Form 4 Biology Chapter 8: Biodiversity (b)

Ecosystem

1. An ecosystem is community of living organisms interacting with one

another and with the non-living environment.

2. Ecosystems are dynamic. All biotic communities are continually

changing. They change in response to external factors such as changing

climates as well as internal factors caused by the organisms themselves.

Ecological terms

Habitat

A habitat is the natural environment in which s organism lives.

A habitat provides plants and animals with the basic resources of life:

food, shelter, living space, nesting and breeding sites. Example, a single

tree can be a habitat for beetles, caterpillars and birds.

Species

A species consists of a group of organisms that look alike and have

similar characteristics, share the same ecological niche and are

capable of interbreeding and producing fertile offspring.

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Form 4 Biology Chapter 8: Biodiversity (b)

Population

A population consists of a group of organisms of the same species

living in the same habitat at the same time, for example, a population

of grasshoppers living in paddy fields.

The individuals of a population rely on the same resources and are

affected by similar environmental factors

Community

A community is a natural collection of plant and animal species

living within a defined area or habitat in an ecosystem.

The members of a community are interdependent and interact with

one another. A change in the population of one species will affect the

population and distribution of the other species within the community.

Niche

Every organism has an important role to play. The function of an

organism or the role it plays in an ecosystem is known as the

ecologies niche.

This includes its habitat, its interactions with other organisms, the types

of food it consumes, the range of temperatures it tolerates and the space

it occupies.

Based on its role in a food chain, an organism can be categorized as a

producer, a predator, or a parasite. For example, the niche of grass is

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Form 4 Biology Chapter 8: Biodiversity (b)

as a producer of carbohydrates and other organic substances, which

provide food for other organisms in ecosystem.

The Process of Colonization and Succession on a Bare Ground

1. An environment of bare rocks and sand left behind by a forest fire is not

suitable for most organisms.

2. After the ground cools down, water reenters the environment. When

air, water, nutrients and sunlight are available, spores and seeds of

certain plants start to germinate and grow. This process is known as

colonization. Colonization is a process whereby a species invades and

occupies a newly formed area where no life has existed previously.

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Form 4 Biology Chapter 8: Biodiversity (b)

This is an example of bare ground. Nothing is there.

3. The early species that appear on a bare patch of ground are called

pioneer species. Pioneer species are typically hardy plants, for

example, grasses, ferns and sedges (grass-like plants).

4. Pioneer species have special adaptations which enable them to survive in

dry and nutrient-poor soil. They change the environment in ways that

favor subsequent communities.

a) Pioneer plants generally produce large numbers of easily dispersed

seeds which help them colonize open spaces.

b) Soil forms gradually from the actions of the pioneer species and

from their decomposed remains.

c) The dense root systems of the pioneer species bind the sand and soil

particles together, and hold water and humus to improve the structure

of the soil.

d) They have a short life cycle. When they die, their remains add to the

humus content of the soil.

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Form 4 Biology Chapter 8: Biodiversity (b)

5. The pioneer species modify the environment, ultimately creating

conditions which are less favourable for themselves but are more

conducive to other species called successor species. The successor

species replaces the pioneer species because they are better adapted to

the new conditions.

6. The successor species grow larger and gradually shaded the original

pioneer species, eventually replacing it altogether.

7. Examples of successor species are herbaceous plants such as

dandelions and weeds.

a) These plants have small wind-dispersed seeds which are able to

spread, germinate and grow rapidly.

b) The successor species also changes the structure and the quality of

the soil, making it more conducive for the growth of bigger plants

like shrubs.

Example of dominant species: Shrub

8. The shrubs become the new dominant species that grow faster and

dominate the slower-growing pioneer and successor species.

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Form 4 Biology Chapter 8: Biodiversity (b)

9. As time passes the shrubs, in turn, modify the environment which

allows large trees to grow.

10. The large trees provide shade and the shrubs cannot compete and are

replaced by forest-floor species which require lower light intensities.

11. The gradual process in which one community changes the environment

so that it is replaced by another community is called succession.

Succession is a gradual and continuous process which can take hundreds

of years.

Colonization in Mangrove Swamp

1. Mangrove swamps are mostly found in the tropical and subtropical

regions where freshwater meets salt water.

2. They have muddy soft soil and are a hostile environment for normal

plants. This is because the soil has very low levels of oxygen and a high

concentration of salt

3. In addition, mangrove swamps are exposed to high intensities of sunlight

and strong winds.

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Form 4 Biology Chapter 8: Biodiversity (b)

Adaptation of mangrove plants

Problems faced by

mangrove plants

Adaptations

Soft muddy soil strong

coastal winds pose

support problems

To support themselves, mangrove trees such as the

Avicennia sp. have long, highly branched

underground cable roots.

Another mangrove species, the Rhizaphora sp.,

have prop roots.

These roots, also known as aerial roots, anchor the

plants onto the muddy soil.

Waterlogged

conditions of the soil

reduce the amount of

oxygen available and

lead to an anaerobic

environment.

The Avicennia sp. has roots called

pneumatophores which grow vertically upwards.

Each pneumatophore has numerous pores through

which gaseous exchange takes place during low

tides.

Gaseous exchange also occurs through pores called

lenticels found on the bark of the mangrove trees.

Direct exposure to the

sun leads to a high rate

of transpiration in the

mangrove plants.

