MPSC 2024 Prelims Paper-I · Question 32 of 100

Correct — option (2), only statement (b). This question prints just two statements, (a) and (b), and they test the two great spatial gradients in the distribution of life. Statement (a) is false because it reverses the latitudinal diversity gradient. Species richness is at its maximum in the equatorial tropics and falls away steadily towards the poles: a tropical rainforest holds more species of tree, bird, insect and amphibian in a few hectares than an entire temperate or boreal country. The standard textbook comparison makes the scale of it clear — Colombia, near the equator, has close to 1,400 recorded bird species, the state of New York at about 41 degrees north has around 105, and Greenland at about 71 degrees north only about 56. Several reasons are offered for the gradient, and they reinforce one another: the tropics receive more solar energy and are more productive, so more species can be supported; tropical environments have been relatively undisturbed for far longer, since the temperate and polar regions were repeatedly scoured by Pleistocene glaciation, giving tropical lineages more uninterrupted evolutionary time to diversify; and the constancy of tropical conditions permits narrow niche specialisation, which allows more species to coexist in the same space. Statement (b) is correct because the altitudinal gradient runs the same way as the latitudinal one: on a mountain slope, diversity is generally greater near the base and declines with height. Rising altitude brings falling temperature, a shorter growing season, thinner air, stronger ultraviolet radiation, poorer and shallower soils and increasingly severe conditions, and the area available also shrinks as the mountain narrows towards the summit — so fewer species can persist. Since (a) fails and (b) holds, the only option that fits is (2).

• (1)Only (a) — This accepts statement (a) and rejects statement (b), which inverts both gradients at once. It would require that high latitudes hold more species than the tropics — contradicted by every comparison from bird counts to tree censuses — and simultaneously that mountain tops are richer than mountain bases, which is contradicted by the plain fact that alpine and nival zones support only a small, specialised flora and fauna while the forested lower slopes support a large one. This is the option a candidate marks who has heard that latitude and altitude affect diversity but has not fixed the direction of either effect.
• (3)Both (a) and (b) — This keeps the correct statement (b) but also accepts statement (a), and statement (a) is the reverse of the observed pattern. Species richness declines from the equator towards the poles, not the other way round; high latitudes are characterised by a small number of species present in very large numbers of individuals, which is a different thing from diversity. The option is tempting because it lets a candidate avoid choosing between two statements that both sound like textbook generalisations, but on a two-statement question one of the pair is very often deliberately inverted, and here it is (a).
• (4)Neither (a) nor (b) — This rejects both statements, which means rejecting statement (b) even though the decline of biodiversity with altitude is a well-established and generally observed pattern, driven by falling temperature, shorter growing seasons, thinner atmosphere, harsher conditions and the shrinking area available towards a summit. A candidate who recognises that statement (a) has been inverted may over-correct and assume the setter has inverted both; but the usual design of a two-statement item is one true and one false, and the correct response to spotting the false one is to evaluate the other on its own merits rather than to distrust it by association.

Biodiversity is not spread evenly over the earth, and its distribution follows two strong and closely related gradients. The latitudinal gradient is the older and better documented: species richness for almost every major group peaks in the equatorial tropics and declines towards both poles. Three explanations are usually given together. First, energy — the tropics receive more solar radiation, support higher primary productivity, and greater productivity supports more species. Second, time and stability — temperate and polar regions were repeatedly overrun by ice during the Pleistocene glaciations and their communities are geologically young, whereas tropical regions have remained comparatively undisturbed for millions of years, allowing lineages to diversify without interruption. Third, specialisation — where conditions are constant and predictable, species can occupy narrower niches, and narrower niches allow more species to fit into the same space. The altitudinal gradient repeats the pattern in miniature on a mountainside: climbing a slope brings the same falling temperature, shortening growing season and increasing severity that moving poleward does, which is why vegetation zones on a tall tropical mountain are often compared to the belts of vegetation crossed on a journey from the equator to the pole.

