Energy Transfer

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Where do we get all our energy from? Food. Where does the energy in food ultimately come from? Plants. Where does the energy in plants ultimately come from? Nowhere, they make it themselves through photosynthesis.

So is all the energy available to all living things on Earth down to photosynthesis? It sure is, my biologist friend, it sure is. Let's take a humbling moment of meditation while adoring this photo of a plant:

One day that weird-looking thing in the middle will be a pineapple ^_^

But wait. Don't plants also use their own photosynthesised goodies (glucose) to provide energy for their own business (growth, reproduction, etc.) via respiration, and waste stored energy in their tissues upon their death? Of course they do. So less must be available for whatever eats the plant. And whatever eats the plant will also lose energy through excretion for example, so whatever eats this herbivore will have even less energy available to themselves.

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Therefore, at each trophic level in the energy transfer (feeding) hierarchy there is a net loss of energy. This results in a pyramid:

The plants at the bottom are the photosynthesising primary producers. They hold the most energy (Joules) and are fed on by herbivores - primary consumers.

Notice only about 10% of that energy is available one trophic level higher. This is taken by carnivores feeding on herbivores - secondary consumers.

At the very top of the pyramid a mere 0.001% of the original 10,000 J remains (10 J). This is taken by tertiary consumers feeding on carnivores.

Because such tiny amounts of energy are left at the highest level, it's rare to find quaternary consumers or above. 

Notice how the above pyramid is based on energy alone. There are 2 other types of pyramid: biomass and numbers. A numbers pyramid is based on simple numbers e.g. 1,000 plants at the bottom eaten by 100 herbivores eaten by 10 carnivores eaten by 1 omnivore.

A biomass pyramid on the other hand takes into account the dry mass of the organisms. This can look something like this:

As you can see, there is a clear correlation between these types of pyramid. However, you can get irregular "pyramids". For example, a single tree can feed several hundred insects, etc.