Why must all living things excrete waste products?

Wich organism obtains its energy from dead or decaying matter

I’m stuck on this question cuz i’m stupid sksksksk

An ecologist studied the same species of deer during the summer and the winter. she noticed that during the summer, when there was plenty of food, the deer were energetic and playful. however, during winter when food was scarce, the deer moved more slowly and did not run unless they needed to escape a predator. which scientific fact is best supported by her observations?