Production by Consumers
Biological communities also include organisms that consume biomass for their nutrition. Such organisms include herbivores, carnivores, and detritivores. These organisms obtain their energy through respiration, a process that releases energy from organic molecules like glucose. The equation for respiration is described below:
C6H12O6 + 6O2 >>> 6CO2 + 6H2O + released energy
The amount of energy actually used by these animal populations is significantly less than the amount consumed. In all animals, digestion is an imperfect process and only a portion of the energy ingested is actually assimilated and then used for body maintenance, growth and reproduction. The remaining portion leaves the organism's body undigested.
We sometimes call the energy assimilated by consumer organisms the gross secondary productivity. Gross secondary productivity can be determined directly, unlike gross primary productivity, by measuring the amount consumed minus the material defecated.
The Grazing Food Chain
The grazing food chain is a model that describes the general flow of energy in communities. For most ecosystems the model begins with the photosynthetic fixation of light, carbon dioxide, and water by plant autotrophs (primary producers) who produce sugars and other organic molecules. Once produced, these compounds can be used to create the various types of plant tissues. Primary consumers or herbivores form the second link in the grazing food chain. They gain their energy by consuming primary producers. Secondary consumers or primary carnivores, the third link in the chain, gain their energy by consuming herbivores. Tertiary consumers or secondary carnivores are animals that receive their organic energy by consuming primary carnivores. The illustation below models this process:
The various levels in the grazing food chain are linked to each other like links in a chain. The levels are often called trophic levels, and they suggest a particular order for the passage of energy through the food chain. Like many very simple models, the idea of a food chain only provides a simple abstraction of the nature of energy flow through communities. The ultimate disposition of the energy assimilated by consumers is by four routes: respiration, biomass accumulation, decay of organic matter by bacteria and other decomposer organisms, and consumption by consumers.
The actual amount of energy incorporated in the tissues of consumers at each tropic level is not determined by the gross amounts consumed. Instead, it is the amount of organic energy converted into actual biomass. Consumers lose significant amounts of consumed energy due to assimilation inefficiencies, morphological and physiological maintenance, reproduction, and the process of finding and capturing food. The energy to perform the latter three processes is supplied by respiration.
Thus, the number of trophic levels that can be maintained in any ecosystem is finite. The limit is reached when consumers can no longer consume enough energy to balance the energy lost in the assimilation process, morphological and physiological maintanence, growth, reproduction, and in finding the food. Normally, ecosystems have about four or five trophic levels.
Finally, some ecosystems, like rivers and lakes, are characterized by large imports of biomass in the form of dead organic matter. This situation can cause consumer production to be higher than the autotrophs found in these systems.