Question

At each level of the food chain, what happens to approximately 90% of the available energy?

Answer 1

90% of the available energy is lost in some form of heat, transferring light energy into different living organisms 

Answer 2

In each stage only 10% is passed !!

As we know not a single animal even human is capable of absorbing whole energy present in glucose !! only 45% of energy present in glucose is used by humans in oxidation !!

so now you can understand the whole food web is not as much efficient !! Thus every time 90% energy is lost as a waste , i am not saying it is lost , it is there but organism is not capable of using that !!