2

1.0 INTRODUCTION

The biosphere is a thin layer around the Earth’s surface in which life exists.

Here, at the surface of the Earth, the signs of life are everywhere. Living

things interact with the physical world.

A wide variety of plants and animals live in the same habitat. Organisms of

the same kind that live in a particular habitat make up a population.

For example, in a freshwater pond habitat, you may find the three populations

as seen on Figure 1.

Figure 1: Three freshwater pond populations

The different plant and animal populations living together in a habitat makes

up a community. For example, duck weed, water snails, dragonflies and other

populations in a freshwater pond make up the freshwater community (see

Figure 2).

3

Figure 2: Freshwater pond community

In this unit, we will be studying about an ecosystem, its components, a food

chain and a food web as well as the different nutrient cycles.

1.1 OBJECTIVES

After going through this unit, you will be able to:

describe an ecosystem;

establish relationships between living and non-living components in

an ecosystem;

describe the role of organisms in a food chain and food web;

identify the different steps in biogeochemical cycles.

4

1.2 WHAT IS AN ECOSYSTEM?

An ecosystem is a naturally occurring assemblage of living and non-living

things. Living things such as plants and animals are referred to as the biotic

component. Non-living things such as water, oxygen, soil, temperature

constitute the abiotic component..

The size of an ecosystem can vary widely. It may be a whole forest, as well as

a small pond. Different ecosystems are often separated by geographical

barriers, like deserts, mountains or oceans, or are isolated otherwise, like lakes

or rivers. As these borders are never rigid, ecosystems tend to blend into each

other. As a result, the whole earth can be seen as a single ecosystem, or a lake

can be divided into several ecosystems, depending on the used scale.

Activity 1

Observing Ecosystems

(a) Take a close look at two different mini-ecosystems near your

school or house. Some examples include a rotten log, a tree, a

woodland park, or a small portion of yard. Identify and compare

the living organisms. Observe, describe, and compare physical

factors such as moisture, light, and soil type. Table 3 is an

example of what you may observe (use the “Key to Animals in

litter and trees” to help you; see Appendix 1).

Table 3: Examination of mini-ecosystem

Name of Living

Organisms Colour Size

Lichens

Caterpillars

(b) Observe an aquatic ecosystem such as a river or the sea. Make a

list of its biotic and abiotic components.

5

1.2.1 Components of an Ecosystem

Ecosystems have three basic components:

Producers

Consumers

Decomposers

Producers

Producers or Autotrophs are generally green plants. They utilise energy

from the sun as well as water and carbon dioxide from the abiotic environment

to produce their own food. This process is called Photosynthesis. Organisms

using the process of photosynthesis to manufacture their food are known as

producers.

Consumers

Consumers or Heterotrophs are organisms that feed on other organisms.

They depend on producers for their energy and synthesis needs. For example,

herbivores such as cows or goats are primary consumers. A carnivore, for

example, a lion that eats only herbivores is a secondary consumer. A tertiary

consumer such a hyena, feeds on secondary consumers.

Decomposers

Decomposers, such as bacteria or fungi, utilise energy from wastes or dead

organisms. During this process, nutrients are returned to the soil or water, and

carbon dioxide is returned to air and water.

6

Activity 2

Extracting Animals from Leaf Litter

Materials: polythene/plastic bag, stiff paper, lamp, sticky tape, glass,

small dish, hand lens.

Steps:

1. Fill a polythene or plastic bag with litter, noting any plants,

fungi or small animals, which you see.

2. Cut out a piece of stiff paper, such as Bristol paper to make a

cone. The narrow end of the cone should be about 5 mm

wide. Use sticky tape to fix the cone.

3. Pour a little water into a glass and fit the narrow end of the

cone into the glass.

4. Tip the litter carefully into the cone and place a table lamp

above it. The lamp must be at least 10 cm above the litter

(see Figure 4).

5. Switch on the lamp and leave the apparatus for 24 hours.

6. Carefully tip the contents of the glass into a small dish

and examine with a hand lens.

7. Use the “Key to animals in litter and trees”

(Found in Appendix 1) to help you identify the organisms

caught.

Figure 4: Extracting animals from leaf litter

7

1.2.2 Food Chains and Food Webs

Food Chains

The interaction of eating and being eaten is referred to as food chain. Figure 5

shows an example of a food chain. For example, plants are eaten by insects.

Insects are eaten by frogs. The frogs may be eaten by snakes and snakes by

eagles. Here the frog is the primary consumer, snake the secondary consumer

and bird the tertiary consumer.

Plants Insects Frog Snake Eagle

Figure 5: Example of a food chain

8

Food web

Interconnected food chains form a food web. Figure 6 shows an example of a

food web. The caterpillar feeds on the plant; the frog feeds on the caterpillar.

