SIJIL PERSEKOLAHAN TINGGI MALAYSIA (STPM) MODULE FOCUS ON ESSAY BIOLOGY PAPER 3 Chapter 2: Ecology FOCUS: Niche vs habitat, biogeochemical cycles*, energy flow*, growth curves, survivorship curve*, K & r-strategies*, carrying capacity & biotic potential*, quadrat, capture-recapture, factors limiting population size*

1. Define the term ecosystem. [ 5 marks ]  An ecosystem comprises biotic components and abiotic components that mutually interact and function together to form a stable and balanced natural system.  The biotic components involve all living things that interact with each other in the ecosystem.  The biotic components in an ecosystem comprise at least three trophic levels, which are producers ( autotroph ), consumer ( heterotroph ) and decomposers.  Energy flow in an ecosystem is continuous and produces an output of heat energy.  Nutrients such as water, sulphur and phosphorus are recycled by biogeochemical cycle.  Ex: pond ecosystem consists of biotic components such as carp, fishes, Daphnia and abiotic components such as pond water, soil and sunlight. 2. (a) Explain niche and habitat (b) State and describe three different types of niche in ecosystems.

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Niche refers the physical area dwelt by an organism and its functional role in the community. The functional role depends on the dwelling and the activities of the organism. Habitat refers to the natural area where an organism lives and grows. Habitat consists of physical and biotic factors that influence the living of an organism.

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The three types of niche are space/ habitat niche, trophic/ food niche and multidimensional/ hyper volume niche Space niche refers to the total overall area or various effective environments dwelt in by an organism even though the environment changes from one stage of the organism’s life cycle to another. Ex: The environment of the seedling of a plant is different from the environment of the mature plant. ( or can use frog as ex: tadpole lives in pond while mature frog live on land and water ). The space niche of the plant is the whole area involved in its life time. Trophic niche is explained by referring to the feeding habit of caterpillars and aphids. Both feed on the same plant but caterpillar eats the leaves while aphids suck the plant’s stem sap. Thus, caterpillars and aphids have different trophic niche. Multidimensional niche occurs when an individual organism or species continues to live in certain area. Multidimensional niche is affected by multiple environment factors such as temperature, light, food source etc. Competition will occurs in the overlapping multidimensional niches if the resources are limited. Competition between species in the overlapping multidimensional niche can be result in the elimination of one of the species or the separation of niche.

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3. Describe how energy is transferred in an ecosystem from the Sun.

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( Give examples for producers, consumers and decomposers if the question specify a named ecosystem )

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Less than 1% of the total amount of incoming solar radiation is used in photosynthesis/ primary production. Most solar radiation is reflected or absorbed by ozone layer, dust particles or clouds. Most energy fails to strike the green plants or absorbed by soil. Of the solar radiation striking the leaves, much is used in evaporating water from the leaves. Sunlight is absorbed by the chlorophyll of the green plants. Some light striked the plants is not in photosynthetically active range/ chlorophyll only absorbs mostly red and blue wavelength. The total amount of energy captured in newly synthesised carbohydrates during photosynthesis is called the gross primary production ( GPP ). Much of the producer materials die and enter the decomposer trophic level. Generally 50% of the plant materials enter the decomposer food chain rather than consumer food chain. Producers are eaten by primary consumer ( herbivore ). Some of the plants eaten will be egested as faeces and will available to decomposers. Some of the plant materials eaten and digested by primary consumer will be assimilated. Some of the energy assimilated will be respired and lost as heat in all trophic level. Some energy may be lost in urine. The productivity/ biomass of one trophic level is less than that of the previous trophic level. Only about 10% of the energy consumed by one trophic level is available for the consumption of the next trophic level. This results in a limit in the number of trophic level in an ecosystem to four or five by the fifth trophic level, there is not sufficient energy to support the next trophic level.

