Here is your essay on the Role of Temperature as an Ecology Factor !
Temperature is one of the essential and obvious changeable environmental factors. It varies not only between climatic regions, but also in the temporal changes of all habitats. It is one of the most extensively studied environmental factors.
It penetrates into every region of the biosphere and profoundly influences all forms of life by exerting its action through increasing and decreasing some of the vital activities of organism such as metabolism, behaviour, reproduction, embryonic development and death.
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Temperature has a universal influence and acts as a limiting factor for the growth and distribution of plants and animals. The interaction of temperature with certain other abiotic environmental factors such as humidity, air, etc., cause into many other climatic changes which influence the living organisms in one way or another.
Nature of Temperature:
Temperature is a measure of the intensity of heat in terms of a standarized unit, and is commonly expressed as degrees either in the Fahrenheit or Celsius scale (centigrade). Heat is a form of energy and is called thermal energy. The same influx of energy on which photosynthesis depends is also the source of thermal energy that characterizes the physical environment of life.
Thermal energy is exchanged between animal and environment by radiation, conduction, convection and evoporation (Fig. 11.4). These four basic modes of heat transfer occur within the organism, in the interface between the organism and its environment (a thermal boundary layer), and between the environment and the thermal surface of an organism.
Radiant energy exchange:
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Radiant energy exchange or radiation occurs between two surfaces as each surface emits energy at wavelengths that are dependent on the temperature of the emitting surface. Radiant energy travels through those media that are transparent, neither reflecting nor absorbing the radiation. A complete vacuum presents no obstructions to radiant energy exchange. The radiant heat exchange can be understood well by discussing its following basic characteristics:
1. The electromagnetic spectrum:
The electromagnetic spectrum includes wavelengths as long as hundreds of kilometers and as short as 1 × 10-10 cm (0.0000000001 cm). Between these two extremes there are, in order of decreasing wavelength, the radio waves that are received by standard radio sets, the short waves, the infrared portion (perceived as heat), visible light, ultraviolet rays, X-rays and gamma rays (Fig. 115).
The solar radiant energy that is important in the maintenance of homeothermy and warmth of biosphere includes the wavelengths in the visible portion (i.e., light) and the longer wavelengths in the infrared portion of the electromagnetic spectrum.
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Further, both the amount of radiations from an object and wavelengths emitted are functions of temperature of the object. The Stefan-Boltzmann law states that the amount of radiation emitted from a black body is directly proportional to the fourth p6wer of the absolute temperature (°K) of the object. A black body is an object that absorbs all the radiant energy that reaches its surface.
If a surface is not a black body, the amount of energy that can be absorbed is expressed as a coefficient ranging from zero to one. The absorption coefficient for long-wave radiation is also equal to the emissivity of that surface, since it is equally as good an emitter of long wave radiation as it is an absorber. This relationship can be expressed with equation (11-1).
Qr = ɛσT48 … (11.1)
Where:
Qr = radiant energy emitted in kcal m-2 hr-1
ɛ = emissivity (range from zero to one)
σ = Stefan-Boltzmann constant=4.93 × 10-8 kcal m-2 hr-1
T8=surface temperature of the object in °K
The Wien displacement law states that the wavelength (λ) of maximum intensity that is emitted from the surface of a black body is inversely proportional to the absolute temperature of the body (equation 11-2).
λmax (µ)=2897 T-1 …(11-2)
Thus, a very hot surface emits shorter wavelengths, while a cooler surface emits longer wavelengths.
2. Solar radiation:
The wavelength of maximum emission from the sun is 0.5µ, which is in the visible portion of the spectrum (Sellers 1965). Sellers points out that 99% of the sun’s radiation is the wavelength range of 0.15µ to 4.0µ, including 9% in the ultraviolet (<0.4µ), 45% in the visible (0.4µ to 0.74µ) and 46% in the infrared (>0.74µ).
The amount of solar energy reaching a surface perpendicular to the rays of sun at the outer limits of the earth’s atmosphere is called the solar constant. The value of solar constant is 2.00 calories per square centimeter per minute. Of this quantity about 1.00 cal/cm-2/min1 reaches the earth’s surface.
