The study of human adaptability focuses on functional and structural features of human populations that facilitate coping with and transforming the physical environment, particularly under conditions of environmental change and stress. This chapter considers the ecosystem concept and the distinction between adaptation and adjustment. It examines the ecological system or ecosystem, describes the interaction between living and nonliving components of a given habitat. The chapter argues that the ecosystem can be subdivided into the three components that structure it: energy, matter, and information. Regardless of the topic of investigation or the level of a model's complexity, the parts of a system are always linked by flows of energy, matter, and information. These flows connect the components of ecosystems, and they must be understood if we are to understand the structure and function of ecosystems and human adaptability within those systems. How energy is stored and how it performs work are best described by the laws of thermodynamics.