Ecology questions in MPSC papers frequently take the two-statement form, in which the reliable design is one statement stated correctly and one stated in reverse. That makes direction the thing to check first: not whether the factor matters, but which way it runs. Both statements here name a real relationship, and only the sign of the first is wrong. It is also worth registering that this question prints only two statements, not the usual four — a candidate who reads the option list expecting four items and reasons about a statement (c) or (d) that was never printed will lose time and may mis-mark. Read the printed list, count the statements, and match the option wording to what is actually on the page. It is also worth reading the hedge in each statement. Both statements here say 'generally', which means a local exception does not falsify them — a claim about a general gradient is not refuted by pointing to one unusually rich temperate forest or one unusually poor tropical patch. Statement (a) fails not because exceptions exist but because the general pattern runs the other way.

• The latitudinal diversity gradient means species richness is highest in the equatorial tropics and declines towards the poles, a pattern that holds for almost every major group of organisms.
• Colombia, near the equator, has close to 1,400 recorded bird species, against about 105 for the state of New York at roughly 41 degrees north and about 56 for Greenland at roughly 71 degrees north.
• The tropics are richer partly because they receive more solar energy and are more productive, partly because they escaped the repeated Pleistocene glaciations that reset temperate and polar communities, and partly because constant conditions permit narrow niche specialisation.
• Biodiversity generally declines with increasing altitude on a mountain slope, as temperature falls, the growing season shortens, ultraviolet radiation increases, soils thin, and the area available towards the summit contracts.
• The altitudinal gradient mirrors the latitudinal one, which is why the vegetation belts crossed while climbing a tall tropical mountain resemble those crossed while travelling from the equator towards the pole.

• Getting the direction of the latitudinal gradient the wrong way round; high latitudes have large numbers of individuals of few species, which is abundance, not diversity
• Reasoning about a statement (c) or (d) that this question never printed — only statements (a) and (b) appear here
• Over-correcting after spotting one inverted statement and rejecting the other by association, which is exactly what option (4) is set to catch
• Confusing species richness with biomass or productivity; a boreal forest can hold enormous biomass in very few species

MPSC's environment and ecology section asks biodiversity through short two- or four-statement items in which one statement is a correct textbook generalisation and another is the same generalisation reversed. Latitude, altitude, moisture and area are the variables most often used, and the illustrative comparisons — bird counts by country, tree counts per hectare of rainforest, the extent of India's hotspots — recur from standard textbook material. The efficient preparation is to hold each gradient as a direction with two or three reasons attached, since the reasons are what the harder versions of the question ask for.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following is NOT among the usual explanations offered for the greater species richness of tropical regions ?

  • (a)Tropical environments escaped the repeated Pleistocene glaciations that disturbed temperate regions
  • (b)Greater solar energy supports higher productivity and therefore more species
  • (c)Constant and predictable conditions permit narrower niche specialisation
  • (d)Tropical species have larger geographical ranges than temperate species

  Answer(d) Tropical species have larger geographical ranges than temperate species — this is not an explanation of tropical richness, and the general observation runs the other way, with tropical species tending to have narrower ranges. The other three — evolutionary time undisturbed by glaciation, higher energy and productivity, and greater niche specialisation under constant conditions — are the standard explanations given together.

• practice — not a real PYQ

  Biodiversity generally declines with increasing altitude on a mountain slope chiefly because of which combination of factors ?

  • (a)Falling temperature, shorter growing season and shrinking available area
  • (b)Rising temperature and increasing soil depth
  • (c)Increasing rainfall and longer growing season
  • (d)Falling ultraviolet radiation and thickening atmosphere

  Answer(a) Falling temperature, shorter growing season and shrinking available area — with thinner air, stronger ultraviolet radiation and poorer, shallower soils reinforcing the effect. The conditions named in the other options describe the opposite of what happens on ascent, which is why the altitudinal gradient mirrors the latitudinal one.