Which level of organization encompasses both populations and communities?

The correct choice is indeed the ecosystem level of organization. An ecosystem is defined as the complex set of interactions among living organisms (including various populations) and their physical environment within a certain area.

At this level, populations (groups of the same species that live in a particular area) interact with each other and with different species, forming a community. A community comprises all the different populations that coexist in a specific habitat. Therefore, the ecosystem encompasses both the populations and the communities, along with the abiotic (non-living) components of the environment such as air, water, and soil.

In contrast, other levels of organization such as biome focus more broadly on larger areas characterized by specific climates and types of organisms; population refers only to a group of individuals of the same species; and organism pertains to an individual living entity. Thus, while these other levels represent important aspects of biological organization, it is the ecosystem that captures the relationship between populations and communities, integrating them into a functional unit.