An ecosystem is a functional unit of nature in which living organisms interact with one another and with the physical environment. It includes biotic components such as plants, animals, microorganisms and human beings, as well as abiotic components such as air, water, soil, sunlight, temperature, minerals and nutrients. Ecosystems may be natural, such as forests, grasslands, rivers, lakes and oceans, or artificial, such as agricultural fields, gardens and urban environments. The proper functioning of ecosystems is essential for maintaining environmental quality, supporting life and protecting human health. The major functions of an ecosystem are energy flow, nutrient cycling, productivity, decomposition, regulation of ecological processes, habitat provision and maintenance of environmental balance. One of the most important functions of an ecosystem is the flow of energy. The ultimate source of energy for most ecosystems is the sun. Green plants and other photosynthetic organisms capture solar energy and convert it into chemical energy through photosynthesis. These organisms are called producers because they prepare organic food from carbon dioxide and water. Herbivores obtain energy by consuming plants, while carnivores obtain energy by feeding on herbivores or other animals. Omnivores may obtain energy from both plants and animals. Decomposers obtain energy from dead organic matter. Thus, energy passes through different trophic levels in a food chain or food web. Unlike nutrients, energy is not recycled continuously; a considerable amount is lost as heat during metabolic activities at each trophic level. Another major function is primary and secondary productivity. Primary productivity refers to the rate at which producers, mainly green plants, convert solar energy into organic matter. Gross primary productivity represents the total amount of organic matter produced through photosynthesis, while net primary productivity is the amount remaining after plants use some energy for respiration. This net production becomes available to herbivores and other organisms. Secondary productivity refers to the formation of new organic matter