Here's why:

* Population: A group of individuals of the same species living in the same area.

* Community: All the different populations of species living and interacting within a particular area.

* Ecosystem: A community of living organisms (biotic factors) in conjunction with the nonliving components of their environment (abiotic factors).

So, you can think of it like this:

* Ecosystem: The whole picture, including the living and non-living parts.

* Community: The living part of the ecosystem, made up of different populations.

* Population: A single species within the community.