The greatest diversity of animal and plant species is found in:

Correct — D, Tropical moist forests. Species richness on land peaks at the equator and falls away steadily towards both poles — the latitudinal diversity gradient, one of the oldest and best-documented patterns in ecology. Five separate advantages stack up in the wet tropics and nowhere else. Warmth and moisture are available in every month, so growth never pauses for a season and net primary productivity is the highest of any land biome; more energy entering a system supports more individuals, and more individuals means more populations large enough to survive. The forest is layered — floor, shrub layer, understorey, closed canopy, emergent crowns — and each layer is a distinct set of living places, so one hectare offers many more niches than a flat grassland can. Low seasonality lets a species specialise narrowly on one fruit or one host instead of having to survive a lean period as a generalist. The equatorial belt escaped the Pleistocene ice sheets, so its lineages have had far longer to diversify without being erased and rebuilt. And richness feeds on itself: every pollinator, seed disperser, herbivore and parasite is another partner to specialise on. NCERT puts numbers to the gradient — equatorial Colombia has roughly 1,400 bird species, New York at 41 degrees north about 105, and Greenland around 56 — and names the Amazonian rainforest as the most biodiverse place on Earth.

• (a)Temperate forests — Rich by comparison with tundra, and temperate coniferous forest covers a larger share of the world's forested land than any other forest type — but area is not diversity. Temperate forests are often dominated by a handful of tree species across enormous tracts, while a single hectare of lowland tropical forest can carry several hundred tree species, a total whole temperate countries do not reach. The annual winter shutdown also forces species towards being generalists that can survive a lean season, which suppresses the fine specialisation that multiplies species in the tropics.
• (b)Deserts and Savannas — The trap in this question, and it is a trap about visibility rather than about ecology. The African savanna carries the large, conspicuous mammals that most people picture when they hear the word wildlife, so it feels like the richest place on Earth. Counting species tells a different story. Water limits life in a desert, and a savanna's strongly seasonal rainfall imposes a long dry period every year, so both biomes have low net primary productivity and support far fewer species than a wet tropical forest. Deserts are in fact among the least species-rich land biomes there are. A fair qualification is that some arid regions hold remarkable endemic floras — the Succulent Karoo is even a recognised biodiversity hotspot — but endemism is not total richness, and no desert or savanna approaches a tropical moist forest on species count.
• (c)Arctic and Alpine systems — The cold end of the same gradient, and the poorest of the four. Short growing seasons, low temperatures and, in the Arctic, deglaciation only a few thousand years ago leave very few species; a tundra plant community may be counted in dozens where a tropical forest hectare runs into the hundreds. High latitudes compensate with enormous population sizes in a small number of species, which is precisely the reverse of the pattern the question is asking about.

Species richness is the plain count of species in a place, and its strongest global control is latitude. Three explanations are standardly given and they reinforce one another rather than compete. The tropics have had longer undisturbed evolutionary time, having escaped the repeated Pleistocene glaciations that scoured higher latitudes. Their climate is less seasonal and more constant, which favours narrow niche specialisation. And they receive more solar energy, giving higher productivity and so a larger resource base to divide among more species.

Two distinctions rescue this question from being answered on impression. The first is richness against visibility: an ecosystem of large, easily seen animals reads as diverse, while a forest whose species are mostly small, mostly high in the canopy and mostly insects reads as merely green. Counted properly, tropical moist forests occupy only a few per cent of the land surface and are estimated to hold more than half of all terrestrial species. The second is richness against the biodiversity hotspot label, which trips a lot of candidates. A hotspot is defined by high endemism combined with severe threat, not by raw species count, which is why the list includes the California Floristic Province and the Mediterranean Basin alongside tropical forests. Answering this question from a half-remembered hotspot map is what sends people to the wrong option.

• Species richness is greatest near the equator and declines towards both poles — the latitudinal diversity gradient.
• Tropical moist forests have the highest net primary productivity of any terrestrial biome, and their vertical layering multiplies the number of available niches.
• NCERT's illustration of the gradient: about 1,400 bird species in Colombia, about 105 in New York, about 56 in Greenland.
• Tropical forests cover a small percentage of the land surface yet are estimated to hold more than half of all land species.
• A biodiversity hotspot is defined by endemism plus threat rather than by total richness, so not every hotspot is a tropical forest.
• The equatorial belt escaped Pleistocene glaciation, giving its lineages far more undisturbed time to diversify.

• Equating the visibility of large animals in a savanna with the number of species living there.
• Reading the area a biome covers as a measure of how many species it holds.
• Assuming every biodiversity hotspot is a tropical forest, when the criteria are endemism and threat rather than richness.

Asked as a single superlative across four biomes, with the wrong options chosen so that one of them looks right to anyone answering from a wildlife documentary rather than from a species count.

• practice — not a real PYQ

  As one moves from the equator towards the poles, species diversity generally

  • (a)increases steadily
  • (b)decreases steadily
  • (c)stays unchanged
  • (d)increases up to the tropics of Cancer and Capricorn and then falls

  Answer(b) decreases steadily — this is the latitudinal diversity gradient, driven by falling productivity, rising seasonality and shorter undisturbed evolutionary time away from the equator.

• practice — not a real PYQ

  A region qualifies as a biodiversity hotspot mainly on which one of the following grounds?

  • (a)The largest total number of species
  • (b)High endemism combined with serious habitat loss
  • (c)The highest annual rainfall
  • (d)The largest protected area network

  Answer(b) High endemism combined with serious habitat loss — which is why regions such as the Mediterranean Basin appear on the list even though they are not the richest places on Earth.