4. Describe the energy flow in tropical rainforest. [ 10 marks ]  The sun is the source of all energy in the food chain of a forest ecosystem.  Solar energy is trapped by green plants and trees by photosynthesis.  The primary producers convert approximately 1-5% of all solar energy to chemical energy in the form of organic food molecules.  This is known as the gross primary productivity ( GPP ) of the forest ecosystem.  Energy is used by plant for respiration and growth, large amount of energy is lost as heat to the surrounding.  The remaining biomass that available for herbivores is known as net primary productivity ( NPP ).  Herbivores such as deer are the primary consumer which feed on plants.  Energy is transferred from the producers to the primary consumer ( from first trophic level to second trophic level ).  In herbivores, energy is utilised for respiration and reproduction.  Large percentage of energy is lost through respiration, egestion and excretory waste.  Secondary consumers ( carnivores ) such as tiger feed on herbivores.  Energy is transferred from second trophic level to the third trophic level.  Decomposers such as saphrophytic bacteria and fungi and detritivores will consume the dead producers and consumers from each trophic level.  As energy is transferred from one trophic level to another, large amount of energy is lost as heat.  Each trophic level receives less energy than that of the previous trophic level. *STPM Biology Term 3 ( Focus on Essay )

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After the fourth or fifth trophic level, the remaining energy is insufficient to support any more trophic level. Therefore, the tropical rainforest trophic level is only four to five levels.

5. Discuss the density-dependant/ biotic factors which influence the population size. [ 8 marks ]  Competition for limited resources such as food, space and mates will increase as the population density increases.  Individuals who are unable to compete die which then leads to population decline.  Overcrowding in large population causes excessive stress, abnormal or aggressive behaviour and hormonal change.  This results in decline in health and survivorship reduces, increase infertility and parental neglect.  Parasitism and disease spreads is prevalent in high density population.  Individuals in the population succumb to infections in large number, causing high mortality.  As population size increases, toxic waste products from metabolism accumulate in the ecosystem.  This may poison the population, leading to higher mortality.  In a prey-predator population, interspecific interaction will result in cyclic fluctuation of both populations.  An increase in prey population supports a larger predator population, which in turns lead to a drastic decline in prey population.

6. Discuss the climatic/ density-independent factors which affect population density. [ 5 marks ]  Temperature influences the action of enzymes in organisms.  Region with optimum temperature is more densely populated.  Light intensity affects the rate of photosynthesis in plants ( producers ).  Regions with optimum light intensity have more plants, which indirectly increase the number of consumers.  Water supply in an area is important as organisms cannot survive without water, area with good water supply has more organisms.  Water current influences the distribution of nutrients in water and strong current may move organisms to other areas.  Oxygen supply is needed in aerobic respiration in organisms, areas with good oxygen supply usually have more organisms.  Winds affect evaporation rate. Less organisms are found in strong wind regions.

7. Explain how the population of rats in rice field can be estimated. Give the assumptions need to be considered. [ 10 marks ]  The rats population in rice field can be estimated by capture-recapture method.  Traps with baits are placed randomly in the study area and left overnight since the rats are primarily nocturnal animals.  In the next morning, the traps are collected and the total number of rats caught is counted and recorded as first sample.  Each rat caught is marked with suitable marker such as non-toxic Indian ink.  The marked rats are released back to the study area.  After suitable period, traps with baits again are placed in the study area and left overnight. *STPM Biology Term 3 ( Focus on Essay )

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The next day, the traps are collected and the total number of rats caught is counted and recorded as second sample. The total number of rats with marking is counted and recorded. The rat population in the rice field can be estimated by the formula: Number of rats in 1st sample x number of rats in 2nd sample Population size = Number of marked rats recaptured

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The assumptions need to be made are: i. The rats are captured randomly. ii. Changes to the initial population size during the experiment is negligible, that are immigration, emigration, death and birth are not significant. iii. The marking does not hinder the movement of rats or make the rats conspicuous to predators. iv. The marked rats mix randomly with their original population. v. Sufficient time is used between the first captur and second capture to ensure the marked rats mix with their original population.

