6S – Energy economy in natural systems

Supporting article S: Trophic dynamics and energy economy of natural systems


Trophic dynamics
In ecology, trophic dynamics is the system of trophic levels (Greek τροφή, trophē, food or feeding), which describes the position that an organism occupies in a food chain: what an organism eats, and what eats the organism.

Energy economy

An arctic food web. Each arrow represents a step-up in trophic level.

Ecologists study the energy economies of natural systems. Foundation species (also known as primary producers) harvest an energy source such as sunlight and turn it into biomass by fixing carbon dioxide. Organic compounds such as carbohydrates, fats, and proteins are high-energy substances consumed by other organisms (primary consumers), which are in turn consumed by others. Each link in this chain of consumption is termed a trophic level. Because only a fraction of the energy used by a level is converted to biomass, less energy is available at higher levels. Plants, algae, and some bacteria can perform photosynthesis and combine water and carbon dioxide to make organic compounds using the Sun’s energy.

Most ecosystems ultimately rely upon the Sun for energy and upon autotrophs to fix carbon and harness that energy. There are only a few exceptions to this, such as chemosynthetic archaea and bacteria, which derive energy from the breakdown of sulfur compounds such as hydrogen sulfide around deep sea hydrothermal vents and acid mine drainage. These organisms can utilize hydrogen sulfide in lieu of water to make organic compounds, and as the reaction between hydrogen sulfide and carbon dioxide is a spontaneous one, they do not need energy from sunlight. Lithotrophs can use inorganic compounds as electron donors to manufacture organic compounds or produce ATP. Sulfur-oxidizing bacteria, for example, can consume hydrogen sulfide, elemental sulfur, sulfite, and thiosulfate as energy sources instead of carbohydrates, fats, and proteins. Using sulfite oxidase, sulfur-oxidizing bacteria can obtain electrons from sulfur compounds and form ATP through an electron transport chain (ETC).

In terrestrial ecosystems, plants such as grass are the primary producers and form the first trophic level. Next are herbivores (primary consumers) that eat the grass, such as rabbits. Next are carnivores (secondary consumers) that eat the rabbits, such as a bobcat.

Every time there is an exchange of energy between one trophic level and another, there is quite a significant loss due to the fundamental laws of thermodynamics. Living organisms utilize energy from their food for cellular processes, growth, and development. Animals need energy to move and digest food. A high metabolism also reduces the efficiency of the energy transfer by causing more energy to be lost as heat. Less energy is lost in the body of a fish than in the body of a small mammal. Energy is also excreted in urine and feces. Therefore, so many units of grass can only support a much smaller number of units of rabbits, who can only support a smaller group of bobcats, who can only support a smaller group of mountain lions. This is why trophic levels are usually portrayed as a pyramid, one that places grass on the bottom and mountain lions on top. The top is generally much smaller than the bottom, although certain factors can produce an inverted pyramid if it is a pyramid of number (which counts the number of organisms in each level) or biomass (which measures the biomass at each level). A pyramid of energy (which measures the energy or kilojoules) is never inverted. Each level implies a loss of energy and efficiency and less life that can be supported by the sun.

There is no in-principle limit to the number of levels in a trophic system, but as only a fraction of the energy of each level can be processed by the next (about ten to twenty percent), there are rarely more than four or five links of consumption.