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Chapter 20
Two species of sea snakes, Laticauda laticaudata and Laticauda colubrina,
spend most of their lives living in the sea. They must be able to move, hunt and
survive in the water, but for short periods they need to come ashore to breed.
They inhabit some of the Pacific islands which have low coral cliffs that the
snakes need to climb in order to get onto dry land for courtship, mating and
laying eggs.
Diagram to show a cross-sectional view of cliff used in the study
height
muscle strength
(arbitrary unit)
The scientists studied the ability of snakes to climb the low cliff. They measured
the body mass of the snakes and relative strength of the snakes. The mean results
are shown in the bar charts below.
body mass (g)
1.
Ecosystems
females
males
females
males
a
State which sex of snake has the greater mean mass.
(1 mark)
b
c
State which species of snake is the strongest.
(1 mark)
The error bars represent standard deviation. Deduce which groups of snakes
show the greatest variability in body mass.
(1 mark)
d
For L. colubrina calculate the difference between the strength of the male
and the female snakes.
(1 mark)
The scientists placed male snakes onto the coral cliff near its base. They
measured their ability to hold onto the coral cliff. The L. colubrina males
climbed much faster than the males of L. laticaudata. The graph below shows
the ability of snakes to cling to the coral cliff.
snakes still
clinging to the
cliff (%)
time (s)
e
f
i
Compare the ability of the two species of snake to cling to the cliff.
(2 marks)
ii
Suggest two reasons for the difference in the ability of the snakes to
cling to the cliffs for 120 seconds.
(2 marks)
Discuss the advantages and disadvantages of body mass in these two
species of snake.
(3 marks)
2.
The diagram below shows part of the carbon cycle.
carbon dioxide
in air
P
producers
Q
R
consumers
S
fossil fuels
a
State the names of processes P, Q, R and S.
(2 marks)
b
As the population increases, the level of carbon dioxide in the atmosphere
also increases.
i
With reference to the above diagram, explain why the level of carbon
dioxide in the atmosphere increases.
ii
State one environmental problem caused by the increase in carbon
dioxide level.
iii
(4 marks)
(1 mark)
State two actions that we can take to reduce the effect of human
activity on the atmospheric carbon dioxide level.
(2 marks)
a
Explain the meaning of the terms population and community.
b
Different organisms in the same community interact with one another. The
graph below shows the numbers of four species of plants in a certain area of
a forest in a 40-year period after the introduction of species W.
number of plants
3.
100
90
80
70
60
50
40
30
20
10
0
(4 marks)
W
X
0
10
20
year
30
Y
40 Z
i
Describe the change in the numbers of different species in these 40
years.
(4 marks)
ii
Explain why the population of species W increases rapidly in the first
25 years.
(1 mark)
iii
Why the number of species W becomes steady after year 25?
(1 mark)
iv
What is the relationship between species W and Y? Explain your
answer with reference to the graph.
(2 marks)
4.
The diagram below shows a Galapagos tortoise.
Galapagos tortoises are able to eat leaves on bushes above the ground while
other species of tortoise can only eat food on the ground.
5.
a
State how the body structure of Galapagos tortoises allows them to eat
leaves on bushes.
(2 marks)
b
Explain why they have a higher survival chance than other species of
tortoise.
(2 marks)
c
Some goats were introduced to the habitat of the Galapagos tortoises.
Explain how this affects the tortoise population.
(2 marks)
d
Scientists warned that the increase in human population may lead to a
decrease in the number of Galapagos tortoises. Explain why.
(2 marks)
The diagram below shows part of the nitrogen cycle that occurs in the farmland.
nitrogen gas
in air
plants
dead plants
nitrogen
compounds in soil
a
Which nitrogen compounds do plant roots absorb from the soil? (1 mark)
b
i
Why does the nitrogen content of the soil in the farmland decrease?
(1 mark)
ii
How can the farmers increase the nitrogen content of the soil?
(1 mark)
c
Describe how the nitrogen in dead plants can be used for the growth of
other plants. Name the organisms involved.
(5 marks)
6.
Scientists carried out an investigation into the interaction between different
species of organisms. The table below shows the numbers of two different
species of beetles on the same tree. The beetles were given a constant source of
food throughout the investigation.
