What term describes a complex of living things and their environment functioning as a unit?

The term that describes a complex of living things and their environment functioning as a unit is "ecosystem." An ecosystem includes all the interactions that occur among the various organisms—plants, animals, fungi, and microorganisms—and their physical environment, such as soil, water, and climate. This interconnectedness of living (biotic) and non-living (abiotic) components highlights the dynamic relationships that sustain life and energy flow within that environment.

A biome, while related, refers to a larger regional classification that encompasses many ecosystems sharing similar climate conditions, flora, and fauna. It focuses more on specific environmental conditions rather than the intricate interrelations within those systems.

A community represents only the living organisms in a given area, excluding the physical environment, and therefore does not encompass the complete system. The term habitat refers to the specific conditions or environment in which an organism lives, rather than the complex interactions among multiple organisms and their surroundings.

Thus, the definition and scope of "ecosystem" accurately capture the essence of a unit where living organisms and their environment interact comprehensively.