Ecology is a branch of biological science that is concerned with the relationships and interactions between living organisms and their physical surroundings or environment. Living organisms and the environment with which they exchange materials and energy together make up an ecosystem, which is the basic unit of ecology. An ecosystem includes biotic components – the living plants and animals – and abiotic components – the air, water, minerals, and soil that comprise the environment. A third and essential component of most natural ecosystems is energy, usually in the form of sunlight.
Familiar examples of land-based or terrestrial ecosystems include forest, deserts, jungles, and meadows. Water-based or aquatic ecosystems include streams, rivers, lakes, marshes, and estuaries. There is no specific limitation on the size or boundaries of an ecosystem. A small pond can be studied as a separate ecosystem, as can a desert comprising hundreds of square kilometers. Even the entire surface of Earth can be viewed as an ecosystem; the term biosphere is often used in this context.
If Earth is imagined to be about the size of an apple, then the layer of air surrounding it would not be much thicker than the skin of that apple. This thin envelope of air and the shallow crust of land and water just beneath it provide the abiotic components that support life in the biosphere. It is a closed ecosystem because there is essentially no transfer of material into or out of it. Only the constant flow energy from the sun provides power to sustain the life cycles within the biosphere. Nutrients are continually recycled and reused.
The biosphere seems so big that it is sometimes difficult to believe that humans can affect or disrupt its natural balances. But global problem related to environmental pollution, such as acid rain, the ozone hole, and the greenhouse effect, are significant and must be controlled before irreversible environmental changes occur. These and other pollution problems are discussed later in the text.
In addition to natural ecosystems, such as lakes or forest, several types of artificial ecosystems are of particular importance in environmental technology. For example, one of the most common methods of wastewater treatment is based on a biological system called the activated sludge process. This is an engineered ecosystem comprising a steel or concrete tank, a suspended population of microorganisms in wastewater, and a constant input of air. The microbes are the abiotic components. The system removes organic pollutants from the wastewater.
Source : Basic Environmental Technology (Nathanson)