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NCERT Solutions for Class 9th Science Chapter 12 Let us read a case study — Activity 12.2

Book page 232 Updated on2026-09-08

Q1.
How are species distributed within a forest? Which plants and animals are closely linked?
Answer

Species are not spread evenly through a forest — each one sits where its particular requirements are met, so the forest is a mosaic of small, specific zones rather than one uniform block of trees.

What the Pakke data show: the four hornbill species are found in different parts of the same forest, and the case study names the two variables that decide where — tree size and fruit availability. These large birds nest only in large, old trees with suitable cavities, and each species feeds on particular fruits. Wherever a big enough tree and the right fruiting trees occur together, that hornbill occurs; elsewhere it does not.

Closely linked plants and animals in the Pakke example:

  • Large old trees ↔ hornbills. The tree supplies the nest cavity; without a cavity of the right size the bird cannot breed at all.
  • Fruiting trees ↔ hornbills. The tree supplies food; the hornbill swallows the fruit and later drops the seed far from the parent tree, so the bird disperses the seed. The link runs both ways — this is why hornbills are called the "farmers of the forest".
  • Tiger ↔ deer ↔ grasses and shrubs. The reserve's food chain rests on the plants at its base.
Tip: a link that runs both ways — each partner needed by the other — is the strongest kind. Break it at one end and both ends fail, which is what question (iii) below asks you to work out.
Q2.
How does classifying the four hornbill species help us understand biodiversity?
Answer

Because until the four are separated, the whole pattern is invisible. If a scientist records only "hornbill", Pakke has one bird found everywhere. Once the four species are told apart, the same forest turns out to hold four different birds, each in its own part of the wood, using different trees and different fruits.

What classification buys here:
  • It converts a name into a count. "Nearly 300 bird species in Pakke, out of about 1,300 in the whole of India" is a measurement of richness — and it is only possible because every one of those birds has been distinguished and named.
  • It lets you ask precise questions. The case study lists them: how are species distributed within a forest, which plants and animals are closely linked. You cannot ask either question of an unsorted heap.
  • It shows how the species share the forest. Four related birds coexisting means they are not competing head-on; each has found a slightly different set of trees and fruits. That division of resources is one of the mechanisms that lets a forest hold so many species at once.
  • It makes conservation specific. "Protect hornbills" is a slogan; "protect large old cavity trees of these species in this valley, because the Rufous-necked Hornbill nests only there" is an action.
Q3.
How can scientists keep track of so many species?
Answer

By using a system instead of memory — the same three tools the rest of this chapter builds.

  1. A hierarchy. Every species is filed under Kingdom → Phylum → Class → Order → Family → Genus → Species. Like a postal address, this narrows a search from millions to a handful in seven steps. All four Pakke hornbills sit in the family Bucerotidae, so anything true of the family is already known for all four.
  2. A unique two-part name. Binomial nomenclature gives every species one Latin name used worldwide, so a scientist in Arunachal Pradesh and one in France mean the same bird — no confusion between bagh, puli, tiger and tigre.
  3. Identification keys and records. Field guides, museum specimens, photographs, call recordings and now DNA barcodes let a new individual be matched against what is already known. Anything that does not match is a candidate new species.
Did you know? The Purple Frog of Kerala, Nasikabatrachus sahyadrensis, was described only in 2003 — it spends most of the year underground and comes out only in the monsoon to breed. Its existence had been missed for exactly as long as nobody had looked in the right place at the right time.
Q4.
The four hornbills look similar in some ways. What features can help scientists distinguish them from one another?
Answer

All four are large fruit-eating birds with the same body plan, so they must be separated on fine, repeatable characters rather than on general appearance.

  • The casque and the bill. The horn-like casque on top of the bill differs in size, shape and colour between species — this is the single most useful character in hornbills, and it is why the group is named after it.
  • Plumage colour and pattern. The rufous (reddish) neck of the Rufous-necked Hornbill; the black-and-white body of the Oriental Pied Hornbill; the yellow bill and neck of the Great Hornbill; the pouch and bar pattern of the Wreathed Hornbill.
  • Body size. The Great Hornbill is much the largest of the four; the Oriental Pied is the smallest.
  • Call. Each species has its own loud call, which is often how it is detected before it is seen.
  • Ecological characters. Which fruits it eats, which tree species and cavity size it nests in, which part of the forest it occupies — as the case study says, these differ between the species.
  • Genetic similarity. Comparing DNA settles the relationships when appearance is ambiguous.
Why appearance alone is not enough: in many birds the male and female look different, and a young bird looks different again. A character that changes with sex, age or season is a bad character. Casque shape, call and DNA stay constant, so they are the ones a taxonomist relies on.
Q5.
What would happen if the large, old trees disappeared from the forest?
Answer

The hornbills would stop breeding, and their loss would then work its way outwards through the forest. The case study is explicit: these large birds nest only in large, old trees with suitable cavities.

The chain of consequences, step by step:
  1. No cavity → no nest. A hornbill cannot dig its own hole; it must find one. A big cavity takes many decades to form. Remove the old trees and the birds have nowhere to lay.
  2. No nest → no chicks. The adult birds may survive for years, so the forest still looks as though it has hornbills — but the population stops replacing itself.
  3. No hornbills → no seed dispersal. Hornbills swallow fruits whole and drop the seeds far from the parent tree. Without them, the seeds of many large-fruited forest trees fall directly under the parent, where they are crowded and heavily eaten.
  4. No dispersal → fewer new trees. Over decades the tree species that depended on hornbills decline — including, eventually, the very kinds of tree that would have become the next generation of nest trees.
  5. Other cavity users suffer too. Owls, parakeets, squirrels, bats and many insects share these cavities. Losing old trees removes housing for all of them at once.
Notice the shape of it: the damage is delayed and self-reinforcing. Nothing dies on the day the tree is cut, which is precisely what makes this kind of loss easy to miss and hard to reverse.
Tip: a species like the hornbill, whose removal changes the whole forest, and a resource like the old cavity tree, on which many species depend, are why conservation protects habitat structure and not only individual animals.
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