What does Productivity mean? (Ecology) Watch

CoronationStreet
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My defintion is:
Rate of energy flow through each trophic level in a food chain, it givesan idea of how much energy is available to the organisms at a particulartrophic level per unit area in a given amount of time. Its measured in KJ or MJof per energy per square meter per year.�!

But I dont understand it really...
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JordanL_
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Productivity is a measure of the energy used by organisms. In plants, they take in energy in the form of light - the amount of energy they take in is their gross productivity. In animals, they take in chemical energy in the form of food - they eat either plants or other animals, and the matter that they eat is equivalent to some amount of energy. Again, this is the gross productivity. So gross productivity is the amount of energy absorbed by an organism.

The trophic level stuff refers to the productivity of a whole trophic level. So trophic levels go producers, primary consumers, and so on. At each level, only a portion of the energy from the previous level is absorbed. For example, you have the first level, producers. You could have a field of lettuces. Then you have rabbits that eat the lettuces. But not all of the lettuces get digested by the rabbits: some matter can't be digested (which comes out as faeces or urine), and some parts aren't eaten. So of all the energy available in the first tropic level, only a percentage of it is absorbed by the next level. So the gross productivity (the energy absorbed) gets smaller at each level.

Net productivity is different to gross productivity. Gross productivity is all the energy absorbed - so everything that's eaten and digested, or all the light energy used in photosynthesis. But of this energy that's absorbed, some is used up in respiration to produce energy (for movement, reproduction, heat production and so on). The energy used up is called the respiratory loss. The net productivity is the energy that's left over. The net productivity is stored as compounds like fat in mammals or starch in plants, and this is the energy that's available to the next trophic level.

The productivity is measured per unit area per unit time. So it's usually given as KJ/m^2/year.

An example to help explain all of that:

There's a field of grass. This is the producer, so it's the first trophic level. Let's say that 100,000 KJ of light energy shines on 1 square meter of grass over a year. So there's 100,000 KJ of energy available to 1m^2 of grass in one year. But not all of this energy is absorbed. Plants reflect some wavelengths of light, and photosynthesis can be limited by factors other than light availability. So let's say that only 70% of this energy is absorbed and used in photosynthesis. This means that the gross productivity - the energy absorbed (and converted to chemical energy by photosynthesis) - is 70,000 KJ/m^2/year. Some of this chemical energy is used by the plant in respiration - so the sugars produced in photosynthesis are broken down in respiration to release energy for reproduction, for example. Maybe 10% of this is used for respiration (don't take any of these numbers as accurate, they aren't). So the respiratory losses are 7,000 KJ/m^2/year, and the net productivity is 63,000 KJ/m^2/year.

So there's 63,000 of energy in the organic matter of 1 square meter of grass each year. This is in the form of sugars, starch and other compounds. Now some sheep come along and eat the grass. Some of the matter isn't digested so comes out as faeces, and some of it isn't eaten. If they eat and digest 70% of the grass, their gross productivity is 44,100 KJ/m^2/year. Just like the grass, the sheep respire. If their respiratory losses are 30%, the net productivity will be 30870 KJ/m^2/year. The net productivity is the energy available to the next trophic level (lions?), but again, not all of that energy will be taken in, and then some of it will be lost in respiration.

Let me know if any of this doesn't make sense or if you need more help!
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