The methodology of human ecology is firmly rooted in the human and the natural sciences. Investigations must consider ecosystem interactions, human physiological responses to environmental stresses, and sociocultural adjustments. Chapter 1 presented a brief overview of the role of energy-matter-information flows between system components, or state variables. This chapter begins with a discussion of ecosystem relationships that examines population ecology, primary production, and the importance of soils in plant productivity. It continues with aspects of human biology, including physiological responses (thermoregulation, biological rhythms, and work capacity), growth and development, and nutrition and disease, including a discussion of gender differences. This chapter and the next focus on methodological approaches, including an integrated study of population, soils, human biology, and information theory, that are not part and parcel of the training received by environmental social scientists. A discussion of remote sensing (using digital satellite data) and geographic information systems (GIS) techniques applicable to contemporary human-environment studies is included in Chapter 4. These techniques are not yet common among ecologists or social scientists (except for geographers) but are growing in usefulness across the social and biological sciences.