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Chapter 21

Chapter 21. Ecology. Objectives. Students should be able to: Briefly describe the non-cyclical nature of energy flow Explain the terms producer, consumer and trophic level in the context of food chains and food webs .

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Chapter 21

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  1. Chapter 21 Ecology

  2. Objectives Students should be able to: • Briefly describe the non-cyclical nature of energy flow • Explain the terms producer, consumer and trophic level in the context of food chains and food webs

  3. Explain how energy losses occur along food chains, and discuss the efficiency of energy transfer between trophic levels • Describe and interpret pyramids of numbers and biomass • Describe how carbon is cycled within an ecosystem

  4. The Ecosystem • An ecosystem is an ecological system formed by the interaction of living organisms and their non-living environment • In any ecosystem, the living organisms are identified as producers, consumers or decomposers

  5. Producers • Mainly green plants, some algae and bacteria • They convert light energy to chemical energy and store it as food during photosynthesis • Provide food substances, energy and oxygen to other organisms Consumers • Obtain energy from other organisms • Herbivores feed directly on plants (primary consumers) • Carnivores feed on herbivores (secondary consumers)

  6. Decomposers • Act on dead organisms, faeces and excretory products and enable the materials locked up in them to be returned o the physical environment to be used by plants • E.g. Fungi, bacteria and earthworms

  7. Fig 21.13 Flow of energy and nutrients through an ecosystem

  8. Food Chain and Food Web • A series of organisms through which energy is transferred in material form (food) constitutes a food chain

  9. Food Chain A food chain: Primary Consumer (herbivore) Secondary Consumer (carnivore) Tertiary Consumer (carnivore) Producer (green plant) grasshopper spider green plant bird

  10. Food Chain & Food Web A food chain: Primary Consumer (herbivore) Secondary Consumer (carnivore) Tertiary Consumer (carnivore) Producer (green plant) A food web: caterpillar green plant grasshopper spider bird aphid aphid ladybird ladybird

  11. Each stage in a food chain is known as a trophic level • A predator is an animal that feeds on another animal called the prey • In a community, food chains are linked up together to form a food web or food cycle

  12. Ecological Pyramids Pyramids of numbers • The number of organisms at each trophic level can be used to construct a pyramid of numbers • The pyramid will be broad at the base and narrow towards the top

  13. Grass  rabbits  snakes  Hawks

  14. Pause and ponder • In a pyramid of numbers, all the organisms at each trophic level are counted, regardless of size and stage of development • This might cause the pyramid of numbers to be inaccurate

  15. Pyramid of biomass • Allows us to compare the mass of organisms in each trophic level at a particular time • The pyramid of biomass is constructed based on the dry mass of organisms in each trophic level at any one time. • The pyramid of biomass has a similar shape to that of the pyramid of numbers

  16. How can we construct a pyramid of biomass? • E.g. there are 1000 rabbits in a given area at one time. To determine the biomass of rabbits at that time: • Dry 10 rabbits in an oven at 100 °C until a constant mass is obtained. If the dry mass is A gram, then 1000 rabbits is 100A gram. • Repeat step 1 for all other trophic levels. • Construct a pyramid of numbers using information obtained in steps 1 and 2.

  17. Variations in ecological pyramids • A pyramid of numbers may be upside down or inverted if: • Organisms of one trophic level are parasitic on organisms of another trophic level. • Many small organisms of one trophic level feed on a large organism of another trophic level. Eg.

  18. 2. Pyramid of biomass for rapidly producing organisms are also not pyramid-shaped. This is because the pyramid of biomass does not take into account the rate of reproduction of organisms. E.g. Phytoplankton  zooplankton  small fish  large fish

  19. Large Fish Small Fish Zooplankton Phytoplankton Pyramid of biomass for food chain involving phytoplankton

  20. Pyramid of Energy • Total energy in the various trophic levels of a food chain being represented • Need to determine the total energy content in each trophic level over a period of time. • Have to take into consideration the rate at which the organisms in each trophic level reproduce • Average energy content of each trophic level can then be calculated using special techniques

  21. Pyramid of Energy • A lot of energy is lost to the environment as food is transferred from one trophic level to the next. Energy level lost : • As heat during respiration at every trophic level • In uneated body parts • Through undigested matter egested by consumers • Through excretory products (urea) • Heat energy is wasted energy as it cannot be recycled in anyway in the ecosystem • Total energy level decreases progressively along the food chain

  22. A pyramid of energy is always broad at the base and narrow towards the top. • A large number of producers may be required to support just a few consumers at the end of a food chain

  23. About 90 % of energy lost when it is transferred from one trophic level to the next • Greatest amount of energy lost : from producer to primary consumer • A shorter food chain means more energy is available to the final consumer

  24. Non-cyclic Energy Flow in the Ecosystem Impt pts • Ultimate source of energy is the sun • Flow of energy through the eco-system is non-cyclic in nature • Energy lost as heat energy • Heat energy cannot be recycled in any way

  25. Energy Flow and Its Relation to Nutrition and Respiration in the Ecosystemc 5 excretion 4 respiration Secondary consumers (carnivores) egestion energy lost in uneaten body parts, faeces and excretory products (usable energy) feeding (holozoic nutrition) heat lost to environment 3 Primary consumers (Herbivores) 5 excretion 4 respiration egestion heat lost to environment feeding (holozoic nutrition) 3 faeces and excretory products + dead bodies of organisms 4 Producers (Green plants) respiration decomposition photosynthesis 2 + CO2 CO2 Sun (light energy) heat released to environment 1

  26. The Carbon Cycle respiration Animals Green plants photosynthesis feeding respiration Dead bodies and faeces decomposing death decomposition Bacteria and fungi act on dead animals combustion Coal, oil, gas, etc

  27. Release of carbon dioxide into the atmosphere • Carbon dioxide is released into the environment in 3 ways • Respiration • Combustion • decomposition

  28. Importance of carbon cycle • Ensures that there is continuous supply of inorganic carbon dioxide for plants to carry out photosynthesis • Enables a linear flow of energy through the ecosystem. The carbon compounds are the vehicles through which trapped solar energy passes from one organism to organism in the food chains

  29. Structured Question Wk Bk 1 (a) Kilogram per unit area (b) Pyramid of numbers maybe inverted if organisms of one trophic level are parasitic on organisms of another trophic level. Hence it is not an accurate representation of the transfer of energy

  30. (c) (i) Trees (ii) Sparrows

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