This book outlines a system that subdivides the Earth into a hierarchy of increasingly finer-scale ecosystems that can sever as a consistent framework for ecological analysis and management. The system consists of a three-part, nested hierarchy of ecosystem units and associated mapping criteria. Delineating units involves identifying the environmental factors controlling the spatial geography of ecosystems at various levels and drawing boundaries where they change significantly. Macroscale units (ecoregions) are climatically controlled and delineated as Köppen-Trewartha climate zones. Nested within these are landscape mosaics, the mesoscale units, controlled by landform and delineated by Hammonds landform regions. At the microscale are individual sites controlled by topographically determined topoclimate and soil moisture regimes.Features of the Second Edition: There are new sections on how ecoregion boundaries were determined, ecoregion redistribution under climate change, ecosystem processes (such as fire regimes), empirical versus genetic approaches, and human modification (such as invasive species). The book has been updated throughout with new text and figures and with new literature reviewed. More examples drawn from outside the North American environment, particularly at the meso and micro scale. Discussion of recent, alternative mapping systems and how they relate to the system presented in this book. Richly illustrated with many new diagrams, photographs, and tables. The new edition also includes two color maps showing the ecoregions of the oceans and continents, and a completely updated guide to further reading.