A small bird can also feed on the same caterpillar.

Figure 6: Example of a food web

Trophic Levels

The different levels in a food chain provide a definite structure called trophic

level. For example, producers are the lowest trophic level and consumers are

the highest trophic level.

9

Activity 3

Field activity: Food chain and Food web

Materials: Sticks, string, glass jars or sealable plastic bags, metre

ruler/ruler, measuring tape.

Steps:

1. In a park or yard, measure out a 1-square metre area. You may

use sticks and string to mark off the area.

2. Using glass jars or sealable plastic bags carefully collect plant

and animal organisms.

3. Identify the different organisms. Return the animals to

their original location.

4. Draw a possible food chain or food web that includes

the organisms you have collected.

Check Your Progress

1. What is an ecosystem? Give some examples of ecosystems.

_____________________________________________________

_____________________________________________________

_____________________________________________________

_____________________________________________________

2. Write down the names of four producers and four consumers found

in a pond/river or the sea. Join some of these producers and

consumers together to form a food chain.

_______________________________________________________

_______________________________________________________

_______________________________________________________

_______________________________________________________

10

3. What is a food chain? Why must it begin with a plant?

______________________________________________________

______________________________________________________

______________________________________________________

______________________________________________________

4. In an area of woodland there are foxes, grass and rabbits.

a. Place these organisms in their correct order in a food chain.

b. Add the name of one more herbivore to make a branched

food chain.

c. Make a food web by adding the name of a carnivore which

feeds on both the herbivores.

______________________________________________________

______________________________________________________

______________________________________________________

______________________________________________________

1.3 BIOGEOCHEMICAL CYCLES

A biogeochemical cycle is a circuit or pathway by which a chemical element

or molecule moves through both biotic and abiotic components of an

ecosystem. In effect, the element is recycled, although in some such cycles

there may be places where the element is accumulated or held for a long

period of time.

Biogeochemical cycles always involve equilibrium states: a balance in the

cycling of the element between biotic and abiotic components.

Biogeochemical cycles of particular interest in ecology are:

nitrogen cycle

carbon cycle

phosphorus cycle

water cycle

11

1.3.1 Nitrogen Cycle

Nitrogen gas makes up 78% of the volume of the earth’s atmosphere. There

are five main steps in the cycling of nitrogen (Figure 7)

Figure 7: Schematic representation of the Nitrogen cycle

1. Nitrogen fixation: the conversion of gaseous nitrogen to ammonia. The

enormous energy of lightning breaks nitrogen molecules to form nitrates.

2. Nitrification: the conversion of ammonia to nitrate by soil bacteria.

3. Assimilation: the uptake of nitrate and/or ammonia by plants.

4. Ammonification: the decomposition of the nitrogen-containing waste

products of organisms to ammonia by bacteria in soil.

5. Denitrification: the conversion of nitrate to gaseous nitrogen.

The nitrogen cycle is highly dependent on soil bacteria. Without them, life

would not exist.

12

1.3.2 Carbon Cycle

The atmosphere contains 0.03 % of gaseous carbon dioxide (CO2). Figure 8

represents the carbon cycle.

Carbon is present in the atmosphere as carbon dioxide gas. Carbon is also

present in the ocean as carbonates and bicarbonates and in rocks as limestone.

Plants produce food by the process of photosynthesis to produce oxygen. This

oxygen is used during respiration and combustion processes to produce carbon

dioxide. During photosynthesis, plants remove carbon dioxide from the air to

produce food such as glucose. Carbon dioxide then returns to the atmosphere

by the process of respiration.

Figure 8: Representation of the Carbon cycle

Sometimes carbon in biological molecules is not recycled back to the abiotic

environment for some time. It is stored in trees. Vast coal beds are formed

from bodies of ancient trees. Fossil fuels are formed from bodies of ancient

animals (aquatic and terrestrial).

13

1.3.3 Phosphorous Cycle

Phosphorus, mainly in the form of certain types of phosphate ions, is an

essential nutrient of both plants and animals.

Figure 9: The Phosphorous Cycle

Various forms of phosphorus are cycled mostly through water, soil and living

organisms by the phosphorus cycle, shown in Figure 9. In this cycle,

phosphorus moves slowly from phosphate deposits on land and shallow ocean

sediments to living organisms and back to the land and ocean.

Phosphorus released by the slow breakdown, or weathering, of phosphate rock

deposits is dissolved in soil water and taken up by plant roots.

Animals get their phosphorus by eating plants or by eating animals that have

eaten plants. Animal wastes and the decay products of dead animals and

plants return much of this phosphorus to the soil, to rivers, and eventually to

the ocean bottom as insoluble deposits of phosphate rock.

