The largest, most inclusive biological level is the ecosystem. Biological levels are arranged like a pyramid, the bottom being the largest and most encompassing, all the way up to the smallest and most exclusive level.
Ecosystems are based upon the interactions between groups of species, the individuals within them and the processes of their respective lives. Ecosystems involved the interaction of everything from the simplest plants to the complex human world. After ecosystems comes organisms, and this includes every living thing from bacteria to animals. Next are the organ systems, which are the various systems that make it possible for living things to undergo the processes they need to live, such as the reproductive system. Next come organs, which are the individual parts of the organ systems. In descending order of size we then have cells, molecules and then atoms.