One of our favorite stories read so far, in this period of exploring the concept of "systems", is an old one called The Old Ladies Who Liked Cats. It is an out-of-print book, but I was able to buy a used copy on Amazon. This book is a variation of a story told by Charles Darwin about cats and clover, an ecological chain tale. In this version, there were unforeseen consequences when a mayor of a town ordered all cats to stay inside at night. This small act breaks the ecological chain, which leads to almost certain disaster. However, the old ladies, "who knew how things work together", make everything okay again by their wise thinking.
I asked the children to create a map of sorts, showing the chain. It is clear they understood the ecological moral of the story, and are certainly getting a clearer idea of what a system is. A lovely read, if you can ever get your hands on a copy.