The amount of solar radiation that reaches a place depends on the moisture, concentration of ozone and dust particles in the atmosphere and altitude and location on the planet. On a clear day, a high percentage of the solar radiation is transmitted through the atmosphere with a completely overcast sky; no direct solar radiation penetrates the cloud cover.
Likewise, as the altitude above sea level increases, the amounts of water vapour, dust, air molecules and carbon dioxide in the path of the sunlight diminish. Thus at higher altitudes solar radiation is more intense.
The Solar radiation absorbed as short wave energy by the earth and reradiated as long wave energy is responsible for heating the earth’s atmosphere. Long wave radiation, prevented from escaping into space by the ozone layer of the stratosphere, warms the upper atmosphere. Thus the atmosphere receives most of its heat directly from the earth and indirectly from the sun.
Solar radiation also affects climate of earth. Sunlight does not strike the earth uniformly. Because of the earth’s shape and tilt on its axis, the sun’s rays strike it more directly on the equatorial than on the Polar Regions. The lower latitudes are heated far more than the polar ones. Air heated at the equatorial regions rise until it eventually reaches the stratosphere, where the temperature no longer decreases with altitude.
There the air, whose temperature is the same or lowers than that of stratosphere, is blocked from any further upward movement. With more air rising, the air mass is forced to spread out north and south toward the poles. As the air masses approach the poles they cool, become heavier and sink over the arctic regions. This heavier cold air then flows towards the equator, displacing the warm air rising over the tropics.
3. Solar radiation and organisms:
There are three possible pathways for radiant energy to take once it reaches a plant or animal (Moen 1973). It may be reflected from the surface, it may be absorbed by the surface, or it may be transmitted through the material (Fig 11.7).
Energy that is reflected from the surface is of no thermal benefit to an animal or plant. Reflected energy in the visible portion of the spectrum is detected as shades of gray or ash colour, depending on the receptors of the organism detecting the light energy. Transmitted solar energy is of no value to an animal, and all animals except the smallest protozoans are essentially opaque.
The leaves of plants, however, transmit some solar energy. Absorbed energy becomes a part of the thermal and physiological regime of an organism, and the quantity and distribution of absorbed radiant energy is of interest to the physiologist and the ecologist.
For an instance the amount of energy absorbed by the hair surface of a mammal depends on the spectral characteristics of the hair and the angle at which the solar energy strikes the surface.
Riemerschmid and Elder (1945) measured absorption coefficients for cattle and found that white coats absorb less and reflect more solar energy; black coats absorb the most solar energy. Further, the greatest amount of energy is absorbed when the solar radiation strikes perpendicular to the surface and no absorption takes place when the rays are parallel to the surface.
Inclination of hair, the smoothness or curliness of the coat, and seasonal changes in the characteristics of the coat has little effect on the absorptive. The distribution of solar radiation on the surface of an animal is called the solar radiation profile (Moen, 1973).
Moreover, the effect of direct solar radiation on the radiant temperature over the entire surface of a plant or animal is not uniform because both the colour of this surface and the angle of the rays striking the surface are important in determining just how much solar energy is absorbed.
For example, a deer bedded in the sun (Fig. 11.8) might have 40 per cent of its surface exposed to direct solar radiation, 80 per cent exposed to indirect solar radiation, and 80 per cent exposed to infra-red radiation. These differences result in different radiant temperatures, but they act in combination with the distribution of tissue metabolism beneath- the hair surface as well as with the distribution of blood.
The radiant temperature distribution of an animal exposed to only infra-red radiation at night is much simpler since the hair surface is almost a black body and infra-red energy is much more uniformly distributed in the environment than is solar energy.
In fact, radiant temperature and air temperature are much more similar to plants than to animals because only a small amount of metabolic heat is released inside the homeo- thermic animals, however, there is heat flow from the many exothermic reactions inside the animal to the outside through a layer of insulating hair or feathers.
