Ecosystem

An ecosystem is a functional unit of nature in which living organisms interact among themselves and with their physical environment. In this chapter you will study the structure of an ecosystem, including its biotic and abiotic components, species composition and vertical stratification, as in a forest with trees, shrubs and herbs in different layers, and you will use a pond as a simple, self-sustaining example. You will learn the meaning of primary productivity, the difference between gross primary productivity and net primary productivity (NPP = GPP - R), and why the oceans produce only about 55 billion tonnes of the biosphere's annual 170 billion tonnes of organic matter despite covering 70% of the surface. The chapter then describes decomposition through fragmentation, leaching, catabolism, humification and mineralisation, and the factors that control its rate. It explains the unidirectional flow of energy through grazing and detritus food chains, trophic levels, standing crop and Lindeman's ten percent law, and ends with ecological pyramids of number, biomass and energy, including the inverted pyramid of biomass in the sea.

Detailed Worksheet: Ecosystem Section A - Definitions (10 marks) 1. Define an ecosystem. Distinguish between natural and man-made ecosystems with one example each. (2 marks) 2. Define gross primary productivity and net primary productivity. Write the relation between them. (2 marks) 3. What is detritus? Name the first step of decomposition and the organisms responsible for it. (2 marks) 4. What is meant by standing crop? In which units is it measured? (2 marks) 5. What is photosynthetically active radiation (PAR)? What fraction of incident solar radiation is PAR, and what percentage of PAR is captured by plants? (2 marks) Section B - Calculations and Applications (15 marks) 6. In a grassland, producers fix 20,000 J of energy. Using Lindeman's ten percent law, calculate the energy available to primary, secondary and tertiary consumers. (3 marks) 7. A forest has a gross primary productivity of 25,000 kcal per square metre per year, and plants use 15,000 kcal per square metre per year in respiration. Calculate the net primary productivity, and state what fraction of GPP is available to herbivores and decomposers. (3 marks) 8. The annual net primary productivity of the whole biosphere is about 170 billion tonnes (dry weight) of organic matter, of which the oceans contribute about 55 billion tonnes. Calculate the percentage contributed by the oceans and by land, and give a reason why oceans are less productive despite covering 70% of the surface. (3 marks) 9. A field receives 10,00,000 J of sunlight, of which 50% is PAR. If plants capture 2% of the PAR in photosynthesis, calculate the energy fixed by producers and the energy expected at the third trophic level. (3 marks) 10. Classify the following as producers, primary consumers, secondary consumers or decomposers, and arrange those in the grazing food chain: grass, earthworm, frog, grasshopper, snake, fungi. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram showing the flow of energy through the different trophic levels of an ecosystem, indicating the loss of energy as heat at each level. (4 marks) 12. Draw a labelled pyramid of energy and an inverted pyramid of biomass in a sea, and explain why the pyramid of energy can never be inverted. (3 marks) 13. Draw a diagrammatic representation of the decomposition cycle in a terrestrial ecosystem showing detritus, fragmentation, leaching, catabolism, humification and mineralisation. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain how temperature, soil moisture, aeration and the chemical composition of detritus affect the rate of decomposition. Predict and explain the rate of decomposition of (i) leaves rich in lignin and chitin in a cold, dry region and (ii) leaves rich in nitrogen and sugars in a warm, humid region. (5 marks) 15. In a forest, a single large banyan tree supports thousands of insects, which feed hundreds of small birds. Draw the pyramid of numbers for this situation, explain why it is inverted, and analyse three limitations of ecological pyramids. (5 marks) 16. In an aquatic ecosystem the detritus food chain may be a minor conduit of energy, while in a terrestrial forest much more energy flows through it. Analyse the reasons for this difference and explain how the grazing and detritus food chains are interconnected in a food web. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Draw all diagrams neatly in pencil and label every part. Show units in all calculations.