An ecosystem is a dynamic system formed by the interactions of organisms with one another and with the non-living environment.
It is a dynamic system where the living organisms are in balance with each other and with the abiotic components.
Habitat A habitat is the natural environment in which an organism lives and obtains its basic resources such as food and shelter. Species A species is a group of organisms which can interbreed to produce fertile offspring. Population A population consists of organisms of the same species living in the same habitat at the same time. Community A community consists of different populations of plants and animals living and interacting in the habitat of an ecosystem. Niche The niche of an organism is the roles and activities of the organism in its habitat. Two organisms sharing the same habitat may have different niches.
Colonisation and succession
Natural phenomena or human activities such as volcanic eruptions, fires, earthquakes and uncontrolled mining activities leave the land with no living organisms.
Later, some organisms will come to occupy the bare land.
The process in which living organisms arrive at a new habitat, live, reproduce and take control of the habitat is known as colonisation .
The first species of organisms to colonise a new habitat is called the pioneer species .
The pioneer species have special adaptations to survive in unfavourable land conditions.
The pioneer species gradually changes the conditions of the habitat, making it no longer suitable for itself but more suitable for other species, called the successor species . Gradually, the successor species takes over the place of the pioneer species.
The process whereby a pioneer species is gradually replaced by other successor species is called succession .
Succession will carry on until a relatively stable community is formed. This type of community is known as the climax community .
In Malaysia, the tropical rainforest is the climax community.
It usually takes hundreds of years to form a climax community. After that it has little or no changes in its species structure. Therefore, we should treasure our forests.
Colonisation and succession in a mangrove swamp
Swamps are formed by deposition of mud and silt carried down by the river. It is found at the estuary, that is where the river meets the sea.
Only mangrove trees are able to colonise the soft, waterlogged, muddy soil which has a low oxygen level but high salt concentration.
Mangrove trees have adaptive structures to overcome the harsh conditions in a swampy area.
A mangrove swamp
Adaptations of mangrove trees Problems faced by mangrove trees Adaptive structures of mangrove trees
Ground too soft to provide support
Have long, branched cable roots or prop roots to support the plants in soft ground.
Very little oxygen in waterlogged mud
Have breathing roots called pneumatophores which grow upwards and protrude out of the ground.
Gaseous exchanges also occurs through lenticels on the bark of mangrove tress.
The root systems of mangroves Pneumatophores of Avicennia sp. Prop roots of Rhizophora sp.
Buttress roots of Bruguiera sp .
Problems faced by mangrove trees Adaptive structures of mangrove trees
High salt content of sea water
The cell sap in root cells has a higher salt content. Sea water enters the roots by osmosis. Excess salt from the sea water is eliminated through hydathodes found at the lower epidermis of leaves.
Seeds sink into the mud and die due to insufficient oxygen
Have viviparous seeds . A radicle grows from the germinated seed when it is still attached to the parent tree. When the seedling is released, the radicle holds the shoot above the mud.
Exposure to strong sunlight and intense heat leads to a higher rate of transpiration
Leaves with thick cuticle and sunken stomata to reduce transpiration
Store water in succulent leaves
Avicennia sp . and Sonneratia sp . are the pioneer species of a mangrove swamp. Avicennia sp . grows in areas facing the sea while Sonneratia sp . is found in more sheltered areas.
The extensive cable root system of these plants traps more mud and slit as well as organic matter from decaying plant parts.
As time passes, the soil becomes more compact and the shore level is slightly raised. The soil becomes firmer and less waterlogged. Such conditions favour the growth of another kind of mangrove tree, namely Rhizophora sp .
Gradually, Rhizophora sp . replaces the pioneer species.
The prop root system of Rhizophora sp . continues to trap more slit and mud. Humus is formed from the old pioneer species as well as decaying leaves of Rhizophora sp . The soil becomes firmer, more compact and fertile. The shore level is raised and is less saline. The condition now is more suitable for Bruguiera sp .
The buttress root system of Bruguiera sp . Traps more silt and mud causing the shore to extend further to the sea.
As time passes, coconut trees, Nipah and Pandanus sp . gradually replace the Bruguiera sp . when the soil becomes more like terrestrial ground.
Eventually a tropical rainforest, which is the climax community, is formed.
Distribution of different mangrove species at the mouth of a river
Colonisation and succession in a pond
Colonisation by pioneer species
(a) Submerged plants such as Hydrilla sp ., Elodea sp . and Cabomba sp . as well as phytoplankton are the pioneer species in a pond.
(b) These submerged plants have adaptive features such as long fibrous roots which penetrate deep into the soil to absorb nutrients and hold the sand together. Fine leaves enable the plants to flow with the water.
(a) When the pioneer species die, they settle to the bottom of the pond and become humus. At the same time, the soil eroded from the sides of the pond makes the pond shallower.
(b) Such a condition becomes unfavourable for the submerged plants but more suitable for floating plants such as Nymphaea sp . (lily), Lemna sp . (duckweed) and Eichornia sp . (water hyacinth) which gradually replace the pioneer species.
Floating plants Water lily Water hyacinth Pistia sp .
Succession by emergent (amphibious) plants
(a) The floating plants reproduce rapidly as they receive enough sunlight for photosynthesis. They cover a large area of the surface of the pond. This prevents sunlight from reaching the bottom of the pond.
