The diagram above shows how energy moves up the food chain through feeding.  Remember that if you are asked what the arrows represent in a food chain, there is only one possible correct response.  “Arrows in a food chain show the flow of energy from one trophic level to the next”

The big idea here is that not all the energy in one trophic level can ever pass to the next.  The specification suggests that only 10% of the energy is transferred from one level to the next (but in fact the percentage varies between 0.1% to around 15%)

kCal is a unit of energy and the pyramid shows that only 10% of the energy in one level is found in the next.

So there is a big question here – where does all the other 90% of the energy in one level end up?

There are a whole load of different ways energy is lost.  Consider the transfer of energy between mice and owls.

• The mice use up energy in the process of respiration.  The glucose molecules that mice oxidise to provide the energy to move around are not available to an owl if the mouse is eaten.
• Not all mice are eaten by owls or other predators. Many die of disease, starvation and exposure and a few might even live long enough to die peacefully in their sleep.  All these “dead mice” will have energy in their bodies that cannot pass up a food chain but instead passes to decomposers.
• Even the mice that are eaten by owls are not eaten in their entirety.  The owl might only eat the energy-rich parts of a mouse and regurgitate out the bones and fur.  So some  energy is lost as not all the mouse is eaten and digested by the owl.
• There will be parts of the mouse that even when swallowed and digested are not accessible.  Owl faeces will contain some molecules from the mice eaten that contain energy.   This energy is perhaps found in molecules that the owl digestive system cannot digest.  The energy present in the owl faeces is lost to the food chain and like the example above will pass to decomposers.

This energy adds up to around 90% of the energy in any trophic level.  Ultimately though where does it all go?  All the energy in all the organisms in an ecosystem has the same fate:  it ends up as heat that is dissipated into the system.  Energy can only enter an ecosystem in one way (as sunlight trapped in the process of photosynthesis in producers) and in the end, it all ends up leaving the system as heat energy.  This heat energy is a waste product of respiration.