Biologists have identified more than 1.4 million species of living things, from microscopic bacteria to giant blue whales. Every living thing depends on others for its survival and is at the center of a web of relationships. Ecology looks at the interactions among different organisms, how they relate to their environments, and how human activities are affecting these relationships. This series covers the fundamental life-science concepts of biology, ecology, evolution, genetics, cell biology, and life-forms, including humans. From the building blocks of cells and the basics of genetic theory through the structure, anatomy, and behavior of living organisms, this series tells the story of life in all its forms.