Temperature
Temperature is a major physical environment factor which profoundly influences the vital activities of living organisms like, metabolism, growth and reproduction. The primary effect of temperature is on the stability and activities of enzymes which carry out and regulate the biochemical reactions in the cells. Temperature also affects the properties of biomembranes.
We know that there are large temperature differences over the earth to which organisms must adapt. Temperature and availability of water in an area largely determine the type of plants and animals that can grow, survive and reproduce there. Every organism has a certain range of tolerance for temperature delineated by an upper and lower lethal temperature, which vary from species to species. Thus, temperature is one of the factors that limit the geographical distributions of plants and animals. Temperature also indirectly influences the availability of water which itself is an important ecological factor. Before we describe the adaptations in organisms due to temperature stress let us discuss the temperature variation at different variation at different latitudes and examine the global picture of temperature.
Latitudinal Variations
The latitudinal variation of temperature over the earth is the result of two main variables
- i) Incoming solar radiation and
- ii) The distribution of land and water masses
What do you think could be the other factors influencing temperature?
The movement of wind and water, and the direction of slopes of hills and mountains also modify the temperature.
We know that the length of the day varies at higher latitudes. That means the incoming solar radiation varies. However, if we compute the average, every point on the earth gets the same total hours of daylight each year, an average of 12 hours per day, but not the same amount of heat. The latter depends upon the amount of radiant energy delivered/unit area/hr and this depends upon the angel of landing sunlight. You have learnt in the previous section that vertical beams are more intense than the oblique beams that land on the higher latitudes. Hence we find a gradual decrease in the amount of incident radian energy/unit area along the increasing latitudes, horizontally. Let us now take into account the outgoing infrared radiation. The absorbed solar radiation with the outgong infrared radiations at various latitudes. Notice that at lower latitudes the rate of warming by absorption of solar radiations is greater than the rate of cooling by outgoing infrared radiations. You can see that the reverse is true for higher latitudes where that rate of cooling far exceeds that rate of absorption. The poles get only 40% of the radiant energy of that delivered to the equator. The heat changes in various latitudes are also brought about by the movement of warm and cold water masses and by the exchange of cold and warm ocean currents.
Besides the difference in temperature at different latitudes, we find a great difference in temperature of places located at the same latitude. For example, Calcutta and Nagpur are at same latitude but the temperature at Calcutta is moderated by its proximity to the sea.
Can you tell the reason for this difference? This is because land and water absorb heat differently and this produces more contrast even within same latitude. The temperature of land locations/areas/stations has greater daily diurnal and seasonal temperature fluctuations.
On the basis of temperature variation three distinct heat zones are distinguished in each hemisphere. The hot torrid zones are near the equator, moderate or temperate zones are in the middle, the cold zones are at the poles. Each zone is characterized by typical plants and animal population found in the area.
Altitudinal Variations
We know that temperature decreases with increasing altitude. This is mainly due to convection currents in the troposphere—the lowermost region of the earth’s atmosphere. As you know that the surface of the earth gets heated up on account of solar radiation and it also heats the air which is in immediate contact with the surface. This gives rise to convection currents which continuously transport air from lower region to higher ones and vice versa. When the air from sea level rises to the upper atmosphere of lower pressure it expands i.e. the volume increases. While expanding, the molecules push aside the neighboring molecules. In doing so the molecules lose their kinetic energy and it is this energy loss which is reflected in a decrease in temperature. The same amount of energy is gained by the gas molecules when they are compressed while descending and thus the temperature increases. Such a change in temperature where no addition or subtraction of heat takes place between the system and the surrounding is called Adiabatic change.
Global Temperature
This isothermal map of the world shows the pattern of annual mean temperature on the earth. We find that the continental land masses have relatively broad temperature ranges. The southern hemisphere has less temperature variation than northern hemisphere because of more water mass in the former.
Temperature Stress
We have mentioned earlier that temperature along with water is an important influence on the geographical distribution and range of organisms. Every organism is restricted to a definite range of temperature, which may be quite dissimilar for different species. In other words, there is an upper and lower lethal temperature above and below which a given organism cannot perform the normal life activities and may suffer irreversible damage or may die.
The lower minimum and upper maximum lethal temperatures and optimum temperature are termed as cardinal temperatures and these vary from species to species. Some organisms have narrow limits of tolerance to high or low temperature while some tolerate a wide of temperature. Besides tolerance temperature range, organisms have different optimum temperature for grown, development and reproduction i.e. at the specific temperature; they perform specific function with maximum efficiency. The characteristics of terrestrial biotic communities are determined by the temperature and rainfall. Thus we find that different biomes distributed throughout the globe in accordance with the temperature which in turn is influenced by other factors. Since the temperature varies with altitudes as well as with latitude the variation in the vegetation from equator to higher latitudes is somewhat similar to that from planes to higher altitude.
