I would say energy myself, and this is because energy plays a key role in all of these. With trophic levels energy is being transferred amongst the levels. Food chain's is a map of how the energy is passed along the chain. Food web is a web of how energy is transferred, and this is why I would say energy is a keyword.
Answer:
Your 100% Correct Im an advanced scientist in highschool!
Well Done! Also Im new so sorry if you done trust.
Explanation:
The body is to could to function properly and has to spend all it energy on trying to keep you warm
