Energy Flow And Nutrient Cylcing In Ecosystem

ENERGY FLOW AND NUTRIENT CYLCING IN ECOSYSTEM The two major functions within an ecosystem are the transfer of energy through, and the recycling of nutrients within the ecosystem. ENERGY FLOWS IN ECOSYSTEMS PHOTOSYNTHESIS 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 (‘selffeeders’). 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 carbohydrates produced by photosynthesis are: 1

Combined with elements such as nitrogen, phosphorous and sulphur to produce proteins and nucleic acids.

2

Converted into starch and stored in the plant.

3

Converted into cellulose (the main plant structural material). 4

Used by the plant for respiration i.e. biochemical processes, cell maintenance and growth.

Levels of photosynthesis vary considerably. As photosynthesis is a multi-stage process, involving the interaction of several factors, it can be limited by the one factor, which is nearest to its minimum value, this is known as ‘The Law of the Minimum’. The limiting factors which affect the rate of photosynthesis are temperature, light intensity, carbon dioxide concentration and the availability of water. ENERGY FLOW THROUGH AN ECOSYSTEM 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. NUTRIENT (GASEOUS AND BIOGEOCHEMICAL) CYCLES 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 concept of the recycling of nutrients between three compartments or stores is shown in Gersmehl’s Model.