The biomes in different regions have been named according to the kind of vegetation they support such as coniferous forest, deciduous forest, grassland, savanna etc. The desert and the tundra’s experience extremes of temperature. The organisms of such biomes survive at the limits of the temperature tolerance. We will now describe the temperature stress on the organisms living in these biomes.
Extremes of Heat and Cold
Desserts are regions of aridity with rainfall of less than 20cm per year and the soil, though fertile, is too porous to retain any water. In summer there is great diurnal variation in temperature from 40°C during the day to below 15°C the same night. The light intensity reaches higher peak due to lack of cloud cover. The true deserts are Sahara in Africa and Great desert in Australia where the annual rainfall is less than 2cm. Plants and animals which survive under such conditions have special features that allow them to withstand high temperature and lack of water.
The other extreme of temperature is seen in the arctic and alpine tundras. The word tundra means bare mountain tops in Finish. Tundras are frozen most of the year round and are very similar to deserts as the rainfall here is also about 20cm per year. But during the short summers, water is plentiful as the topmost layer of ice melts. The ground gets covered with short grass and patches of moss and lichens. Beneath the thawed layer of 3-5 cm, the soil called permafrost remains permanently frozen. Since the roots cannot penetrate deep in the soil trees do not grow here.
Adaptations
Every organism can live and reproduce within a certain range of climatic conditions. Organisms that live in hot or cold environments have behavioral and physiological features that enable them to survive extremes of temperature. As a survival strategy organisms either tolerate these extreme conditions or evolve ways of avoiding them altogether. For example, plants cope up with high temperatures in the desert by developing a thick layer of cuticle, succulence i.e., water storage tissue in the leaves and stems. In many cacti the stem is green and carry out the functions of leaf and makes food by photosynthesis. These plants also have physiological adaptations. The stomata remain closed during the day to prevent the loss of water due to transpiration. CO2 diffusion cannot occur in stomata closure in the day. To carry on photosynthesis these plants have evolved special physiological adaptation. During night when their stomata open the trap CO2 and store it in the form of four carbon acid. The CO2 trapped in the leaf at night is subsequently released during the day and used in photosynthesis. This type of metabolism is called crusstulacean acid metabolism. The details will be covered in physiology course. Comparative study of plant responses to constant favorable temperature region verses alternating temperature show that seed germination, vegetative growth or fruit production is best under the letter conditions. This shows that plants are adjusted to natural rhythmic diurnal cycle temperature changes. The regulation of plant responses to periodic thermal changes is called thermoperiodism.
Let us now see what short of strategy animals adopt to cope with temperature stress. Animals have advantage over plants as they can move from one place to another. They cope with temperature stress by regulating their internal and external environment by physiological and behavioral means. Probably you know that birds and animals are capable of maintaining constant body temperature. They do so by using the energy of metabolism released during cellular respiration. They are called homeotherms or endotherms because they control constant body temperature by internal means. The body fat, feather, fur or hair etc. helps to retain this heat. Some animals use a number of behavioral mechanisms to regulate their body temperature. This type of regulation is termed as behavioral Thermo regulation. For instance, they can move to shady areas or take a dip in water during the hot period of the day. The desert animals such as snakes, lizards, scorpions, and rats are mostly nocturnal i.e. they remain hiding during the day to avoid the scorching sun and roam in search of food at night or early in the morning when temperature is generally low. Reptiles like lizard And snake are considered cold blooded because they cannot control their body temperature. However, experiments on these animals have revealed that they can also control their body temperature effectively by behavioral means. Reptiles move in and out of burrow in such a way that their body temperature remains fairly constant. It has been found that in spite of great temperature fluctuation of environment, desert lizard can maintain is body temperature between 31°C to 39°C. These animals are called poikilotherms or exotherms because they control body temperature to a considerable range by behavioral means. Animals also regulate their body temperature by losing excess heat by sweating and evaporation.
In colder climates animals have adaptations to gather heat. Birds warm up their body by increasing the muscular activity in their wings by shivering. The chameleon changes their colour to back, thus increasing their heat absorbing capacity, the ectotherms black in the sun. The animals also manipulate by exposing a certain portion of their body so as to acquire desirable heat.
Another way to avoid adverse climate conditions is through migration. Probably you know that birds of northem or colder regions migrate to warmer southern regions during the winter season. Fishes also swim long distance until they reach water masses which have suitable temperature for their survival.
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