Final answer:
A key biotic component of a forest ecosystem is the trees, which provide structure, regulate temperature, support wildlife, and participate in carbon and water cycles.
Step-by-step explanation:
An ecosystem is a dynamic complex of plant, animal, and microorganism communities and the nonliving environment interacting as a functional unit. In a forest ecosystem, a key biotic component could be the trees themselves, as they provide the structural framework for other interactions. Trees are vital for creating the canopy that regulates temperature, supports diverse wildlife, and plays a critical role in the carbon and water cycles within the ecosystem.