The expression of net primary production in terms of dry matter, organic matter, carbon, or energy converts all units of plant production to an equivalent basis. However, net production differs in chemical composition and nutritive value depending upon species, plant age, and habitat. The nutritive quality of net primary production is very important in determining the efficiency of food utilization by herbivores. Nutrient content is equally important in determining the rate of decomposition of organic residues. Carnivores consume food of high nutrient content, especially with respect to protein, and adequate nutrition for these animals is likely determined by the amount of food consumption rather than by nutritive quality. Omnivores and detritivores fall somewhere between carnivores and herbivores with respect to the importance of food quality in regulating their growth and numbers in natural systems. The use of energy flow or transfer in ecosystem analysis is a gross oversimplification of the relationship between organisms and their food. Energy utilization by consumers is regulated by nutritive quality of the food and all other factors regulating population sizes.