Ecology, study of the relationships between living organisms and their environment. The earth is covered from pole to pole with a thin, intricate web of interdependent living organisms called the biosphere. Within the biosphere there are many cleaerly defined subunits, or ecosystems. Ecosystems (for example, ponds, forests, fields of grass, deserts, or oceans) vary greatly in size, but in each case the animals and plants within them have a pattern of feeding relationships called a food chain or web. The ultimate foundation of the food chain is plant life and the process of photosynthesis, by which energy from the sun is converted to organic material. Within an ecosystem, every species occupies a distinct ecological niche. The nature of the niche is determined by the space required by the species to survive, and the nature and availability of its food. Over a period of time, the numbers and types of animals and plants in the system change. This change is actually a process of succession and is accompanied by continually increasing complexity. The process begins with an elementary ecosystem and culminates in what is called a climax community, an ecosystem capable of supporting an enormous number of species. Climax communities, such as tundra, forests, or deserts, are stable until humans or natural geological upheaval destroys them, thereby starting an entirely different ecosystem. The growth and stability of ecosystems depend entirely on the cycles that expand and renew the chemicals that are essential to life. Among the most common of these are the carbon and nitrogen cycles and photosynthesis. Any serious or prolonged disturbance of these vital cycles threatens the existence of an ecosystem and of the species it contains. The environment, left to itself, can continue to support life for millions of years. The single most unstable and potentially disruptive element in the scheme is the human species. Human beings with modern technology have the capacity to bring about, intentionally or unintentionally, far-reaching and irreversible change. In addition to studying nature, ecology also considers the effects upon the environment and particular ecosystems of various human activities. The body of knowledge and insights unique to ecology are not only significant in their own right, but are also important in the development of environmentalism.