What term describes organisms that live in or on their food source?

The term that describes organisms that live in or on their food source is substrate feeder. Substrate feeders obtain their nutrients by consuming organic material that is part of their habitat, such as decaying leaves or wood. This feeding strategy allows them to take advantage of the resources in their immediate environment, often leading to efficient energy use.

For instance, many types of worms and certain larvae are considered substrate feeders as they literally feed on the material in which they live, breaking it down and extracting nutrients. This contrasts with fluid feeders, which extract nutrients from liquids; filter feeders, which sift food particles from water; and pseudopod feeders, which utilize extensions of their cells to capture food, typically seen in certain types of protozoa. Each of these terms refers to different feeding adaptations and ecological niches, but substrate feeding specifically pertains to organisms that directly utilize the substances they inhabit.