Environmental science is an interdisciplinary field that studies the interactions between humans and the natural environment, while ecology is the narrower study of relationships among organisms and between organisms and their surroundings. The living world is organized in nested levels. A species is a group of organisms that can interbreed and produce fertile offspring; a population is all the members of one species living in a defined area at the same time. A community consists of all the populations of different species living together in an area, and an ecosystem is that community together with its physical environment. Each organism occupies a habitat, the physical place where it lives, and a niche, its role within the ecosystem, which together describe both where it lives and how it functions.
Biodiversity refers to the variety of life at three levels: genetic diversity, species diversity, and ecosystem diversity. Within ecosystems, organisms are classified by how they obtain energy. Producers, including plants and many microbes, make their own food through photosynthesis or chemosynthesis. Consumers obtain energy by eating other organisms, and decomposers break down dead matter, recycling nutrients back into the system. These feeding relationships form linear food chains and interconnected food webs, with each feeding position called a trophic level. Producers sit at the first trophic level, primary consumers at the second, and so on.
Energy flows through these levels but is lost at each transfer. The 10% rule states that only about 10 percent of the energy at one trophic level is passed on to the next; the rest is lost largely as heat through metabolism. The fundamental processes that move carbon and energy are photosynthesis, in which plants convert sunlight, carbon dioxide, and water into glucose and oxygen, and cellular respiration, in which organisms convert glucose and oxygen back into energy, carbon dioxide, and water. Together these processes link living systems to the atmosphere and underlie every ecological and climate phenomenon discussed in the chapters that follow.