8. State the meaning of survivorship curve, describe the three types of survivorship curves with named example for each. [ 10 marks ]  Survivorship curve represents the probability that a given individual in a population will survive to a particular age.  Human shows Type I survivorship curve.  Human shows low mortality rate during young/ infant age due to good parental care.  Most individuals able to reach the physiological age to reproduce.  The individuals live for a long period of time and die due to senescence.  Hydra exhibits Type II survivorship curve.  The death rate is constant throughout the lifetime.  The population size declines at a steady rate over time.  Oyster shows Type III survivorship curve.  Oyster shows high mortality rate during infancy due to predation and no parental provided.  The survive rate becomes constant once the organism reaches maturity.  The organism dies due to senescence.

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9. Compare K-strategies and r-strategies. [ 6 marks ]  Organisms performing K-strategies live in stable environment while organisms performing r-strategies live in unstable environment.  Organisms that perform K-strategies cannot adapt well to the environmental changes but organisms that perform r-strategies have little adaptational features to ensure survival.  Organisms performing K-strategies have long lifespan while organisms performing r-strategies have short lifespan.  Organisms performing K-strategies provide good parental care to the offsprings but organisms performing r-strategies provide little or no parental care to the offsprings.  Organisms performing K-strategies usually have long gestation period while organisms performing rstrategies have short gestation period.  Organisms performing K-strategies produce small number of offspring while organisms performing rstrategies produces large number of offspring.  Organisms performing K-strategies have large body size while organisms performing r-strategies have small body size.  Organisms performing K-strategies have strong competitive ability while organisms perfoming rstrategies lack of competitive ability/ mostly are opportunistic organisms.  Organisms performing K-strategies show low mortality rate at early age while organisms performing rstrategies show high mortality rate at early age.

10. Explain carrying capacity and biotic potential. [ 8 marks ]  Carrying capacity is the maximum number of organisms that can be supported by the environmental resources in an ecosystem.  The area occupied by a population has limited the resources and this will limits the population growth by maintaining an equilibrium between natality and mortality rate.  The population size increases until it approaches the saturation ( carrying capacity ) of its ecosystem.  As the population size approaches carrying capacity, the rate of increase in population size decreases.  The carrying capacity of an ecosystem is not constant as it is affected by environmental conditions.    

Biotic potential is the maximum growth rate of a population. Biotic potential will be achieved when all individuals in a population survive and reproduce at maximum rate. Full biotic potential is reached when there is no environmental resistance. The population which reproducing at biotic potential produces a J-growth curve/ exponential growth curve.

11. Explain the meaning of biogeochemical cycle. [ 5 marks ]  Biogeochemical cycle is the circulation of chemical elements through living ( biotic ) and non-living ( abiotic ) components of an ecosystem.  Most chemical elements are found in the main reservoir.  The main reservoir of the elements can be in the form of organic or inorganic materials.  The main reservoir in organic material can be found in living organisms, detritus and fossilised deposits such as peat and coal.  The main reservoir in inorganic material form can be found dissolved in water, soil, air or rocks.  Only a small portion of elements from the main reservoir are released to be available to the ecosystem. *STPM Biology Term 3 ( Focus on Essay )

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The elements are circulated from producer to consumer and become available again through decomposers and detritivores. The cycle prevents the depletion of the resources.