Number of beetles
Time (month)
Species X
Species Y
0
20
50
5
50
80
10
40
120
15
30
160
20
10
180
25
0
190
a
Plot a graph to show the change in the numbers of different species of
beetles throughout the investigation.
(4 marks)
b
Explain why there is a rapid increase in the numbers of both species during
the first 5 months.
(1 mark)
c
d
What kind of relationship exists between the two species?
(1 mark)
Explain why the number of species X decreased after 5 months. (2 marks)
e
What is the maximum carrying capacity of the environment for the two
species of beetles?
(1 mark)
f
If a kind of disease infected species Y but not species X at the beginning of
the investigation, how would the result of the investigation change? Explain.
(2 marks)
7.
The diagram below shows a food chain.
grass → grasshopper → mouse → fox
a
b
Name one herbivore and one carnivore in the food chain.
(2 marks)
The diagram below shows the energy flow between the grass and the
grasshopper.
23 000
-2
-1
kJ m year
i
ii
c
19 500
-2
-1
kJ m year
grasshopper
Describe the role of grass in the energy flow of ecosystem. (2 marks)
Calculate the energy loss when energy is transferred from the first to
the second trophic level.
(1 mark)
iii How is this energy lost?
(2 marks)
A farmer killed all the mice on his farm. How would this affect the
population of
i
ii
8.
grass
grasshoppers?
foxes?
(2 marks)
(2 marks)
The diagram below shows a pyramid of biomass for a tropical forest.
tertiary consumers
secondary consumers
decomposers
primary consumers
producers
a
With reference to the above diagram, explain why food chains usually
contain no more than four trophic levels.
b
Describe how the biomass of consumers and producers becomes the
biomass of decomposers.
c
(2 marks)
Apart from the energy obtained by the decomposers, how is energy lost
from one trophic level to another?
d
(3 marks)
(2 marks)
Which trophic level has the greatest effect on atmospheric carbon dioxide
level? Explain.
(2 marks)
9.
The food chains below are found in a woodland.
(1) tree → insect → spider → bird → hawk
(2) tree → insect → bird → hawk
(3) tree → rat → hawk
(4) tree → insect → rat → hawk
a
Construct a food web to show all the feeding relationships among the
organisms concerned.
b
Along which food chain would there be most energy available to the
hawks? Explain.
c
(2 marks)
Draw a pyramid of numbers for the first food chain. Explain the shape of
pyramid.
d
(3 marks)
(4 marks)
There is a group of organisms very important in nutrient recycling not
shown in the food chains. Name this group of organisms and state the
importance of them in nutrient recycling.
(3 marks)
10. The diagram below shows a food web in grassland.
hawk
small bird
grasshopper
snake
rabbit
grass
a
Give two food chains shown in the above food web that involve four
members.
(2 marks)
b
One member important in the energy flow is not shown in the food web.
What is it?
(1 mark)
c
How can light energy from the sun be incorporated into the grass as organic
matter?
(1 mark)
d
Name one producer, primary consumer, secondary consumer and tertiary
consumer in the above food web.
(4 marks)
11. a
At which trophic level are you feeding on when you eat the following?
(2 marks)
b
c
i
Steak
ii Rice
iii Eggs
iv Grapes
Is all chemical energy contained in the food consumed converted to
chemical energy of your body materials? Explain.
(3 marks)
Describe how a nitrogen atom in the soil becomes part of a protein molecule
in your body.
(4 marks)
12. A forest contains 100 oak trees, 75 000 primary consumers and 60 000 secondary
consumers.
a
Draw a labelled pyramid of biomass for the forest. (No need to draw to
scale)
(2 marks)
b
A scientist estimated the total amount of energy passed through the forest
per year. The result is shown in the table below:
Energy transfer
Light energy incident on oak trees
Energy
(kJ per m2 per year)
2 000 000
Energy stored in cells of trees
20 000
Energy transferred to primary consumers
2 000
Energy transferred to secondary consumers
500
i
Calculate the percentage of light energy captured by trees. Show your
calculation.
