Food webs, energy and nutrient cycles

Food-web arrows conventionally point from the food resource to its consumer, showing transfer of matter and chemical energy. Producers form organic compounds; consumers obtain them by feeding; decomposers break down dead material. Available energy usually decreases between trophic levels because organisms respire and because some material is not consumed or assimilated. Energy is transferred and dissipated as heat rather than recycled indefinitely.

Matter cycles through living and non-living reservoirs. Photosynthesis fixes carbon; respiration and combustion release carbon dioxide. Nitrogen fixation makes atmospheric nitrogen biologically available; nitrification forms nitrates, while denitrification returns nitrogen gas. Phosphorus cycles through rocks, soil, water and organisms. Evaporation, condensation and precipitation transfer water. Fertiliser runoff can change nutrient availability and aquatic productivity.

Watch out: Nutrients cycle through reservoirs; energy enters, transfers and is eventually dissipated as heat, requiring continuing input.