The leaves of the mangroves are covered by a thick

layer of cuticle which reduces transpiration during

hot days. In addition, the leaves are thick and

succulent, and able to store water.

The high salinity of the

sea water makes the

The cell sap in the roots cells of the mangrove trees

has higher osmotic pressure than the soil water

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Form 4 Biology Chapter 8: Biodiversity (b)

surrounding water in

the soil hypertonic

when compared to the

cell sap of the root

cells.

that surrounds them.

This ensures that the roots do not lose water by

osmosis. Instead, the excess salt in the hypertonic

solution of the soil enters the roots and is then

excreted as crystalline salt from the hydathodes,

the pores on the lower epidermis of leaves.

Seeds which fall onto

the ground die because

they are submerged in

the soft and water

logged soil.

Mangrove seeds are able to germinate while still

attached to the mother plant. This phenomenon is

called viviparity.

Viviparity increases the chances of survival of the

mangrove as the seedlings can float horizontally on

the water and subsequently get washed up on the

sand or mudflats where they settle to establish a

new population.

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Form 4 Biology Chapter 8: Biodiversity (b)

Avicennia sp. with its pneumatophore

Rhizaphora sp. with its prop roots

Bruguiera sp. and its buttress root system

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Form 4 Biology Chapter 8: Biodiversity (b)

Colonisation and succession in a mangrove swamp

1. The pioneer species of a mangrove swamp are the sonneratia sp. and

Avicennia sp.

2. The presence of these species gradually changes the physical

environment of the habitat. The extensive root system of these plants

traps and collects sediments, including organic matter from decaying

plant parts.

3. As time passes, the soil becomes more compact and firm. This

condition favors the growth of the Rhizaphora sp.

4. Gradually the Rhizophora sp, replaces the pioneer species.

5. The prop root system of the Rhizaphora sp. traps silt and mud ,

creating a firmer soil structure over time.

6. The condition now becomes higher. As a result, the soil is drier become

it is less submerged by sea water.

7. The condition now becomes more suitable for another mangrove

species, the Bruguiera sp., which replaces the Rhizaphora sp.

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Form 4 Biology Chapter 8: Biodiversity (b)

8. The buttress root system of the Bruguiera sp. forms loops which

extend from the soil to trap more silt and mud.

9. As more sediments are deposited, the shore extends further to the sea.

The old shore is now further away from the sea and is like terrestrial

ground.

10. Over time, terrestrial plants like the nipah palm and Pandanus sp.

begin to replace the Bruguiera sp.

11. The gradual transition and succession from a mangrove swamp to a

terrestrial forest and eventually to a tropical rainforest, which is a climax

community, takes a long time.

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Form 4 Biology Chapter 8: Biodiversity (b)

Colonisation and Succession in a Pond

1. Succession in a disused pond begins with the growth of pioneer species

such as phytoplankton, algae and submerged plants like the Hydrilla

sp., Cabomba sp. and Elodea sp.

2. These plants have special adaptive features which enable them to

colonise the pond. Their fibrous roots penetrate deep into the soil to

absorb nutrients and bind sand particles together.

3. Plenty of sunlight penetrates through the clear water to allow

photosynthesis to take place.

4. When the pioneer species die and decompose, more organic nutrients

are released into the pond. The organic matter is converted into humus

at the pond base.

5. The humus and soil which erode from the sides of the pond are

deposited on the base of the pond, making the pond shallower.

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Form 4 Biology Chapter 8: Biodiversity (b)

6. The condition becomes unfavourable for submerged plants but more

suitable for floating plants such as duckweed (Lemna sp.), water

hyacinths (Eichornia sp.) and lotus plants (Nelumbium sp.).

7. These plants float freely on the surface of the water. Since these plants

receive sunlight directly and can reproduce rapidly by vegetative

propagation, they spread to cover a large area of the water surface

and prevent sunlight from reaching the submerge plants .

8. As a result, the submerged plants die because they cannot

photosynthesise.

9. The decomposed remains of the submerged plants add more organic

matter on the base of the pond.

10. At the same time, more erosion occurs at the edge which results in

more sediments being deposted on the base of the pond.

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Form 4 Biology Chapter 8: Biodiversity (b)

11. As a result, the pond becomes more and more shallow which makes it

unsuitable for the floating plants.

12. The floating plants are subsequently replaced by emergent

(amphibious) plants which can live in water as well as on land, for

example, sedges and cattails.

13. The rhizomes of these plants grow horizontally across the habitat. Their

extensive roots bind the soil particles together and penetrate deeply to

absorm more mineral salts. These plants spread rapidly and colonize

the habitat, changing it.

14. The emergent plants grow from the edge of the pond towards the middle

of the pond as the pond become more shallow.

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Form 4 Biology Chapter 8: Biodiversity (b)

15. When these plants die, their decomposed remains add to the

sediments on the base of the pond. This further reduces the depth of

the pond.

16. The condition of the pond now becomes more favourable for land plants

like small herbaceous weeds, for example, Ageratum conyziodes,

Euphorbia hirta and Oldentandia dichotoma.

17. As time passes, the land becomes drier.

18. Land plants such as shrubs, bushes and woody plant become numerous.

19. A primary forest emerges and eventually turns into a tropical rainforest

which is also known as a climax community.

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