3. The consequence — why narrow niches mean MORE species …

Step 1 — What "stability" means for a community.

A stable community, by definition:

1. shows little year-to-year variation in total productivity/biomass,
2. is resistant — not much altered by occasional disturbance (natural or man-made),
3. is resistant to invasion by alien species.

So the operational measure of stability in a long-term plot experiment is the variance in total biomass across years: low variance = high stability.

Step 2 — Tilman's experiment.

David Tilman ran long-term ecosystem experiments using outdoor plots in the Minnesota prairie. He deliberately varied the number of plant species sown per plot and then tracked total biomass over many years. Two findings emerged:

• Plots with more species showed less year-to-year variation in total biomass — i.e. greater stability.
• Increased diversity also contributed to higher productivity.

Step 3 — Why more species means less fluctuation (the mechanism). …

Concept — Why are there more species in the tropics? (Biodiversity and Conservation). NCERT gives three hypotheses for the latitudinal gradient in diversity, and each of statements 1–3 is one of them.

Step 1 — Test statement 1. "Speciation is generally a function of time." Correct. Tropical latitudes have remained relatively undisturbed for millions of years, so they have had a long, uninterrupted evolutionary time in which species could diversify.

Step 2 — Test statement 2. "Tropical environment is less seasonal, relatively more constant and predictable." Correct. Such constancy promotes niche specialisation and hence higher species diversity.

Step 3 — Test statement 3. "Solar energy contributes to high productivity." Correct. More solar energy is available in the tropics, which contributes to higher productivity and indirectly to greater diversity. …

Step 1 — The geometry behind the biology

For a body of characteristic length $L$:

So as an animal gets smaller, its surface-area-to-volume ratio gets larger.

Step 2 — Why that matters in the cold

Heat is produced in proportion to body volume (metabolising tissue) but lost through the body surface. A small mammal or bird therefore loses heat far faster, per unit of body mass, than a large one. In the polar cold this loss is severe.

Step 3 — The energetic consequence …

Step 1 — What is special about grass leaves anatomically?

Grasses are monocots. In the upper epidermis of a grass leaf, at intervals between the vascular bundles, sit groups of large, empty, thin-walled cells called bulliform (or motor) cells. Hibiscus is a dicot and has no such cells.

Step 2 — The mechanism of rolling.

• When water is plentiful, the bulliform cells are turgid and push the leaf blade flat/open.
• At noon, transpiration is at its peak and the leaf loses water. The bulliform cells, being thin-walled and highly vacuolated, are the first to become flaccid.
• Their loss of turgor makes the upper surface contract relative to the lower, so the leaf rolls/folds inwards.

Step 3 — Why this is adaptive.

Rolling encloses the stomata-bearing surface inside the roll, trapping a humid pocket of air and reducing the water-potential gradient. Transpirational loss falls sharply. This is a classic xerophytic/mesophytic water-conservation adaptation of grasses.

Step 4 — Eliminate the distractors (all describe conditions that are true elsewhere or are effects, not causes). …