Organisms that must feed on other organisms are called heterotrophs.

Here's why:

* Autotrophs are organisms that can produce their own food through processes like photosynthesis (using sunlight) or chemosynthesis (using chemicals). Examples include plants, algae, and some bacteria.

* Heterotrophs cannot make their own food and must obtain it by consuming other organisms. This includes animals, fungi, and many bacteria.

So, any organism that doesn't make its own food through photosynthesis or chemosynthesis must rely on consuming other organisms to survive.