14

1.3.4 Water cycle

The water cycle, also known as the hydrologic cycle, is the circulation of

water within the earth's hydrosphere, involving changes in the physical state of

water: liquid, solid, and gas states (Figure 10).

Figure 10: The Water Cycle

The water cycle involves four main physical actions: evaporation, precipitation,

infiltration and runoff :

Evaporation is the transfer of water from liquid to gaseous state.

Precipitation is atmospheric moisture that has previously condensed to form

clouds. This mostly occurs as rainfall, snow, sleet, etc.

Infiltration into the ground is the flow of water from the ground to form

aquifers.

Runoff includes the variety of ways by which land surface water moves down

slope to the oceans for example in rivers, canals, etc.

15

Check Your Progress

5. Draw the carbon cycle using the following terms:

photosynthesis, respiration, combustion, fossil fuels, coal,

erosion, dissolved CO2 in water.

____________________________________________________

____________________________________________________

____________________________________________________

____________________________________________________

____________________________________________________

____________________________________________________

____________________________________________________

____________________________________________________

____________________________________________________

____________________________________________________

6. Name the different processes A, B, C and D in the given diagram:

A

D B

C SOIL

SOIL

_______________________________________________________

_______________________________________________________

Nitrogen in the air

Ammonia in the soil Nitrates in the soil

16

1.4 SUMMARY

Ecology is the study of the interrelationship of organisms in the

environment

An ecosystem includes a community of living organisms (biotic

components) and non-living factors (abiotic factors).

The sun is the only source of energy to living things. There is

unidirectional flow of energy from the sun to the ecosystem.

In an ecosystem, green plants are producers which make food by

photosynthesis. Herbivores depend on producers and carnivores depend on

herbivores.

Nutrients circulate in a circular path in an ecosystem.

Thus we get carbon, nitrogen, phosphorous and water circulating in the

ecosystem in cyclic paths.

1.5 UNIT-END EXERCISES

1. Why is a tree called a ‘producer’? Write down the names of four more

producers which all grow in the same habitat as a tree.

2. a. Name an animal, which feeds as an herbivore and state its normal food.

b. Name an animal, which feeds as a carnivore and state its normal food.

3. List a food chain in your immediate environment including at least four

trophic levels. You can prepare a chart by writing the names of organisms

and connecting them by lines to show the feeding process. What do you

observe?

17

4. Think of a situation where all plants are removed from the ecosystem.

a. What will happen to the solar energy?

b. What will happen to herbivores?

5. Look at the diagram of the water cycle. Can you think of other sources

from where water evaporates into the atmosphere?

1.6 ANSWERS TO CHECK YOUR PROGRESS

1. An ecosystem encompasses all interactions between living organisms and

their abiotic factors.

Some examples of ecosystems are forests, ponds, rivers, seas.

2. Phytoplankton Zooplankton “Tilapia” “Capitaine”

3. It is the interaction of organisms feeding upon each other. Plants

(producers) provide food and energy for the other organisms in the food

chain.

4. grass rabbits foxes

goats

Lions

5. Diagram of carbon cycle – refer to figure 8

6. A – Nitrogen fixation

B – Denitrification

C – Nitirification

D – Assimilation

18

1.7 APPENDIX

Key to Animals on trees and in litter

1. Worm-like body, no shell

or tentacles, less than 1

mm long.

nematode worm

(a)

2. Shell present

No shell

snail (b)

slug

3. Around twenty segments

woodlouse (c)

4. Two pairs of legs per

segment

One pair of legs per

segment

millipede (d)

centipede (e)

5. No legs, less than 15

segments

insect larvae (f)

6. Body clearly divided into

two parts

Body not clearly divided

spiders (g)

mites (h)

7. Slender body, long legs,

piercing mouth parts

Squat hairy body, short

antennae

gnat or mosquito

(i)

other two-winged

fly (j)

8. Long thin body, biting

mouth parts, complicated

network of veins on the

wings

dragonfly (k)

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

19

9. No distinct waist, sucking

mouth parts tucked under

head

greenfly

10. Large wings covered with

scales, long antennae

Small wings

butterfly or moth

11. Very narrow waist, long

abdomen, with egg-laying

tube

Hairy body, short

abdomen, often striped

ichneumon fly (l)

bee or wasp (m)

12. Forewing covers most of

abdomen

ladybird or other

beetle (n)

13. Abdomen 6 segments or

less, jumps by spring under

abdomen, 1mm long

More than 6 segments in

abdomen, no springs

springtail (o)

other groups of

small insects

14. Long, rather soft-bodied

organisms

Definite waist

insect larvae (p)

15. Small (2 cm long)

Large (up to 15 cm long)

whiteworms

earthworm(r)

(k)

(l)

(m)

(n)

(o)

(p)

(q)

(r)