Autotrophs are self-nourishing organisms, which are mainly green plants capable of manufacturing their food from simple inorganic substances with the help of sunlight. The process is known as photosynthesis. (See the above image)
Sunlight is the main or ultimate source of energy in an ecosystem and is used by green plants to produce chemical energy (food) to nourish themselves, and non-producer organisms (consumers) in the ecosystem. Autotrophs are the only organisms that can produce food in an ecosystem, hence, they are called primary food producers or simply primary producers.
Primary productivity is controlled by a number of factors: light intensity, temperature, moisture, air, soil nutrient and carbon dioxide (CO2). If these factors operate in the appropriate proportions, productivity is high. If however, one or mere of the factors are deficient, productivity is reduced accordingly. Some of the food they produce is used up for their physiological activities and in building their body tissues (growth).
Others are trans-located and stored in various parts of their bodies as chemical energy for consumers. In an aquatic ecosystem, the main producers are green protozoans (euglena), algae, diatoms, water hyacinths, water lilies, ceratophyllum, morning glory (ipomea aquatic), spirogyra and other phytoplankton. In terrestrial ecosystems, producers range from grasses, shrubs, and trees, e.g., maize, rice, yam and paw-paw.