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A - Structure of Ecosystem 1- What is a system ? System is a group of interdependent items (elements) that interact regularly to perform a task. This task is always requiring input (almost energy). 2- What is an ecosystem ? An ecosystem can be defined as the basic functional unit of nature including biotic (living) and abiotic (nonliving) constituents. A community with interacting organisms of different species, and their relationships with the associated chemical and physical systems, can be considered as ecosystem.
A - Structure of Ecosystem 3- An ecosystem components Air, rocks, and water are the basic components of the abiotic environment, while biotic components are classified according to the way they get their food energy to: The producers: the green plants, algae and some bacteria are so called because they are capable of producing their food from materials of the abiotic environment through the photosynthesis process. The consumers: are organisms that are incapable of producing their own food and that must use (consume) other organisms for food. All animals fall into this category. There are three types of consumers: herbivores, which can eat plants; carnivores, which eat animals; and omnivores, which are both herbivores and carnivores e.g. human beings. Decomposers: also called microorganisms, include bacteria, fungi and some of invertebrates (animals without backbone). Like consumers, these organisms cannot produce their own food, instead they obtain their food energy by breaking down dead plants and animals to their basic components.
A - Structure of Ecosystem 4- Types of ecosystems Any area that contains the four environmental components: rocks, water, air and living organisms is said to be an ecosystem. Classification of ecosystems may be based on: Types of biotic components. There are, for example, forest ecosystem, grasslands ecosystem, tundra ecosystem, or desert ecosystem. Sometimes they are classified according to abiotic components – the freshwater or saltwater (marine) ecosystems, for instance. It should not be thought that the ecosystem concept applies only to natural areas. A corn field, for example, is also an ecosystem. Even a city can be considered as an ecosystem.
A - Structure of Ecosystem 5- Concept of sphere Instead of subdividing the Earth into ecosystems, we can see itas a one large ecosystem composed of four spheres: The atmosphere, that composed of the air The lithosphere, that composed of soils and rocks The hydrosphere, that composed of water The biosphere, that composed of living organisms
A - Structure of Ecosystem Each sphere is a reservoir for, or located within another sphere. For example, fish (member of the biosphere) live in water (hydrosphere), yet their bodies also contain a considerable amount of water. Thus, the biosphere here serves as a reservoir for hydro sphere. Similarly, the soil (lithosphere) contains many pores spaces; some contains air (act as reservoir for atmosphere). In addition, many types of organisms (bacteria, fungi, earthworms, etc) live entirely in soil, where plants roots are also present (soil here acts as a reservoir for biosphere).
B- Functions of Ecosystems 1- Energy movement: A basic function of ecosystems is that of energy movement. Energy moves through biotic portion of ecosystem in characteristic patterns called food chains. a- what are food chains? In a wild area, for example, an insect that is considered as an herbivore, feeds on a part of grasses (producer), so it is categorized as a primary consumer. The frog (secondary consumer) feeds on this insect. This frog is also fed by the snake (tertiary consumer), which is also being fed by the eagle. The above sequence is called a food chain in which living organisms depends upon each other for their food energy.
B- Functions of Ecosystems b- The significance of food chains It is important to note that energy movement through food chains is in one direction only. There is never energy movement from herbivores to producers. Each link is dependent upon the link directly below it for energy. Although human beings can function as herbivores or/and carnivores, the fact that they ultimately cannot obtain food energy directly .from the sun makes them dependent upon plants for food energy
B- Functions of Ecosystems c- Role of Producers The Sun is the ultimate source of more than 99% of the energy available at the Earth’s surface. Some of this energy is transformed by green plants into chemical energy (food energy) by the process of photosynthesis. Plants use light energy to combine low-energy substances from theirsurrounding environment (carbon dioxide from atmosphere and water and nutrients from the soil) to produce food of high-energy content. The food produced by photosynthesis is sugar known as glucose. The plants can change glucose (a carbohydrates) into other types of food, including proteins, fats and other carbohydrates such as starch.