(2 marks)
ii
iii
Explain why a large proportion of the light energy is lost. (2 marks)
Give three reasons to explain why some energy stored in the primary
consumers is lost and not passed on to the secondary consumers.
(3 marks)
iv
Explain why food chains with more than three trophic levels are
uncommon.
(2 marks)
13. The graph below shows the changes in population sizes of some phytoplankton
and zooplankton in a lake throughout the year from January to December. The
variations in temperature and light intensity are also shown.
quantity
month
a
Describe the effects of temperature and light intensity on the changes in
population of phytoplankton.
(3 marks)
b
Discuss the relationship between the shapes of curves of phytoplankton and
zooplankton.
(5 marks)
14. State the relationship between the following organisms and describe how they
interact with each other.
a
Algae and fungi in lichens
(3 marks)
b
Barnacles living on the shell of crabs and the crabs
(3 marks)
c
Tapeworms living inside the human body and the humans
(3 marks)
15. The diagram below shows a food web in grassland.
lion
horse
hawk
fox
small bird
rabbit
grasshopper
grass
a
What is the energy source of grass? Name the process through which grass
captures energy.
(2 marks)
b
c
d
e
Write down all the food chains that involve three consumers.
(2 marks)
Name all the secondary consumers in the food web.
(2 marks)
What is the relationship between fox and hawk?
(1 mark)
Suggest and explain one adaptation of grasshopper that is useful for
escaping from predation by small bird.
(2 marks)
f
If horses are removed from the grassland, explain why
i
the number of lions immediately decreases.
(1 mark)
ii
(2 marks)
the number of rabbits increases later.
16. The graph below shows the population sizes of predators and prey in the same
food chain over a period of time.
a
b
Which organism, X or Y, is the predator? Why?
(3 marks)
Explain why the peak of population size of X is always smaller than that of
Y in the same cycle.
(4 marks)
c
What would happen to Y if all X are hunted and killed by man? Explain
your answer.
(3 marks)
17. The tables below show the populations in two ecosystems, X and Y, in South
East Asia.
Ecosystem X
Organism
Number
Total dry weight of organisms (kg)
Grass
1 000 000
25 000
Rabbit
100
150
Owl
3
15
Organism
Number
Total dry weight of organisms (kg)
Tree
1
20 000
Herbivorous insect
5 000
200
Insect-eating mouse
50
50
Owl
3
15
Ecosystem Y
a
b
Sketch and label
i
the pyramid of numbers of ecosystem Y.
ii the pyramid of biomass of ecosystem Y.
Owls can obtain food from both ecosystems.
i
c
(2 marks)
(2 marks)
In which ecosystem can owls obtain more energy from the producers?
Explain your answer.
(2 marks)
ii State three ways that energy is lost between trophic levels. (3 marks)
In South East Asia, there is little rainfall in winter. Describe and explain
how this affects the rabbit population in ecosystem X.
(3 marks)
Essays
1.
After volcanic eruption, a barren area is formed. Ecological succession occurs on
the area.
Describe how this area would change in the following few thousand years.
(10 marks)
2.
Discuss the various ways in which human activities influence the nitrogen cycle.
(10 marks)
3.
Describe the role of bacteria in the main processes of the nitrogen cycle.
(12 marks)
Answers
1.
a
Female
1m
b
Laticauda colubrina / L. colubrina
1m
c
Female Laticauda colubrina / L. colubrina
1m
d
25.5 - 20 = 6.5 arbitrary units
1m
e
i
At start, there are slightly more L. laticaudata clinging to the cliff.
0.5m
At 40 s both species hold on the cliff equally.
0.5m
After 40 s, there are more L. colubrina clinging to the cliff.
0.5m
The difference between the two species is greater at the end than at the
ii
f
beginning.
0.5m
The muscles of L. colubrina are stronger,
1m
and their size is smaller, thus they can cling better.
1m
Any three from:
1m x 3
Advantages:
Snakes with smaller body mass can reach the top of cliff faster, providing
reproductive advantage.
Snakes with greater body mass are better in competing for nesting sites and food.
Females with greater body mass have more resources for reproduction.
Snakes with greater body mass have a greater ability to capture prey.
