Sunlight is the original source of almost all energy in most ecosystem.
Photosynthesis is the process by which plants use light energy to convert carbon dioxide and water into sugars...Respiration occurs when glucose combines with oxygen to produce useable cellular energy. This energy is used to fuel growth and all of the normal functions.
Although there is no context for this question, it would be safe to say that that population would have reached its carrying capacity, if not become overpopulated.
I think the answer is renal medullae