B- Functions of Ecosystems 2- Cycling of materials Second major function of ecosystems is cycling of materials among four spheres (atmosphere, lithosphere, hydrosphere, and biosphere). These materials may include: carbon, oxygen, nitrogen, and water among many other components of the environment. These cycles may also composed by sub-cycle. We take carbon cycle as an example.
B- Functions of Ecosystems a- Atmosphere-biosphere carbon sub-cycle A major carbon sub-cycle is that of the interaction between atmosphere and biosphere. Carbon is present in atmosphere in the form of carbon dioxide (CO2). Green plants (producers) remove CO2 from the atmosphere and, by the process of photosynthesis, incorporate carbon into such food substances as sugar, fats, and protein. Thus, in the process some carbon is tied up in the biosphere and portion of this fixed carbon is transformed to CO2 as a product of respiration by producers and consumers. CO2 then moves from these organisms back into atmosphere. The carbon that is tied up in dead plants is also eventually returned to atmosphere by respiration of decomposers (microorganisms). The sub-cycle tends to remain in equilibrium; i.e. about as much carbon is fixed by photosynthesis of producers, as is released by respiration of all the inhabitants of the biosphere.
B- Functions of Ecosystems b- Atmosphere-hydrosphere carbon sub-cycle Another important carbon sub-cycle is that between atmosphere and hydrosphere. Large amounts of carbon dioxide are exchanged between atmosphere and the upper layer of the oceans’ waters by winds and waves. The exchange occurs in both directions through the surface of water. c- Atmosphere-lithosphere carbon sub-cycle By different rock forming processes, carbon becomes incorporated into and released from these components in form of limestone, dolomite, and carbonaceous shale.
C- Limiting Factors Theconcept of limiting factors recognize that any factor of environment complex (such as temperature, light, humidity etc) that approach or exceeds the limit of tolerance of any organism or group of organisms act as a control on them. This is justified, first by the law of minimum and then by the law of tolerance. Justus Liebig, a German ecologist (1840), stated that “ growth of plants is dependent on the amount of food stuffs, which is presented to it in minimum quantity”, or, plant growth is constrained by the most limiting factors. On the other hand, V. E. Shelford (1913) showed through his law of tolerance, that all organisms or group of organisms must live in a range of conditions between the maximum and minimum, which represent the limits of tolerance. This concept is well-illustrated by the following schematic presentation:
C- Limiting Factors Let us take algae population to see the effect of environmental factors (say phosphates in water as nutrients) on it. A minimum concentration of phosphates in water is needed before the algae will grow. As the conc. of phosphates increases, large algal population can be supported. Eventually, a phosphates conc. is reached that is optimal level for algal growth, and any increase beyond this becomes toxic to some members of the algal population. Finally, the conc. is reached a point that will be lethal to all the algae. The three critical points on the curve are: the lower tolerance limit, the optimum conc., and the upper tolerance limit.
D- Toxicity One aspect of limiting factors is the toxicity of certain substances and agents for human beings and other organisms. Toxicity is the measure of how much a chemical substance (e.g. mercury, cobalt, zinc etc) or physical agent (e.g. radiation) is required to cause discomfort, illness, or death. Toxicity is often expressed as LD50, the single dose quantity of substance that will kill 50% from the population (calculated from the population weight not from the population number). It is usually given in the unit of milligrams of poison per 1000 kilograms of animal body weight. For example, the chemical compound that causes food poison (botulism) has an LD50 of 0.0014. hence, if we feed the poison to men each weighting 100 kilogram, a dose of 0.0014 milligrams will kill five of them.
D- Toxicity The values of LD50 are affected by several factors. One variable is the means whereby the toxin enters the animal’s body. For example, the LD50 for DDT (dichlorodiphenyltrichloroethane: C14H9Cl5) in human being is 133 mg if DDT taken orally, but it should be 2510 mg (22 time less) if enters through the skin. It should be remembered that the LD50 values are based upon a single dose, not accumulative effect. Many potentially lethal substances can accumulate in certain body tissue – for example, lead in the liver.
