Which term describes the various populations of species existing together in an ecosystem?

The term that correctly describes the various populations of species existing together in an ecosystem is "community." In ecological terms, a community refers to all the different species living and interacting in a particular area. This includes not just the number of individuals of each species but also their relationships with one another, such as competition, predation, and symbiosis.

The other choices refer to different concepts within ecology. A population specifically refers to a group of individuals of the same species living in a particular area, while a habitat describes the natural environment where an organism lives, encompassing the physical and biological factors that affect it. An ecosystem is a broader term that includes both the community of living organisms and the non-living environmental components they interact with. Understanding these distinctions helps clarify how species interact and coexist within their environments.