All ecosystems have two major processes: a flow (of energy) and a cycle (of nutrients). These two processes, which are driven by the sun’s energy, help to link the component parts of ecosystems.

Energy Flow

     Individual ecosystems, whether we are dealing with the tropical rain forest or a small pond, are sustained by a flow of energy through them. The main source of this energy is sunlight or solar radiation. Light energy enters the ecosystem when it is absorbed by green plants. It is then passed through the ecosystem as food; plants are eaten by animals and animals consume each other. Energy absorbed and transferred through the ecosystem in this way is eventually converted into heat. This heat, which is exactly equivalent to the solar input, finally leaves the ecosystem and is lost in space.

Primary Production

     Green plants are able as they grow to absorb light energy from the sun and raw materials from the environment (nutrients in the soil solution, gases in the atmosphere). The process of absorbing light energy (or light fixation) by plants is called photosynthesis or primary production. It has the formula

                                     6C0₂  +  6H₂0  + light energy    =    C₆H₁₂0₆   +   60₂                                                 Carbon Dioxide +  Water + Light energy    =  Sugar (Carbohydrate) + Oxygen

     A distinction is made between gross primary production (GPP) and net primary production (NPP). GPP is the total amount of energy absorbed or fixed by green plants (see Figure 4.4). Some of the GPP goes into the production of new plant tissue. This is the NPP. The remaining part of the GPP is re-used by plants to carry out normal functions, e.g. absorbing nutrients, repairing cell damage. This re-used energy, which is made available by a process known as respiration, is eventually lost as heat. As shown in the formula below, respiration is the converse (opposite) of photosynthesis:

                          C₆H₁₂0₆       +     60_{2    =}   6C0₂    +      6H₂0      +      release of energy respiratory Heat                                               Sugar                        Oxygen        Carbon                Watetr                                       Lost to Ecosystem                                                                                    (New Production)                                  Dioxide

                                                                                                                               The rate of energy fixation or production is referred to as productivity. It is expressed in units as ‘energy stored per unit of ground area per unit of time’, e.g. tonne~hectare/year or kg/m2/year. As a rate of production of new tissue or biomass, productivity should not be confused with total biomass. As suggested earlier, the standing crop or biomass is the total amount of living tissue accumulated over a period of time, expressed at any one moment. The huge biomass of the tropical rain forest is not equivalent to the annual productivity, which happens to be high, because that biomass probably took hundreds of years to accumulate.

     Plant productivity is determined by light, temperature, water, nutrients and carbon dioxide. It is highest where light, warmth, moisture and key nutrients are abundant, e.g. in swamps, marshes and,  tropical rain forest. It is lowest in areas where light, warmth and/or water are lacking, e.g. in the Arctic tundra and the hot sandy deserts.

Trophic Level

   Plant growth, represented by NPP, is the principal food resource for most animals and soil organisms in the ecosystem. A proportion of the NPP may be stored for a time as new biomass (e.g. in the trunks and branches of trees), while some may be consumed directly by animals above the surface of the ground. On land, the majority of NPP, however, enters the soil where it will form humus and will eventually be eaten by soil organisms. Ultimately, even the long-lasting trees will die and be consumed by animals. In summary, therefore, plant material above and below the ground surface is eaten by herbivores (plant eaters), which may then be consumed by carnivores (meat eaters). Plants and animals are also eaten by soil micro-organisms, especially by bacteria and fungi, which are referred to as the decomposer organisms.

     The movement or flow of energy from plants to animals to decomposers can be represented in its simplest form as a food chain. Each stage in the chain where energy is exchanged is called a trophic level or ‘feeding’ level. Thus, plants (e.g. alfalfa grass) are represented by trophic level 1, herbivores or primary consumers (e.g. beef cattle) by trophic level 2, and carnivores or secondary consumers (e.g. humans) by trophic level 3. The decomposer part of the system can be in trophic levels 2, 3 and 4 depending on the source of the food. Decomposers receive dead material from trophic levels 1, 2 and 3 and waste animal products from levels 2 and 3.

There may be one or two additional levels in a food chain represented by ‘top’ carnivores. These are carnivores, which eat other carnivores, e.g. in other food chains a fox may eat a snake, magpies may eat small cats. Few food chains, however, involve more than five or six trophic levels because a significant amount of energy (as much as 90%) is lost as heat at each step in the food chain. There is seldom enough energy available, therefore, to support an additional level.

     Although simple linear food chains seldom exist in nature, something close to them occurs in species-poor Arctic ecosystems; as in the sequence of algae in the ocean through an algae-eating fish to a sea lion to a polar bear. In reality, the situation is often more complex because many organisms including humans operate at several trophic levels. For instance, a person might eat the bear, the seal, the fish or even the algae, placing him in trophic levels 5, 4, 3 or 2. Or the polar bear might eat the fish or the person. For these reasons it is more realistic to describe energy flow in terms of a food web rather than as a food chain.

[Welcome] [System] [Ecosystem] [Greenhouse] [Review] [Links]