The materials that living things are made from, such as carbon and nitrogen, are not made new. They are recycled through an ecosystem. This happens between the abiotic parts, which are the non-living parts such as air, water and soil, and the biotic parts, which are the living things.
Plants take in materials from the abiotic environment: carbon dioxide from the air and water and nitrates from the soil. They use them to build the compounds that make up their bodies. Animals get these materials by eating plants or by eating other animals, so the materials pass along food chains.
Materials return to the environment when living things respire and excrete waste, and when they die. Decomposers such as bacteria and fungi break down dead organisms and waste, returning nutrients to the soil and carbon dioxide to the air. Plants can then take them in again. Without decomposers, dead material would build up and the materials would stay locked up.