The flow of energy can be modeled in several simple ways. However, in order to discuss these models, we first have to look at some underlying concepts. Let’s look first at the laws of thermodynamics. These are three rules that explain the movement and storage of energy in a system. The first law of thermodynamics is that energy can neither be created nor destroyed. It can change forms, act upon matter, or move from one location to another, but it never simply disappears. For example, if you burn a piece of paper, the flame releases the potential energy in the paper and changes it to heat. The second law states that when energy is transferred or transformed, part of the energy is lost as waste by taking a form that cannot be passed on or do further work. This energy will end up as heat and randomly disperse into the environment, a process called entropy. The third has little impact on this discussion. It states that as a system approaches absolute zero, entropy approaches a constant minimum. However, this can only be achieved under artificial circumstances and will have no direct impact on the following principles.
Next, we need to define a few terms. First we have to differentiate between an open system and a closed system. A system is the unit we are observing. In this case the system will be our ecosystem. An open system is one where new energy sources can readily enter the system, such as individuals immigrating into the area, or existing sources can exit the system, such as individuals leaving the area. A closed system is one where energy is recycled through the system without any sources entering or leaving a system. Very few ecosystems are true closed systems. The best example of this is a deep lake that is not connected to a river.
Each organism in the ecosystem occupies a trophic level, which is a position within the food chain for that ecosystem. Broadly classified, individuals within an ecosystem can be consumers or producers. Producers are able to make their own food through chemical processes. The most well know of these are plants which use photosynthesis to make food from sunlight. There are also some who use other chemical processes, especially among bacteria, that might use other starting materials such as sulfur. Consumers are the individuals who cannot produce their own food. This would be the area that most organisms fall into. Anything that lives off of producers is considered a primary consumer. The next level of consumers is made up of those that eat the primary consumers. They are called secondary consumers. One who eats a secondary consumer would be called a tertiary consumer, and so on. There can be many level of consumer within in an ecosystem and often times lower level consumers will be omnivores, or those that will eat both plants and animals depending upon the time of year and the food availability so their trophic classification will vary. Given this, it would be easy to construct a food chain, for any given ecosystem. Let’s say our ecosystem was made up of grasses, mice, and an owl. We could come up with a chain that looked something like this, where the arrows indicate the movement of energy:
Sunlight → Grasses → Mice → Owl
In this example we have three trophic levels: producers, made up of the grasses, primary consumers, or the mice, and a secondary consumer, the owl. Each time an energy exchange occurs it moves up through the trophic levels. However, with each successive exchange of energy, there is always some energy lost as heat due to entropy. This is always a ten percent loss. This creates what is known a food pyramid when analyzed graphically. There are always more producers than primary consumers and more primary consumers than secondary consumers due to the decreasing availability of transferable energy at each level. In a closed system
Of course this can become much more complicated when we consider that when the owl or the mice die, they in turn provide nutrients for the plants and when we consider that there are also probably both detritivores, organisms that live off of decaying material, and scavengers, those that do not kill their own food. These make the chain become rather circular and cyclic, creating what is called a food web, where every energy exchange is taken into consideration. This model is essentially for a closed system and assumes that no other energy sources are moving in or out of the system. However this gives a basic sketch of how the energy moves in a simple ecosystem.
