Energy Transferred In Ecosystems
Übersicht
Stand in a wheat field on a bright afternoon and an enormous amount of energy is pouring onto every square metre around you. Almost none of it ends up in the bread you will eat. Ninety nine parts in a hundred bounce off, slip past or warm the soil, and of the fraction the crop does capture, most is spent again before anything reaches a cow, a sparrow or you. Ecosystems are not efficient machines. They leak at every joint.
This lesson follows that leak. You will find out why plants and algae hold on to only about one per cent of the light that reaches them, why roughly nine tenths of the biomass at any feeding level never makes it to the level above, and where the missing nine tenths actually goes. You will then use those numbers: building a pyramid of biomass to scale, reading one you are handed in an exam, and working out why a country can feed far more people from a field of beans than from the same field of beef.
Ziele
- Radiation from the Sun is the source of energy for most communities of living organisms. Plants and algae transfer about 1 % of the incident energy from light for photosynthesis. This energy is stored in the substances that make up the cells of the plants.
- Only approximately 10 % of the biomass from each trophic level is transferred to the level above it because: some materials and energy are always lost in the organisms’ waste materials; respiration supplies all the energy needs for living processes, including movement. Much of this energy is eventually transferred to the surroundings. Construction of food webs and chains, and of pyramids of numbers, is not required. An understanding of pyramids of numbers is not required.
- The biomass at each stage can be drawn to scale and shown as a pyramid of biomass. Students should be able to interpret pyramids of biomass and construct them from appropriate information.
- The efficiency of food production can be improved by reducing the number of stages in a food chain.
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- Approximately what percentage of the light energy reaching plants and algae is transferred for photosynthesis? A. 1% B. 10% C. 50% D. 90% Answer: A
- Approximately what percentage of the biomass at one trophic level is transferred to the trophic level above it? A. 1% B. 10% C. 50% D. 90% Answer: B
- Which process does NOT cause biomass to be lost between one trophic level and the next? A. Egestion of undigested food in faeces B. Excretion of urea in urine C. Respiration to supply energy for movement D. Absorption of digested food into the blood Answer: D
- In a pyramid of biomass the scale used is 1 cm to 300 g/m2. The producers have a biomass of 3000 g/m2. How long should the producer bar be? A. 1 cm B. 3 cm C. 10 cm D. 30 cm Answer: C
- A country decides that a wheat crop should be milled into flour for people instead of being fed to cattle. Why does this increase the amount of food available? A. Wheat contains more energy per gram than beef B. One stage is removed from the food chain, so less biomass is lost C. Cattle are unable to digest wheat D. Plants do not respire, so no energy is lost by the wheat Answer: B
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