12Class 12 Biology
Ecosystem short notes
Short notes on productivity, decomposition, energy flow and ecological pyramids. Know which pyramids can be inverted.
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Productivity
- Gross primary productivity (GPP) minus respiration gives net primary productivity (NPP).
- Annual NPP of the biosphere is about 170 billion tons dry weight, of which oceans make only about 55 billion.
- Secondary productivity is the rate at which consumers form new matter.
Decomposition
- Steps: fragmentation, leaching, catabolism, humification and mineralisation.
- Warm and moist conditions speed it up. Lignin and chitin slow it down.
Energy flow
- Energy flows one way, and it's lost as heat at each step.
- Only about 10% passes to the next level (Lindeman's law).
- Grazing food chains start with producers, and detritus food chains with dead matter.
- Only about 50% of incident solar radiation is photosynthetically active.
Pyramids
- Pyramids can show number, biomass or energy.
- A tree ecosystem's pyramid of number is inverted.
- A pond's pyramid of biomass is inverted.
- A pyramid of energy is always upright.
Where students slip up
- Saying an energy pyramid can be inverted. It can't.
- Thinking energy recycles like nutrients. It only flows one way.
- Mixing up GPP and NPP.
Try it on your weakest chapter.
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