Thus the surface temperature of an animal is dependent on blood flow beneath the skin, local tissue metabolism and on the quality of the insulating pelage. In a plant, the surface temperature is dependent primarily on the interactions between thermal parameters alone, with virtually no input from metabolic reactions within the leaf.
Convection:
Thermal energy may be removed from the surface of an object by fluid or air flowing over the surface. This process is called convection. Two types of convection occur convection or natural convection and forced convection.
Free convection occurs when temperature differences in the boundary layer of air surrounding an object cause a movement of the air in response to changes in air density. Forced convection occurs when external pressure differences cause wind to blow past the object.
Convective heat loss:
The amount of heat that is removed from an object by convection is a function of the factors expressed by the following equation (11-3):
Qo = hc At (Ts – To) ………. (11-3)
Where Qc = calories of heat transferred by convection
hc = Convections coefficient
A = area
t = time
Ts = temperature of the surface of the convector
To = temperature of the air (fluid).
Following two kinds of conclusions have been drawn from experiments which were performed to determine convective heat loss from cylinders of different diameters:
(i) Small cylinders are more efficient convectors than large ones, and (ii) low air velocities have a greater relative effect on convective heat loss than do high air velocities. These conclusions are of ecological significance because animals are, geometrically, collections of imperfect cylinders and cones.
Cylindrical hairs are very efficient convectors because of their very small diameters. Thus convective forces can be very effective in removing the radiant heat energy absorbed by the hairs and the heat that is conducted along the shafts of the hairs from the skin surface through the coat.
The relatively greater effect of the lower wind velocities is of ecological interest because so many organisms live in lower vegetation-filled zone marked by low but highly variable wind velocities owing to the effect of the vegetation on the wind flow.
Conduction:
The transfer of heat by conduction results from the exchange of energy when oscillating molecules collide, with a high rate of exchange during more rapid oscillations. Energy dissipation by conduction is from the higher temperatures resulting from more rapidly oscillating molecules to the lower temperatures.
The perfect conductor permits the complete movement of heat energy through the conducting medium, and the perfect insulator prevents all movement of heat energy through the medium. The thermal conductivity (k) of a material is an expression of the rate of heat flow by conduction through the material under a specified set of conditions.
The amount of heat flow by conduction is expressed by following equation (11.4).
Qk = k A t (T1 – T2) / d
Where,
Qk = calories of heat transferred by conduction
K = thermal conduction coefficient
A = area
t=time
T1 = temperature of first surface
T2 = temperature of second surface
d = distance between the surfaces.
Conduction through the hair layer includes heat flow through the hair shafts themselves and through the air trapped between the hairs. Since air is a better insulator than hair, the important function of hair is the stabilization of the trapped air. This trapped air is a more important thermal barrier than the hair shafts themselves. The same relationship is true for feather surfaces.
Heat loss by evaporation:
Thermal energy or beat is lost from the surface of a plant or an animal by evaporation because energy is absorbed as liquid water is changed to a gaseous state. At 100°C the heat of vaporization is 540 keal per gram and at 0°C it is 595 kcal per gram. There is a linear relationship between air temperature and the heat of vaporization of water. Thus, if one knows the amount of water evaporated from an organism, the amount of energy removed by vaporization can be determined:
Evaporative heat loss comes from two sources: evaporation from the skin surface in the form of perspiration from the animal and evaporation from the lungs and lining of the nasal passages.
Heat budget:
The temperature at the earth’s surface is governed by the brightness of the sun; the constancy of brightness of sun has remained virtually unchanged for about 3 billion years. In fact, average terrestrial temperatures now probably do not differ rapidly from those at the earth’s beginning (Sch- warzs Hild, 1967).
The total amount of heat entering the biosphere from the sun must balance the amount lost per unit time if temperatures are to remain unchanged, since the flow of geothermal heat from the interior of the earth is small by comparison and probably has been negligible for at least the past 500 million years.
The estimate of this energy flow is referred to as the heat budget (Vern berg and Vern berg, 1970). Heat budget has been projected for the total surface of earth as well as for special environments.