Disadvantages:
Snakes with greater body mass are less able to climb coral cliffs. This inhibits
breeding.
Snakes with greater body mass move slower.
2.
a
b
P: photosynthesis
0.5m
Q: respiration
0.5m
R: feeding
0.5m
S: combustion / burning
0.5m
i
1m
As population increases, the use of fossil fuels increases.
Burning more fossil fuels increases the amount of carbon dioxide released into
the atmosphere.
1m
As population increases, more land is needed for building houses. This leads
to deforestation.
1m
As the number of trees decreases, the amount of carbon dioxide removed
from the atmosphere by photosynthesis decreases.
1m
ii
Global warming
1m
iii
We can reduce the use of fossil fuels / use alternative energy sources. /
We can reduce the use of vehicles / walk more. /
We can travel by using public transport. /
We can reduce the use of paper. /
We can plant more trees. (any 2 or other reasonable answers)
1m x 2
3.
a
b
Population refers to a group of organisms of the same species
1m
living in the same habitat.
1m
Community refers to all the populations of different species
1m
living in the same habitat.
1m
i
1m
The number of species X remains nearly the same.
The numbers of species Y and Z decrease gradually in the first 25 years.
1m
ii
The number of species W increases rapidly in the first 25 years.
1m
The numbers of species W, Y and Z become steady after year 25.
1m
There is no natural predator in the forest. / Species W may be very efficient
in reproduction.
iii
1m
The resources in the forest may not be enough to support the survival of
more plants. / There is a limited amount of food or space for the growth of
species W.
iv
1m
The number of species Y decreases as the number of species W increases.
1m
This indicates that they may compete for the same resources, e.g. light,
nutrients, space, etc.
1m
4.
a
b
c
Galapagos tortoises have a long neck.
1m
The special shape of their shells allows their heads to rise.
1m
They do not need to compete with other species of tortoise for the food on the
ground.
1m
They can survive even when there is no food on the ground.
1m
The goats compete with Galapagos tortoises for food,
1m
thus reducing the tortoise population.
1m
d
The building of houses may destroy their habitat,
1m
and reduce the space and food for Galapagos tortoises.
1m
a
Nitrates
1m
b
i
5.
Nitrates in the soil would be absorbed by the plants for protein synthesis.
1m
ii
Nitrogen content of the soil can be increased by adding fertilizers / planting
leguminous plants.
c
1m
Dead plants are broken down by the decomposers / putrefying bacteria / fungi.
1m
The nitrogen inside the dead plants is converted into ammonium compounds in
the process ammonification.
1m
The ammonium compounds are then changed into nitrites and then nitrates 1m
by nitrifying bacteria.
1m
Nitrates are absorbed through the roots of other plants for growth.
1m
Correct title
1m
Correct axes and scale
1m
Correct points
1m
Curve passing through all the points
1m
b
There are sufficient food and space for the growth of both species.
1m
c
Competition
1m
d
As the population of both species increases, the resources (e.g. space) can no
6.
a
longer support the growth of both species at the same time.
1m
Species Y has a certain advantage that it exerts stronger control over the
competition.
1m
e
190 beetles
1m
f
The number of species X would increase.
1m
As the number of species Y decreases due to the disease, more food and space
are available for species X to grow.
1m
7.
a
b
Herbivore: grasshopper
1m
Carnivore: mouse / fox
1m
i
Grass captures light energy by photosynthesis and converts it to chemical
energy stored in organic compounds.
1m
This provides food for the organisms at higher trophic levels.
1m
ii
23 000 - 19 500 = 3500 kJ m-2 year-1
1m
iii
Some energy is lost as heat due to respiration. /
Some energy is lost because some grass is not eaten by the grasshoppers. /
Some energy is lost as the undigested and unabsorbed materials pass out as
c
i
ii
faeces. (any 2)
1m x 2
The number of grasshoppers would increase
1m
because there is no mouse to eat them.
1m
The number of foxes would decrease
1m
because killing the mice decreases the food available for them.
1m
8.
a
The amount of biomass in an organism is proportional to the amount of energy
stored in an organism.