12. Outline the phosphorus cycle. [ 10 marks ]  The main reservoir of phosphorus is the inorganic phosphate in rocks.  Phosphate-rich rocks exposed to the environment are undergoes erosion/ weathering.  Rainwater dissolves the inorganic phosphate into the soil, the inorganic phosphate in soil then leach into water ( ocean/ river ).  Inorganic phosphate in soil is absorbed by plants/ producers ( via active uptake ), which will then be assimilated to synthesis inportant biological molecules such as ATP, phospholipid and nuclei acid to become organic phosphate.  The organic phosphate is passed through trophic level/ food chain when producers are eaten by herbivores, and herbivores are eaten by organisms of next trophic level.  Some of the phosphate are egested in faeces or excreted in dry leaves.  When organisms die, the decomposers return the phosphate back to the soil.  Of the inorganic phosphate leach to ocean, some are absorbed by absorbed by aquatic plants while some sediment to the ocean floor.  Aquatic plants are eaten by fishes, which will then be eaten by sea birds. The dropping of sea birds ( guano) containing phosphate is cycled back to the soil.  When the aquatic organisms die, decomposers return the phosphate back to water and sediment on the ocean floor.  After millions of years, geological uplift raise the marine deposit containing phosphate above sea level to become inorganic phosphate rocks.  Mining of rocks for the manufacture of fertilizer returns inorganic phosphate to the soil.

13. Outline the sulphur cycle. [ 10 marks ]  The main reservoir of sulphur is in the form of iron pyrite found in sedimentary rocks.  The sulphur in rocks is oxidised to become sulphate ion, SO42- by bacteria and enter the soil.  The sulphate ions are absorbed by plants and reduced to sulphydryl group, R-SH to form amino acids and proteins.  When organisms dead, the organic sulphate is reduced to hydrogen sulphide, H2S by sulphate-reducing bacteria such as Desulphovibrio sp.  Hydrogen sulphide can be oxidised back to sulphate by bacteria such as Thiobacillus sp.  Green and purple photosynthetic sulphur bacteria, ex Chromatium sp. oxidise hydrogen sulphide to sulphur element.  Iron pyrite, FeS from the rocks is mined and is reduced to hydrogen sulphide gas. The hydrogen sulphide then undergoes series of oxidation to form sulphate.  Sulphate can dissolve in rainwater to form acid rain which returns sulphate into the soil.  In swamp, sulphate-reducing bacteria reduce sulphate to hydrogen sulphide which gives the muddy smell.  Volcanic eruption releases a lot of sulphur dioxide, which then oxidised to sulphate and dissolve in rainwater to form acid rain.  Sulphate in the soil can leach into water ( ocean/ river ).

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14. Outline the carbon cycle. [ 10 marks ]  The main reservoir of carbon is in the sedimentary rocks such as limestone.  Carbon present as carbon dioxide gas in the atmosphere, which made up approximately 0.04% of the atmosphere.  Plants/ cyanobacteria absorb carbon dioxide during photosynthesis and assimilate the inorganic carbon into biological molecules such as glucose, starch, nucleic acids etc.  During aerobic respiration of organisms, glucose is oxidised to form carbon dioxide and released back to the atmosphere.  When producers are eaten by consumers, the organic carbon is passed through the trophic level.  When organisms dead, decomposers act on the dead body. Some dead bodies were buried underground before complete decomposition take place.  After millions of years, these dead bodies are converted to to fossil fuels which include coal, oil or natural gas.  Burning of fossil fuels produces carbon dioxide and return back to the atmosphere.  Some carbon dioxide dissolved in water as carbonate ion, CO32- and bicarbonate ion, HCO3-.  Shell of marine organisms contain large amount of inorganic carbon. After dead, the shell sinks to ocean floor and is covered by sediment.  After millions of years, geological uplift raises the ocean bed deposit to form sedimentary rocks ( limestone ).  Weathering/ erosion returns the inorganic carbon back to water or atmosphere.

15. Describe the human activities which interupt the carbon cycle. [ 5 marks ]  Combustion of fossil fuels increases the carbon dioxide content in the atmosphere.  Deforestation slows down the process of carbon removal from the atmosphere where the rate of photosynthesis by plants is greatly reduced.  Burning of forest / logged trees contributes a large amount of stored carbon ( in the wood ) to the atmosphere as carbon dioxide.  The increase in carbon dioxide content in atmosphere contributes to greenhouse effect which raises the Earth’s temperature.  The rise in temperature causes the melting of ice in polar regions.

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