The movement of a chemical through the biosphere is known as a biogeochemical cycle. All of the chemicals necessary for life are cycled through various organic and inorganic stages, but they are neither created nor destroyed. Energy, on the other hand, is constantly added in the form of sunlight and constantly lost in the form of infrared radiation and light reflected into space. Therefore, matter is described as being "cycled" through an ecosystem whereas energy "flows" from sunlight to producers and consumers -- and is eventually lost as heat.
The Biogeochemical Cycles As Closed Systems
The chemical composition of Earth is essentially constant -- aside from minor leaking of the atmosphere into space and occasional inputs from asteroids, matter neither enters nor leaves the planet. For this reason, the biogeochemical cycles are essentially closed systems.
Energy Flow in Ecosystems: An Open System
The movement of energy through an ecosystem, in contrast with the movement of matter, constitutes an open system. There are inputs and outputs, or gains and losses. Energy enters the system in the form of sunlight. Energy cannot be truly destroyed, but in every chemical or biological reaction some of it is always transformed into heat. This heat leaves Earth in the form of infrared radiation.
Examples of Biogeochemical Cycles
The most important biogeochemical cycles are the water cycle, the nitrogen cycle, the carbon cycle and the phosphorous cycle. Conceptually, the water cycle is the simplest to understand. Powered by sunlight, water evaporates from sources such as oceans, lakes and rivers, then falls to the ground as precipitation. All life takes in water and later excretes it. The whole time, water may change phase from gas to liquid to solid, but it remains chemically the same. In other cycles, such as the nitrogen cycle, for example, the chemical forms various chemical compounds at different stages in the cycle. All life requires nitrogen, but only certain organisms can metabolize the inorganic nitrogen in the atmosphere. All other organisms depend on nitrogen compounds synthesized by these "nitrogen fixers." All of the biogeochemical cycles are ultimately powered by sunlight.
Energy Flow: Trophic Levels
Nearly all energy in the biosphere, with the exception of a few extreme ecosystems like those formed around volcanic vents on the ocean floor, comes from solar radiation. Not all organisms, however, are able to use sunlight directly. Organisms that can use sunlight as their primary source of energy are called "primary producers" or simply "producers." Producers are on the first "trophic level." The second trophic level includes all organisms that feed directly on producers. These are called herbivores or primary consumers. The third level is made up of carnivores, which eat herbivores, or secondary consumers. There is often a fourth level, made up of carnivores that eat other carnivores, and there is sometimes even a fifth trophic level as well.
Energy Loss Between Trophic Levels
Plant photosynthesis is about 1 percent efficient. Also, as a general rule of thumb, only 10 percent of the energy contained in the organisms of one trophic level are available to consumers in the next level. This is because most organisms only use about 10 percent of the energy they consume in the building of their own bodily tissues -- the rest is used for metabolism. In other words, for a person to eat 1 calorie worth of beef, the cow had to eat 10 calories worth of plant matter, and the plant had to absorb 1,000 calories worth of sunlight. Mathematically, this provides a good explanation for why an ecosystem can only support of few high-level predators.
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