1m
From the diagram, the biomass decreases from one trophic level to the next,
indicating that energy is lost when it is transferred between trophic levels.
1m
The amount of energy that can be passed to the fifth level would be so little that it
may not be enough to support the life of the organisms.
b
1m
Decomposers feed on the organic waste and dead bodies of the organisms. 1m
The nutrients from the organic waste and dead bodies are assimilated to build the
body parts of the decomposers.
c
d
1m
Energy is lost through uneaten organisms, undigested and unabsorbed materials,
excretory products and respiration.
2m
Producers
1m
They take in carbon dioxide during photosynthesis to keep the balance of carbon
dioxide in the atmosphere.
1m
9.
hawk
a
bird
spider
rat
insect
tree
b
Energy is lost during transfer of energy between two trophic levels.
3m
1m
There are the fewest number of trophic levels in (3), thus less energy is lost and
more energy is available to the hawks.
c
1m
hawks
birds
spiders
insects
trees
3m
Explanation:
The size of trees is big and one tree can supply food to a lot of insects, thus the
base of the pyramid is narrow. The numbers of other animals at higher trophic
d
levels are larger and so the other parts of the pyramid are upright.
1m
Decomposers
1m
They break down the dead bodies and excretory products of producers and
consumers into inorganic nutrients,
1m
thus returning materials in the form of inorganic compounds to the physical
environment.
1m
10.
grass → grasshopper → small bird → hawk
1m
grass → rabbit → snake → hawk
1m
b
Decomposer
1m
c
Light energy is converted by the grass into chemical energy in the organic food
a
d
during photosynthesis.
1m
Producer: grass
1m
Primary consumer: grasshopper / rabbit
1m
Secondary consumer: small bird / hawk / snake
1m
Tertiary consumer: hawk
1m
i
Third trophic level
0.5m
ii
Second trophic level
0.5m
iii
Third trophic level
0.5m
iv
Second trophic level
0.5m
11.
a
b
No
1m
Some materials are undigested and passed out as faeces.
1m
Some absorbed food substances are broken down to release energy for body
c
activities.
1m
Plants absorb nitrates from the soil.
1m
Nitrogen in nitrates is used to make protein for growth of the plants.
1m
The plants are eaten and the protein in plant cells is digested into amino acids.
1m
Useful amino acids are assimilated to form protein of our body.
1m
12.
a
secondary consumers
primary consumers
oak trees
b
Pyramid in right shape
2m
i
20 000/2 000 000 x 100%
1m
= 1%
1m
ii
iii
The energy is not absorbed by leaves / chloroplasts and some energy cannot
be absorbed because of wrong wavelength (green).
1m
Some energy is lost as heat due to respiration of the trees.
1m
Some energy is lost as heat due to respiration of the primary consumers. /
Some energy is used for activities of primary consumers. /
Some energy is lost as undigested and unabsorbed materials which pass out
as faeces in primary consumers. /
Some energy is lost because some primary consumers are not eaten by the
secondary consumers. /
Some energy is lost through the excretory products of primary consumers.
(any 3)
iv
1m x 3
There is energy loss when transferring energy from one trophic level to
another.
1m
If the number of trophic levels is too large, the energy passed on to the higher
trophic levels will be too little to support the life of organisms at those levels.
1m
13.
a
As the temperature and light intensity increase from January to April, the number
of phytoplankton increases and reaches a maximum in April.
1m
Further increase in temperature and light intensity from April to August leads to a
decrease in the number of phytoplankton.
1m
From August to December, the number of phytoplankton decreases slightly as
b
temperature and light intensity decrease.
1m
Phytoplankton is the food of zooplankton.
1m
The increase in the phytoplankton population from January to April provides more
food for the zooplankton, therefore zooplankton reproduces and increases in
number.
1m
As the zooplankton population increases further, a lot of phytoplankton is
consumed and the reproduction rate of phytoplankton decreases.
1m
As availability of food decreases, the reproduction rate of zooplankton slows
down and the zooplankton population decreases.
1m
When phytoplankton population decreases further in November and December,
zooplankton population also decreases as the amount of food is little.
1m
14.
a
Mutualism
1m
Algae make food by photosynthesis for both organisms.
