Study of Ecosystem Functions: Energy Flow, Productivity and Biogeochemical Cycling!
The functional pattern of an ecosystem is a very important and basic aspect of study, because all its components are dynamic in one way or the other and are responsible for the creation of a unique state of man-environment relationship as well as a habitat, different from other regions.
Ecosystem functioning can be studied under three heads, viz., energy flow, productivity and biogeochemical cycling.
1. Energy Flow:
Energy is defined as the ability to do work and is the essence of life. Without energy transfers, there could be no life and no ecological systems. The energy used for all the life processes is derived from solar radiant energy.
The behaviour of energy is described by the laws of thermodynamics:
(i) Energy may be transformed from one type into another but is never created or destroyed, and (ii) no process involving an energy transformation will spontaneously occur unless there is a degradation of the energy from a concentrated form into a dispersed form, etc.
The radiant energy produced in the sun travels through space in the form of waves. But only a small fraction of solar radiation reaches the earth to provide energy for the biotic components of the ecosystem. Figure 2.2 indicates the fate of radiant energy reaching the earth’s atmosphere.
It becomes clear from the figure that most of the radiation is lost in space by processes of reflection, absorption as well as through scattering. Its energy is greatly altered as it passes through cloud cover, water and vegetation.
The daily input of sunlight to autotrophic layer of an ecosystem varies mostly between 100 and 800 to 300-400 g cal per cm2 (= 3000 to 4000 kcal per m2 in the temperate zone.
The total radiation flux within different strata of the ecosystem varies from season to season as well as with nature of the earth’s surface, thus controlling the distribution and response of organisms accordingly.
The energy reaching the earth’s surface is transformed and/or absorbed by plants and other organisms. It is used by the green plants during photosynthesis by converting the light energy to chemical energy and making it available to other organisms as food.
Such energy transfers along the food chain were studied for the first time by Lindeman in 1942. He stated that all the functions of an ecosystem could be explained in terms of energy by the knowledge of two attributes of each trophic level—the level of energy storage and the efficiency of energy transfer. Odum (1957), Golley (1960), Slobodkim (1959, 1960, 1962), Teal (1962) and Kozlovsky (1968) have made further detailed investigations.
The process of energy flow in an ecosystem has been depicted by Odum in a simplified diagram as shown in Figure 2.3, where the ‘boxes’ represent the trophic levels and the ‘pipes’ depict the energy flow in and out of each level. It becomes clear from this simplified model that the energy flow is greatly reduced at each successive trophic level from producers to herbivores and then to carnivores.
The productivity of an ecosystem refers to the rate of production, i.e., the amount of organic matter accumulated in any unit of time. It is of three types, viz., primary, secondary and net productivity.
(i) Primary productivity is defined as “the rate at which radiant energy is stored by photosynthetic and chemosynthetic activity of producer organisms (chiefly green plants) in the form of organic substances which can be used as food materials.”
Primary productivity is of two types:
(a) Gross primary productivity is the total rate of photosynthesis including organic matter used in respiration during a particular period.
(b) Net primary productivity is the rate of storage of organic matter in plant tissues in excess of respiratory utilisation by plants during the measurement period.
(ii) Secondary productivity is the rate of energy storage at consumer level. It actually remains mobile from one organism to another and does not live in situ like primary productivity.
(iii) Net productivity refers to the rate of storage of organic matter not used by the consumers. Thus, it is the rate of increase of biomass of the primary producers, which has been left out by the consumers.
3. Biogeochemical Cycling:
“The chemical elements, including all the essential elements of protoplasm, tend to circulate in the biosphere in circular paths from environment to organism and back to the environment.
These moves, in a more or less circular path, are thus known as biogeochemical cycles”. The growth of life process requires about 40 elements. Hydrogen, carbon and oxygen are the basic elements, while nitrogen, phosphorus, potassium, calcium, sulfur, magnesium and iron are the other important elements.
All the elements continue to cycle at the ecosystem level; they are also an integral part of the larger and/or global cycle
The cycling of various elements can be grouped into two categories, viz.:
(i) Gaseous cycles of elements like hydrogen, carbon, oxygen, nitrogen, sulfur; and (ii) Sedimentary cycles of those elements which are present in the soil and organisms.
