The food web depicts feeding connections and is offenly referred to as the consumer-resource system. The food web is an illustration of the methods of feeding that links an ecosystem into a unified system of exchange. Feeding relations can be roughly divided into four categories: herbivory, carnivory, scavenging and parasitism. Ecologists can place all life forms in one of the two categories: autotrophs- who produce biomass from natural elements such as the sun, and heterotrophs- who get their food from other living matter. Energy flow is directional which makes it contrast against the cyclic flows of material through the food we systems. Energy flow typically includes production, consumption, and respiration . In a general sense, energy flow can be defined as the sum of metabolic production . Energy flow diagrams show the rates and efficiency of transfer from one tropic level to another and through the hierarchy.