This edition grew out of an undergraduate and graduate courses introduced in several Universities and the purpose of this book is to offer a conceptual foundation for making human ecology and sustainable development an integral part of their personal and professional lives. It reflects view of human ecology and attempts to relate with social consciousness in clear and understandable and coherent system of concepts for comprehending how ecosystems function and how human societies interact with ecosystems. The edition draws on a variety of existing ideas, integrating long-standing ecological principles with concepts assembled more recently under the rubric of complex systems theory. Most of the concepts in this book are firmly established but a few are still the subject of unresolved scientific debates. Some scientists consider each level of biological organisation to have a life of its own that emerges from the organisation of the parts rather than simply from the characteristics of the parts themselves. Other scientists consider emergent properties to be theoretical constructs that lack rigorous scientific confirmation.