The first category is autotrophs, including all kinds of green plants and chemosynthetic bacteria, called producers. Green plants can convert absorbed water, CO2 and inorganic salts into primary products-carbohydrates through photosynthesis, and further synthesize fat and protein for self-construction. In this way, solar energy will continue to enter the ecosystem through the synthesis and transformation of producers and become the only source of food and energy for other biological groups. Chemosynthetic bacteria can also synthesize inorganic substances into organic substances, but the energy they use comes not from the sun, but from the energy generated when some substances undergo chemical changes. For example, nitrifying bacteria can oxidize ammonia (NH3) into nitrous acid and nitric acid, and use the energy released in this oxidation process to hydrate CO2 and organic matter.
The second category is heterotrophs, including herbivores and carnivores, called consumers. As the name implies, these consumers can't directly use solar energy to produce food, and they can only get energy by directly or indirectly eating green plants. According to the different feeding status, it can be divided into a class of consumers who directly rely on plant branches, leaves, fruits, seeds and litter, such as grasshoppers, rabbits, deer, cattle, horses, sheep and other herbivores; Carnivores that feed on herbivores are secondary consumers, such as weasels, foxes and frogs. There is a law of the jungle relationship among carnivores, and the strong among them become third-and fourth-class consumers. These advanced consumers are the fiercest carnivores in the biological world, such as lions, tigers, eagles and sharks in the water. Some animals eat both plants and animals, which are called omnivores, such as some birds and fish.
The third category is heterotrophic microorganisms, such as bacteria, fungi, soil protozoa and some small invertebrates. They make a living by decomposing animal and plant residues, which are called decomposers. Microorganisms are widely distributed, rich in the surface layer of soil and water, and less in the air, most of which are saprophytic bacteria and molds. Microorganisms are the largest group in biological communities. It is estimated that 1 g fertile soil contains up to 108 microorganisms. Bacteria and fungi mainly survive by absorbing soluble organic matter from animal and plant residues, and release inorganic nutrients from organic matter during digestion and return to the environment. It can be seen that microorganisms play a role in recycling nutrients in the ecosystem. Small invertebrates in soil, such as nematodes, earthworms, etc., crush plant residues and accelerate the decomposition and transformation of organic matter under the action of microorganisms. In addition, these soil animals can be decomposed in the body, transforming organic matter into inorganic salts for re-absorption and utilization by plants.