Energy Flows

The diagram shows how both energy and inorganic nutrients flow through the ecosystem. Energy "flows" through the ecosystem in the form of carbon-carbon bonds. When the carbon-carbon bonds are broken, carbon is combined with oxygen to form carbon dioxide. This process releases the energy, which is either used by the organism (to move its muscles, digest food, excrete wastes, etc.) or the energy may be lost as heat. The dark arrows represent the movement of this energy. Note that all energy comes from the sun, and that the ultimate fate of all energy in ecosystems is to be lost as heat. Energy does not recycle!!

Energy flow in an ecosystem. Dark arrows represent energy movement. Light arrows represent inorganic nutrient flow.

The other component shown in the diagram are the inorganic nutrients. They are inorganic because they do not contain carbon-carbon bonds. These inorganic nutrients include the phosphorous in your teeth; the nitrogen in protein; and the iron in your blood. The movement of the inorganic nutrients is represented by the open arrows. Note that the producers obtain these inorganic nutrients from the inorganic nutrient pool, which is usually the soil. These inorganic nutrients are passed from organism to organism as one organism is consumed by another. Ultimately, all organisms die and become detritus, food for the decomposers. At this stage, the last of the energy is extracted (and lost as heat) and the inorganic nutrients are returned to the soil or water to be taken up again. The inorganic nutrients are recycled, the energy is not.

In the flow of energy and inorganic nutrients through the ecosystem, a few generalizations can be made:

1. The ultimate source of energy (for most ecosystems) is the sun
2. The ultimate fate of energy in ecosystems is for it to be lost as heat.
3. Energy and nutrients are passed from organism to organism through the food chain as one organism eats another.
4. Decomposers remove the last energy from the remains of organisms.
5. Inorganic nutrients are cycled, energy is not.

Living Things Interact

In terms of energy flow in an ecosystem, living things can be either producers or consumers. Producers make their own food by changing energy from the environment into food. Plants take energy from sunlight and use it to change carbon dioxide into sugar. Other producers include algae and some forms of bacteria. Consumers cannot make their own food. They must eat producers or other consumers. There are 4 types of consumers:

Producers are the base of the food chain. Herbivores, or primary consumers, eat producers. An organism that eats herbivores is a carnivore (secondary consumer). A carnivore that eats a carnivore is a tertiary consumer.

Food Chains, Webs, and Pyramids:
A food chain is the path of food from a producer to consumers. For instance, a typical food chain in a field ecosystem might be:

The real world, of course, is more complicated than a simple food chain. While many organisms do specialize in their diets, other organisms do not. Hawks don't limit their diets to snakes, snakes eat things other than mice, mice eat grass as well as grasshoppers, and so on. A more realistic depiction of who eats whom is called a food web.

See example---------------

It is when we have a picture of a food web in front of us that the definition of food chain makes more sense. We can now see that a food web consists of interlocking food chains, and that the only way to untangle the chains is to trace back along a given food chain to its source.

Food Web for a field.--------------------------

An energy pyramid is created from the relationships of living things as they compete for energy. The pyramid is smaller at the top than the bottom because at each level, energy is lost. As much as 90% of the energy at each level is lost and cannot pass to the next level. That means that only 10% of the energy at each level can pass on to the next level. Why is energy lost at each level? Because 1) not everything in the lower levels gets eaten, 2) not everything that is eaten is digested, and 3) energy is lost as heat.

Animals that eat plants are getting only 10% of the original energy the plant consumed. Animals that eat animals are getting only one-tenth of one-tenth (or one-hundredth) of the original energy used by the plant. The further an animal is from the bottom level, the greater the loss of the original amount of energy. Because of this, less biomass (weight of organisms) can exist at the top level than the bottom level.

Matter Cycles

While energy does not cycle through an ecosystem, chemicals do. The inorganic nutrients cycle through more than the organisms, however, they also enter into the atmosphere, the oceans, and even rocks. While all inorganic nutrients cycle, we will focus on only 3 of the most important cycles - water, carbon, and nitrogen.

The Water Cycle:

In the water cycle, energy is supplied by the sun, which drives evaporation and the weather systems that move clouds from one place to another. Precipitation occurs when water condenses from a gas in the atmosphere and falls to earth. Evaporation is the reverse process in which liquid water becomes a gas. Once water condenses, gravity takes over and the water is pulled to the ground. Gravity then pulls water underground (groundwater) or across the surface (runoff). In either event, gravity continues to pull water lower and lower until it reaches the oceans.

Frozen water may be trapped in cooler regions of the Earth (the poles, glaciers, etc.) as snow or ice, and may remain as such for very long periods of time. Lakes, ponds, and wetlands form where water is temporarily trapped.

The Carbon Cycle:

Carbon Cycle
In the carbon cycle, plants absorb carbon dioxide from the air and combine it with water from the soil to make sugar through photosynthesis. The process of photosynthesis uses the carbon atoms in the carbon dioxide in sugar. Animals eat the plants and use the carbon for their needs or to build their own tissues. Animals return the carbon to the air as carbon dioxide when they breathe, or when they die, they return carbon to the soil during decomposition. Carbon atoms in the soil may be used by plants or bacteria. Or when plants and animals die, they may be changed into fossil fuels. When fossil fuels are burned in factories or cars, they release carbon dioxide into the air. When we release more carbon dioxide into the air, some may be dissolved into the ocean, while excess carbon dioxide contributes to global warming.

The Nitrogen Cycle:

The nitrogen cycle is one of the most difficult of the cycles to learn. But nitrogen is important in forming amino acids which form the proteins of your body. Proteins make up skin and muscle and the enzymes that carry out the chemical reactions in your body. Most of the nitrogen is the atmosphere, which is about 78% nitrogen. Nitrogen gas in the atmosphere is composed of two nitrogen atoms bound to each other. In this form, it is not available to living things. It takes a lot of energy to get nitrogen gas to break up and combine with other things, such as carbon or oxygen.
Nitrogen gas can be taken from the atmosphere (fixed) and made available to living things in two basic ways. First, lightning provides enough energy to "burn" the nitrogen and fix it in the form of nitrate, NO3. Nitrogen is also fixed by nitrogen fixing bacteria, who use special enzymes to fix nitrogen. These bacteria make the nitrogen available to plants who use it to make amino acids. Nitrogen fixing bacteria can live free in soil or in plant roots. Animals acquire all of their amino acids when they eat plants (or other animals). When plants or animals die (or release waste), decomposers eat them and the nitrogen is returned to the soil.
The usual form of nitrogen returned to the soil in animal wastes or in the output of the decomposers, is ammonia. Ammonia goes through conversions by two different kinds of bacteria to make it nitrate, which can be taken up by plants to continue the cycle. We now have a cycle set up in the soil, but what returns nitrogen to the air? It turns out that there are denitrifying bacteria which take the nitrate and combine the nitrogen back into nitrogen gas.