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| Abiotic Factors - Affecting an Organism |
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| The properties of temperature, pressure, humidity rainfall, sunshine cloud and wind in a given place and time is called weather. The average weather conditions of an area, which includes atmospheric conditions, seasons. etc. constitutes the climate. |
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| Weather refers to the daily or weakly changes, while climate refers to atmospheric conditions over longer periods, such as seasons. The main climatic regions are: |
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| 1) Tropical |
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| 2) Sub tropical |
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| 3) Temperate and |
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| 4) Arctic and Antarctic |
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 Light and radiations |
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| The radiant energy from the sun is the basic requirement for the existence of life on the earth. It affects both plants as well as animals greatly. It can be measured by a photometer. |
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| Electro magnetic spectrum |
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| Sun is the ultimate source of energy for organisms either directly or indirectly solar radiation before entering the atmosphere carries energy at a constant rate of 2 cal cm-2 min-1,
which is known as the solar constant. Solar spectrum is made up of short
wave radiation, light and long wave radiation short wave radiation are
cosmic rays, X-rays and ultraviolet rays, which have wavelengths shorter than 0.4mm. The ultraviolet radiation is absorbed by ozone layer and only a small fraction reaches the earth's surface. Ultraviolet radiations are of 3 types namely uv-C, uv-B and uv-A, uv-C radiation is lethal and uv-B is harmful to living organisms. |
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| Light has wavelengths of 0.4 to 0.7 mm and it is called photo synthetically active radiation (PAR). |
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| The infrared rays have longer wavelengths, longer than 0.74 mm. |
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| The electromagnetic spectrum of solar radiation |
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| Animals depend on light for orientation, diurnal migration and synchrony of rhythmic activities. Hence receptors for light are common to all animals, in those with better developed photoreceptors, light greatly influences behavior. |
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| Light energy varies with different media. The transparency of air and water is important in regulating the amount and quantity of light that may be available in particular habitats. |
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| When sun remains overhead, the intensity of sunlight over the earths surface will be greatest. The illumination or intensity of daylight is greatly diminished by moisture, clouds and dust in the atmosphere and also by forest vegetation. |
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| Depending upon the penetration of light, oceans are divided into |
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 euphotic zone (upto 50 meters depth) |
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 disphotic zone (upto 80 - 200 metres dpeth) |
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 aphotic zone (below 200 metres of depth) |
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| In the ocean, algae are distributed according to the length of light rays and their colours, which are best suited to absorb and to utilize. |
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| Light energy influences almost all the aspects of plant life directly or indirectly. Thus, it controls plants structure, form, shape, physiology and growth, reproduction, development and local distribution. |
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| Phenology refers to the timing of seasonal activates of plant in relation to changes in environmental condition. |
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| On the basis of light factor certain ecologists have classified plants into sciophytes or shade loving or photophobic plants, which have best growth under low intensities of light. For example beech, spruce, firs and heliophytes or photophilous plants, which have best growth in full sunlight. |
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| Examples: Pine, willows and birch. |
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 Photoperiodism |
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| Actual duration or length of the day (photoperiod) is a significant factor in the growth and flowering of a wide variety of plants. The controlling effect of photoperiod is called photoperiodicity. |
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| In this basis, the angiosperms are divided into 3 categories. They are: |
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| Long day plants |
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| Long day plants, which bloom when light duration is more than 12 hrs per day. |
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| Examples: Beet roots, carrot, oats and rye. |
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| Short day plants |
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| Short day plants, which bloom when light duration is less than 12 hrs per day. |
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| Examples: Tobacco, Dahlia, hemp and cosmos. |
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| Day neutral plants |
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| Day neutral plants, which show little response to length of day light. |
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| Examples: Cucumber, cotton and potato. |
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 Light also affects movement in some plants. This is called heliotropism or phototropism. |
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 Light promotes the growth in most of the plants by promoting the development of chlorophyll, photosynthesis, synthesis of growth hormone and stomatal opening. |
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| Colour |
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| It is the ability to differentiate different light colours and is well marked in fish, birds and primates due to the presence of cones in the retina of the eye. |
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| Pigmentation |
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| Light intensity induces chemical changes leading to pigment formation in the cells. Cave animals lack skin pigments. If they are kept out of darkness they regain skin pigmentation. Tanning occurs in human beings in response to prolonged exposures to bright sunlight. |
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| Bird Migration |
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| In the northern hemisphere, birds migrate towards the north during spring (increasing photoperiod) and towards the south during autumn (decreasing photo period). It also controls the migration of fishes like eels and salmon. |
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| Biological Rhythms |
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| A rhythm is a recurring process which is wave like in character, because maximum and minimum states appear at identical intervals of time. |
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| Operating on an 24 hours day - night cycle of the earth's rotation. The simplest form of this is the alternating periods of activity and sleep which correlate with dark and light cycle. In this cycle, some animals remain most active at sunrise and sunset times. Such animals are known as crepuscular, some animals are active during the night (nocturnal) but most animals are active during the day time and called as diurnal. |
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| The biological rhythms synchronised with the low and high tides in the sea are called as circatidal rhythms. Thus the organisms living in the inter tidal zone are alternatively submerged and exposed to air. |
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| The other rhythms are circa lunar rhythms, semi lunar rhythm and circannual rhythms. |
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| The photoperiod is known to affect breeding behavior of insects, certain birds and mammals. |
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| For example in some birds, gonads become active during summer. Starlings breed in spring (longer photoperiod) while deer breed in autumn (decreasing photoperiod). |
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| Light in some cases, for example in Salmon larvae, accelerates development where as in Mytilus larvae it retards it. |
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| Metabolism |
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| Light increases the enzymatic activities and general metabolic rate. It also increases photo - oxidation and respiration rates. Animals found in caves have low rate of metabolism. |
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| Locomotion |
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| The regulation of speed of locomotion by light is called photokinesis. For example blind larvae of mussel crab moves faster in light intensities. The orientation of animals during locomotion in response to light is called phototaxis. For example Euglena is positively phototactic while earthworms, slugs are negatively phototactic. |
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| Distribution |
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| Light affects the distribution of animals by affecting the vegetation. For example Tundra biome is characterized by insufficient light, low temperature, less vegetation so less animal life. |
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| While tropical area is characterized by high rainfall, strong light, sufficient vegetation and abundant animal life. |
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