How our growing knowledge of the evolution of complex ecosystems, using the latest genetic tools, can help us heal them - and surviveThis is not another Chicken Little book about the environment. Every reader already knows how overpopulation, ignorance and tribalism are contributing to environmental destruction and breakdowns in public health. We are all aware of the grim possibility that during our lifetimes the Earth might "flip" to a new ecological equilibrium, threatening our very survival.Why Ecosystems Matter explores an exciting new way to avoid such threats, by using our exponentially-growing knowledge of how evolution has shaped and is continuing to shape the complex communities of living ecosystems on which we all depend. Throughout this book we will visit ecosystems where the author has worked or has had direct experience, beginning with a tour of the amazing series of ecosystems that span the entire eastern slope of the Peruvian Andes. This journey provides a vivid glimpse of ecosystems' diversity and capacity for rapid change. Next, we trace how Darwin gained a fundamental insight about the origins of such ecosystem complexity. He realized that, when subgroups of the same species inhabit even slightly different ecosystems, these subgroups will evolve in diverging directions. This divergent evolution is primarily driven by interactions with the many other species in each ecosystem, which are themselves evolving in different directions in the different ecosystems. We explore how this subtle and fascinating concept lies at the heart of the evolutionary ferment that powers ecosystem diversity and resilience - the bubbling evolutionary cauldrons of the book's title. This ferment pervades ecosystems, but it is especially active in their microorganism communities. We then examine the evolutionary forces that power these cauldrons, starting with between-species interactions and tunnelling down to their causes. Using real-world examples, we explore how the technologies available for measuring these changes are increasing exponentially in precision and scope. We show how this deluge of new genetic and environmental information can be used to protect and restore a wide variety of damaged ecosystems. Ecosystems have survived dramatic changes in the past, often becoming even more wondrous and diverse than before. We are now learning how this happens, and how we can preserve this astounding ability.