The process of biogeochemical cycles can be understood through following examples:
The hydrological cycle or water cycle is the most important one. It involves interchange of water between earth’s surface and the atmosphere through rainfall and evapotranspiration. The water from water bodies’ like oceans, lakes, rivers, etc., gets evaporated by solar energy. These water vapours, after cooling and condensation form clouds, and result in rainfall, snowfall, etc.
A large part of the rainfall occurs over oceans, and a sizeable part of the water vapours goes back to the oceans via rivers and streams.
Some part of it infiltrates into the soil to become soil or underground water. A small quantity of water is also absorbed by the plants and consumed by other animals. The same is released during respiration and transpiration of plants. Thus, there is a continuous cycling of water as shown in Figure 2.4.
Like the hydrological cycle, the carbon cycle is also very important for man. The small amount of carbon dioxide (CO2—0.03%) in the atmosphere is the only source of all carbon that passes through the organisms along the food chains. As shown in Figure 2.5, carbon moves from the atmospheric pool to green plants (producers), then to animals (consumers) and finally to bacteria and other micro-organisms (decomposers) that return into the atmosphere, through decomposition of dead organic matter.
The carbon dioxide has the unique property of absorbing infra-red radiation and its small quantity helps in keeping the earth warm. A part of this cycle also operates in the ocean.
The oxygen constitutes 21 per cent of the atmosphere. It also occurs in a bound state as oxides and carbonates in rocks and in water. The plants release oxygen in photolysis of water during photosynthesis. Again, gaseous oxygen is used in respiration of all the organisms and in the oxidation of the organic matter. Another phase of the oxygen is the ozone layer of the outer stratosphere which serves an important function of protecting the life from the ionising ultraviolet waves. The pattern of oxygen cycle has been depicted in Figure 2.6.
Nitrogen is an essential element of all forms of life and in atmosphere, its percentage is 79. But nitrogen is never taken directly from the atmosphere. The chief sources of nitrogen for plants are nitrates in the soil. During the cycle, the reduced and oxidised forms are involved at one or other stage.
The chief symbiotic nitrogen fixers are bacteria belonging to the genus Rhizobium found in the root nodules of legumes, while symbiotic nitrogen fixers are some blue-green algae. Plants in turn are eaten by animals.
The dead organic matter formed due to death of plants and animals is decomposed by bacteria, fungi, etc., which releases nitrogen either in Free State to atmosphere or as ammonia gas.
Ammonia gas may also reach the soil as nitrates through the activity of nitrifying microbes. The whole pattern of the nitrogen cycle becomes clear from Figure 2.7.
This is a sedimentary cycle and a simpler one, in comparison to nitrogen cycle. As shown in Figure 2.8, phosphorus, an important and necessary constituent of protoplasm, tends to ‘circulate’, the organic compounds being broken down eventually to phosphates which are again available to plants. The great reservoir of phosphorus is rocks or other deposits which have been formed in past geological ages.
These erode and release phosphates to ecosystems. A major portion of phosphates goes into the sea in the form of sedimentation. Sea birds also have a role in returning phosphorus to the cycle from sea to land. The general pattern of phosphorus cycle is more or less similar to that of other minerals.
The sulfur cycle, as shown in Figure 2.9, links air, water and soil, where microbes play an important role. The reserve pool of sulfur is the soil. It is made available to the plants in the soil by the activity of sulfur bacteria.
Some quantities are added to the atmosphere by the burning of the fossil fuels. Later, sulfur dioxide and hydrogen sulfide return to the soil as sulfates or sulfuric acid with the rain.
In tissues of organisms, sulfur is in the form of proteins. The decomposition of proteins releases sulfur. The sulfur cycle is a good example of the interaction and complex biological and chemical regulation between different mineral cycles.
Apart from above mentioned cycling of elements, other elements to follow an almost similar pattern of cycling. The cycles of these elements are usually restricted to the same ecosystem but some of them are widespread and a part of the global system.