1m
The fungi form a sponge-like body that holds water, provides protection and
b
c
anchorage for both organisms.
1m
Commensalism
1m
The crabs provide transport and food for the barnacles.
1m
The barnacles do no harm or provide any benefit to the crabs.
1m
Parasitism
1m
Tapeworms live inside the intestines of humans and obtain nutrients from the
humans.
1m
Humans are harmed by losing nutrients.
1m
Energy source: the sun
1m
Process: photosynthesis
1m
grass → rabbit → fox → lion
1m
grass → grasshopper → small bird → hawk
1m
c
Fox, lion, small bird, hawk
2m
d
Competition
1m
e
Protective colour / colour matches with the surroundings to avoid being seen by the
15.
a
b
predator. / Strong legs for jumping to escape from predation. / Compound eyes to
f
spot the predator in all directions. (any 1 or other reasonable answers)
2m
i
As horses are removed, less food is available for lions.
1m
ii
As horses are removed, rabbits no longer need to compete with horses for
grass.
1m
More grass is available for rabbits as food.
1m
16.
a
X
1m
The changes in the population size of X always slightly lag behind similar
b
changes in the population size of Y.
1m
This shows that X preys on Y.
1m
X is at a higher trophic level than Y.
1m
When energy is transferred from Y to X, there is energy loss
1m
due to uneaten organisms, respiration / metabolism, undigested / unabsorbed
material and excretion.
1m
c
Therefore, more Y are needed to support X.
1m
The number of Y would increase because of decreased predation by X.
1m
After a period of time, food for the abnormally high population of Y may become
exhausted.
1m
The number of Y would then rapidly decrease to a lower level because of
starvation / spread of infectious diseases in an overcrowded condition.
1m
i
Correct proportion of size of bars
1m
Correct labelling
1m
17.
a
owls
insect-eating mice
50
herbivorous insects
5000
tree
ii
3
1
Correct proportion of size of bars
1m
Correct labelling
1m
owls
15
insect-eating mice
50
herbivorous insects
200
tree
20 000
(unit of biomass: kg)
b
i
Ecosystem X
1m
There are fewer trophic levels and thus less energy is lost to the surroundings.
1m
ii
Respiration / uneaten organisms / undigested and unabsorbed materials /
excretory products (any 3)
c
The population of rabbits would decrease.
1m x 3
1m
There is less water for the growth of grass, and hence less food for rabbits. 1m
The owls keep feeding on rabbits further reduces the population of rabbits. 1m
Essays
1.
The barren area will first be colonized by pioneer community made up of lichens and
mosses.
1m
As they grow, rocks are broken down into small particles and soil is formed.
1m
Grass, ferns and herbs start to grow.
1m
The death and decay of grass, ferns and herbs enrich the soil for the growth of shrubs.
Small animals join the community.
1m
Finally the soil becomes so thick and rich in nutrients that it can support the growth of
trees.
1m
The habitat becomes a woodland. More animals are attracted to the community. 1m
A climax community is reached.
1m
Effective communication
3m
Nitrogen fixing is done artificially in the factory
1m
by chemical processes to make fertilizers.
1m
2.
The chemical fertilizers are added to the soil to increase the amount of nitrates for the
growth of crops.
1m
Animal waste from stock rearing is used as manure.
1m
Sewage discharged into water increases organic nitrogen supplies to water plants.
1m
Intensive growing of crops decreases the amount of nitrates in the soil.
1m
Ploughing improves aeration of the soil, producing aerobic condition which decreases
the activity of certain decomposers.
1m
Effective communication
3m
3.
Putrefying bacteria / decomposers
1m
convert nitrogen inside the organic waste and dead bodies of organisms into ammonium
compounds.
1m
Nitrifying bacteria
1m
convert ammonium compounds to nitrites and then to nitrates,
1m
which can be absorbed by plants.
1m
Nitrogen fixing bacteria in the soil or in root nodules of leguminous plants
1m
convert nitrogen gas into ammonium compounds.
1m
Denitrifying bacteria
1m
convert nitrates in the soil into nitrogen gas, which is then released into the atmosphere.
1m
Effective communication
3m