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. Energy moves, Nutrients and chemicals move. And it happens all the time. 24 hours a day bacteria work to get Nitrogen ready for the plants.