Question 7

Field visit to a forest: forest versus grassland, a forest food web, trophic levels and pyramids of numbers

10 marks

Time to write: About 20 minutes
Topic: Types of ecosystems

Question

Question 7 · 10 marks

As part of your curriculum, suppose you go to a nearby forest area in order to better understand the structure and functioning of a forest ecosystem. On the basis of your understanding, answer the questions that follow:

How does forest ecosystem differ from a grassland ecosystem
Draw a food web made up of organisms found in a forest.
Categorize the organisms shown by you in the food web into the various trophic levels, that is, producers, primary consumers, secondary consumrers and tertiary consumers
Draw a pyramid of numbers which is a) upright and b) inverted, using appropriate food chains found in a forest ecosystem.

Reworded from the university paper; the task is the same. The original paper belongs to the University of Delhi. See DU's question bank.

Before you write

Read off the question and its marks

20 minfor 10 marks

Read from the question

Draw…Two asks in one question: answer both.
differSet the differences out in a table, basis by basis, not as two separate essays.

What the paper leaves open

The paper prints "consumrers" in part (c); read as "consumers". Wording is kept as printed.

Write it in this order

The parts stand alone: start with the one you are surest of.

Read; underline the asks; jot your points 1 min
1 (a) Forest versus grassland 2 marks 4 min
2 (b) Forest food web 2 marks 4 min
3 (c) Trophic levels 2 marks 3 min
4 (d) Upright and inverted pyramids of numbers 4 marks 7 min
Check: every ask answered, key terms underlined 1 min

1234

Short on time? A few lines on every part beat a full answer to one: each part is capped at its own marks.

Try it first. About 20 minutes for 10 marks. Write it out, then open the solution.

Solution

Hover or tap a marked figure, like 72,000, to see how it is made and where else it appears.

(a) How a forest ecosystem differs from a grassland ecosystem

Basis	Forest ecosystem	Grassland ecosystem
Rainfall and soil	Enough rain and soil depth to support trees	Usually low rainfall and/or shallow, poor soil, which keep most trees and shrubs out but let grass grow in the monsoon
Producers	Trees, shrubs, herbs and climbers in layers: trees at the top, shrubs next, herbs and grasses at the bottom (stratification)	Mainly grasses and small herbs, mostly low, though the Terai's patches of tall elephant grass grow to about five metres
Seasonal change	Canopy present all year (or leafless only for a short dry season)	In most grasslands the grass dries and the above-ground parts die in summer; it grows back from roots and seeds in the next monsoon
Animals	Tree-dwelling and shade-loving animals: monkeys, squirrels, deer, tiger, leopard, many birds	Grazing animals and burrowers adapted to open country: blackbuck, hares, rodents, bustards, wolves
Biomass	Large, stored in wood	Small, mostly in roots and grass

(b) A food web from a forest

Several food chains of a deciduous forest of central India, interlinked into a web (each arrow means is eaten by):

Tree leaves  --> caterpillar --> sparrow ----> hawk
Tree leaves  --> deer -------------------------> tiger
Grass        --> deer
Grass        --> rabbit -----> snake ---------> hawk
Fruits/seeds --> squirrel ---> snake
Fruits/seeds --> sparrow
Tree leaves  --> langur -----------------------> leopard
Caterpillar  --> frog -------> snake

The chains are not separate: the deer, sparrow and snake each sit in more than one chain, and this interlocking makes the food web.

(c) Trophic levels of the organisms in the food web

Each step in a food chain is a trophic level: green plants form the first level, herbivores the second, carnivores the third, and the carnivores that eat them the fourth. The table lists them from the top of the food chain down, as they would stand in a pyramid.

Trophic level	Organisms from the food web
Tertiary consumers	Hawk when it eats a sparrow that ate caterpillars, or a snake that ate a rabbit or squirrel; snake when it eats frogs
Secondary consumers	Frog, snake (eating rabbit or squirrel), sparrow when it eats caterpillars, tiger and leopard (eating herbivores)
Primary consumers (herbivores)	Caterpillar, deer, rabbit, squirrel, langur; the sparrow when it eats seeds
Producers (first level)	Trees (leaves, fruits, seeds), grass

A species can occupy more than one level: the sparrow is a primary consumer when it eats seeds and a secondary consumer when it eats insects. The hawk moves in the same way: it is a secondary consumer when it eats a seed-eating sparrow, and a quaternary consumer, one step above the categories asked for, when it eats a snake that ate a frog (leaves → caterpillar → frog → snake → hawk).

(d) Pyramids of numbers from forest food chains

A pyramid of numbers shows how many individuals there are at each trophic level, with producers at the base and the top consumer at the apex.

Upright pyramid: grass → deer → tiger. Many grass plants feed fewer deer, and many deer feed a single tiger, so each level is smaller than the one below; most ecosystems give upright pyramids. (Numbers are our own illustration.)

              [ Tiger: 1]                secondary/top consumer
         [   Deer: 50]             primary consumer
   [     Grass plants: 100,000]     producer

Inverted pyramid: one large tree → insects → parasites. A single big tree supports thousands of insects feeding on it, and each insect carries many tiny parasites, so the numbers grow towards the top. (Numbers are our own illustration.)

   [   Parasites on the insects: 20,000]    secondary consumers
         [   Leaf-eating insects: 2,000]     primary consumers
              [ One banyan tree: 1]           producer

The pyramid of numbers is inverted here because one producer is very large, not because energy increases; a pyramid of energy is always upright.

Where the 10 marks sit

#	Component	Marks
1	(a) Forest versus grassland	2
2	(b) Forest food web	2
3	(c) Trophic levels	2
4	(d) Upright and inverted pyramids of numbers	4
	Total	10

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