Understanding ecosystems is the basis for all of the study of ecology. Students are introduced to the idea of an ecosystem by examining his or her interactions with other living things and the environment. The terraqua column that students build becomes the foundation for the following weeks of exploration of water, soil, living things, and the environment.
Bottle Biology contains a wealth of information about the terraqua column project and further directions to explore. Definitely check out the Bottle Basics section to learn about working with soda bottles and also the Terra Column section for everything you wanted to know about building, maintaining, and studying terraqua columns. This site has a much more detailed step-by-step building guide complete with diagrams and troubleshooting.