1.4 Energy Flow

ENERGY FLOW AND MATERIAL CYCLING IN AN ECOSYSTEM  Energy and Productivity  Energy Flow in an Ecosystem  Material Cycles in an Ecosystem

ENERGY AND PRODUCTIVITY ENERGY When we consider an ecosystem, we must describe the flow of energy and the cycling of nutrients. That is, we are interested in things like how much sunlight is trapped by plants in a year, how much plant material is eaten by herbivores, and how many herbivores are eaten by carnivores. The two ecological processes of energy flow and mineral cycling involving interaction between the physicochemical environment and the biotic communities may be considered the 'heart' of ecosystem dynamics. In an ecosystem, energy flows in non-cyclic manner (unidirectional) from sun to the decomposers via producers and macro consumers (herbivores and carnivores), whereas the materials keep on moving in a cyclic manner.

PRODUCTIVITY OF ECOSYSTEM The productivity of an ecosystem refers to the rate of production, i.e., the amount of organic matter accumulated in any unit time. It is of following types 1. Primary productivity. It is defined as the rate at which radiant energy is stored by photosynthctic and chemosynthetic activity of producers. Primary productivity is of following types: (i) Gross primary productivity. It refers to the total rate of photosynthesis including the organic matter used up in respiration during the measurement period. GPP depends on the chlorophyll content. The rate of primary productivity are estimated in terms of either chlorophyll content as chl/g dry weight/unit area or photosynthctic number, i.e., amount of CO, fixed/g chl/hour.

(ii) Net primary productivity. It is the rale of storage of organic matter in plant tissues in excess of the respiratory utilization by plants during the measurement period. 2. Secondary productivity. It is the rate of energy storage at consumer's levels herbivores, carnivores and decomposers. Consumers tend to utilise already produced food materials in their respiration and also convert the food matter to different tissues by an overall process. So, secondary productivity is not divided into 'gross' and 'net' amounts. Due to this fact some ecologists prefer to use the term assimilation rather than production at this level - the consumers level. Secondary productivity, in fact, remains mobile (i.e., keeps on moving from one organism to another) and does not live in situ like the primary productivity. 3. Net productivity. It is the rate of storage of organic matter not used by the heterotrophs or consumers, i.e., equivalent to net primary production minus consumption by the heterotrophs during the unit period as a season or year, etc.

ENERGY FLOW IN AN ECOSYSTEM

Photosynthesis (or phototrophism) is the process by which light energy from the sun (insolation), is absorbed by plants, blue-green algae and certain bacteria. It is then used to produce new plant cell material, which forms the food source for plant eating animals (herbivores). Plants which are able, through the process of photosynthesis, to convert light energy and inorganic substances (carbon dioxide, water and various mineral nutrients) into organic (carbon based) molecules, are called phototrophs or autotrophs (‘self-feeders’). In a plant, most photosynthesis is carried out by the leaves, and in order for the process to occur they must contain chlorphyll, which is able to absorb enerfy from sunlight. The plant also requires carbon dioxide, from the atmosphere, and water from the soil. As a result of the process, and carbohydrates are produced. 6CO2 + carbon dioxide

12 H2O water

C6H12O6 + 6O2 + 6H2O glucose oxygen water

The energy produced by photosynthesis will pass through the food chains and food webs of an ecosystem, with some of it being stored as chemical energy in plant and animal tissue. Some of it will be lost from the system, as respiration (heat energy) and excreta products. The total amount of energy lost, from all the trophic levels in an ecosystem through respiration, forms the community respiration. Energy is lost at each level in the food chain, with the average efficiency of transfer from plants to herbivores being about 10 per cent, and about 20 per cent from animal. As a result of the loss of energy at each transfer between trophic levels, ecosystems are usually limited to three or four trophic levels. The actual number will depend upon the size of the initial autotrophy (producer) biomass, and the efficiency of energy transfer between the trophic levels.

MATERIAL CYCLES IN AN ECOSYSTEM The nutrients, or elements used by all organisms for growth and reproduction, are termed essential elements or macronutrients, and include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), sodium (Na), sulphur (S), chlorine (Cl), potassium (K), calcium (Ca) and magnesium (Mg). Other elements called trace elements or micronutrients, including iron (Fe), manganese (Mn), copper (Cu), zinc (Zn) and cobalt (C0), are also required, but in smaller quantities. Some organisms also require molybdenum (Mo), slica (Si), and boron (B). The nutrients required by plants are obtained as inputs either from the atmosphere through various gaseous cycles or in precipitation, or from the soil via the weathering of parent rock, through several biogeochemical or sedimentary cycles. The two types of cycle are interrelated, as nutrients pass from abiotic nutrient stores, such as the soil and the atmosphere, into biotic, plant and animal stores (the biomass). The nutrients are then recycled, within the ecosystem, following death and decomposition. Nutrients are lost, as outputs, by surface runoff, leaching through the soil profile or material.

The following cycles are illustrated through the self explanatory diagrams. 1. 2. 3. 4. 5.

Carbon Cycle Oxygen Cycle Nitrogen Cycle Phosporus Cycle Sulphur Cycle

CARBON CYCLE

OXYGEN SCALE

NITROGEN SCALE

PHOSPOROUS SCALE

SULPHUR SCALE