Ecosystems, from life, to the Earth, to the Galaxy

Ecosystems are systems where energy flows and material cycles are maintained in an apparently stable, but non-equilibrium state through a process of self-regulation. Such a definition does just apply to biological systems, it can also apply to systems that involve entirely physical processes. We discuss how four systems, each operating on very different spatial and temporal scales, each exhibit these features of an ecosystem. These are, in order of increasing magnitude, the cell, the forest, the Earth and the Galaxy. In particular, we discuss how the process of star formation across the spiral arms of galaxies works as an ecosystem. The carbon abundance plays a crucial role in both the self-regulation and the evolution of the system. We suggest that spiral galaxies may be the first ecosystems to form in the Universe after the Big Bang