What is an ecosystem ...

An ecosystem is a biological community of interacting organisms and their physical environment.

This is actually more of a biology post but, whatever.

An ecosystem is like a complex community filled with living organisms (animals, plant's etc) that all interact with each other in some way (like a food chain, ants eat grass, something eats ants, something eats something ...).

Ecosystems can be in the water, can be in the air, or in the land, etc etc.