first constructed. After the members of the web are decided, the biomass of each functional group is calculated, usually in kg carbon/hectare. In order to calculate feeding rates, researchers assume that the population of the functional group is in equilibrium. At equilibrium, the reproduction of the group balances the rate at which members are lost through natural death and predation When feeding rate is known, the efficiency with which nutrients are converted into organism biomass can be calculated. This energy stored in the organism represents the amount available to be passed on to the next trophic level. After constructing the first soil flow webs, researchers discovered that nutrients and energy flowed from lower resources to higher trophic levels through three main channels. The bacterial and fungal channels had the largest energy flow, while the herbivory channel, in which organisms directly consumed plant roots, was smaller. It is now widely recognized that bacteria and fungi are critical to the decomposition of carbon and nitrogen and play important roles in both the carbon cycle and nitrogen cycle. Interaction web An interaction web, shown above right, is similar to a topological web, but instead of showing the movement of energy or materials, the arrows show how one group influences another. In interaction food web models, every link has two direct effects, one of the resource on the consumer and one of the consumer on the resource. The effect of the resource on the consumer is positive, (the consumer gets to eat) and the effect on the resource by the consumer is negative (it is eaten). These direct, trophic, effects can lead to indirect effects. Indirect effects, represented by dashed lines, show the effect of one element on another to which it is not directly linked. For example, in the simple interaction web below, when the predator eats the root herbivore, the plant eaten by the herbivore may increase in biomass. We would then say that the predator has a beneficial indirect effect on the plant roots. Food web control Bottom-up effects Bottom-up effects occur when the density of a resource affects the density of its consumer. For example, in the figure above, an increase in root density causes an increase in herbivore density that causes a corresponding increase in predator density. Correlations in abundance or biomass between consumers and their resources give evidence for bottom-up control. An often-cited example of a bottom-up effect