So if we look at the puddle example you might be looking at the ground beneath the puddle, temperature, depth, turbulence, sunlight, atmospheric pressure, weather patterns, wind, nutrients, etc. Those are just the non-living things in the ecosystem of a puddle. When you add on all the living interactions you have a good idea how complex an ecosystem can be. It's those complexities that make it so interesting. Ecologists have to study all of the interactions and then use the idea that no ecosystem exists alone.
Ecosystems interact with each other.
So that puddle is interacting with the vacant lot. The vacant lot interacts with the ecosystem of the city block. Nothing just sits there in nature. Ecosystems are living things, constantly changing and moving. Not only do the animals move, but so do the things that they need.