(b) Without sunlight, the submerged plants cannot perform photosynthesis. As a result, these plants die and become humus.
(c) The amount of humus deposited at the bottom of the pond increases. More soil erosion occurs which results in the pond becoming shallower. This makes the pond too shallow for the floating plants.
(d) Floating plants are gradually being replaced by emergent plants such as Fimbristylis sp . and Lepironia sp .
(e) Emergent plants can live in water as well as on land. Their extensive rhizomes grow rapidlly to bind the soil together and to absorb nutrients, changing the habitat. They grow from the edge of the pond to the middle of the pond.
(a) The death of emergent plants as well as deposition of more organic matter make the pond even shallower. Evaporation of pond water finally dries the pond.
(b) Terrestrial plants such as creepers, grasses, ferns and herbaceous plants begin to grow.
(c) Later, shrubs and woody plants begin to grow.
Over hundreds of years, a tropical rainforest which is a climax community is formed.
Which of the following describes a population?
A. The function of an organism or the role it plays in an ecosystem
B. The natural environment in which an organism lives
C. A group of organisms of the same species living in the same habitat at the same time
D. A natural collection of plant and animal species living within a habitat in an ecosystem
Answer : C
Phytoplankton, zooplankton and algae are often among the first species to establish themselves in a mining pond. As time passes, submerged and floating plants will grow, followed by amphibious plants, grasses, small shrubs, bushes and eventually trees.
This is an example of
Answer : D
Over many years a forest can be found on an initially barren piece of land left behind by a volcanic eruption. What is the correct sequence of ecological processes that have taken place?
A. Colonisation, succession, climax community
B. Colonisation, climax community, succession
C. Succession, colonisation, climax communiity
D. Succession, climax community, colonisation
Answer : A
Which gives the correct sequence of plants involved in the process of succession in a disused pond?
A. Emergent plants floating plants
land plants submerged plants
B. Floating plants submerged plants
land plants emergent plants
C. Submerged plants floating plants
emergent plants land plants
D. Land plants emergent plants
floating plants submerged plant
Answer : C
Diagram 1 shows the seed of Rhizophora sp . which germinates while it is still attached to the parent plant.
This phenomenon is know as
A. double fertilisation
D. vegetative reproduction
Answer : C
8.3 Population Ecology
Population is a group of organisms of the same species living in a habitat. The number of organisms in a population is called the population size .
The study of the measurement of population size and the factors affecting the population size is know as population ecology .
The quadrat sampling technique is mainly used to estimate population size, densiity and distribution of plants and immobile animals.
The capture, mark, release and recapture technique is used to estimate the population sizes of mobile animals.
Capture, mark, release and recapture technique
To estimate the population size of animals which move freely such as snails and woodlice, the capture, mark, release and recapture technique is used.
Capture, mark, release and recapture technique
A number of animals are captured at random and marked with waterproof paint, ink or a ring.
The marked animals are released.
A second capture is carried out after a few days.
The number of animals captured in the second sample and the number of marked animals are recoded.
The following assumptions are made in this technique:
The size of the population does not change and is stable throughout the period of investigation.
The marked animals are not harmed or predated upon.
The animals are captured at random.
The marked animals are able to mix randomly with the other animals before the second capture.
Each marked animals has the same probability of being recaptured as an unmarked animals.
To increase the accuracy of this technique,
more animals must be captured.
the animals must be captured at random.
the markings must be permanent.
enough time must be allowed for the marked and unmarked animals to mix.
Quadrat sampling technique
A quadrat sampling technique is used to determine the distribution of plants whereby the density , frequency and percentage coverage of the plants can be determined.
A quadrat has a square or rectangular frame made of wood, metal or rope. It is subdivided into smaller squares.
The size of the quadrat depends on the size and density of the plants sampled.
In this technique, a number of quadrats are placed randomly in the area being studied.
The species found within the quadrat are counted and recorded.
The distribution of the plants can be determined in three ways:
The percentage coverage is the percentage of how much of an area is covered by the plants.
The frequency is the number of times a particular plant is found from the total number of quadrats used.
The density is the number of individuals per unit area. This value is difficult to determine for the population of plants which reproduce vegetatively.
Influence of abiotic factors on the population distribution of an organism
Abiotic factors refer to the non-living components of an ecosystem which include pH, temperature, light intensity, humidity, topography and climate.
The population distribution of an organism is influenced by the changes in the abiotic factors.
The quadrat sampling technique can be used in the study of the populations of
Answer : B
Which of the following steps need to be taken to improve the accuracy of the capture, mark, release and recapture technique ?
A. Put a bigger mark.
B. Capture more animals.
C. Capture the bigger animals.
D. Do recapture after one month.
Answer : B
The following information is about the capture, mark, release and recapture technique.
P – Mark the captured animals.
Q – A second sample is captured.
R – Release the marked animals.
S – An initial sample is captured.
Which of the following is the correct sequence of the capture, mark, release and recapture technique?
A. P R S Q
B. P S R Q
C. S P R Q
D. S R Q P
Answer : C
The table shows the results obtained from an investigation into the distribution of mimosa plants on a school field.
The percentage frequency of the mimosa plants is
Answer : D Quadrat 1 2 3 4 5 6 7 8 9 10 Number of plants 5 2 0 1 8 2 0 3 4 5