Light is the most important and indispensable physicochemical, abiotic ecological factor without which life cannot exist. Organisms get light from the Sun, Moon, stars, lightning, volcanoes and bioluminescent organisms. Among this light energy from the Sun is the most important in nearly all ecosystems. It is the energy that is used by green plants (which contain chlorophyll) during the process of photosynthesis; a process during which plants manufacture organic substances by combining inorganic substances.
The energy from the sun comprises of short, high-energy radiations to long, low energy radiations. The amount of energy in the sunrays just before entering the atmosphere is about 2 cal/cm2/min. It is called solar constant. As the sunrays travel through the atmosphere a large amount of energy is absorbed.
Sunlight is formed of cosmic rays, gamma rays, X rays, Ultraviolet rays, visible light, infrared rays, radio waves, etc. (Fig. 3). Of these the ultraviolet rays, visible light and infrared rays are biologically significant. Ultraviolet rays have a wavelength of 100 nm-390 nm. They are further classified into three categories as shown in Table 1. UV-C and UV-B are absorbed by the ozone layers in the atmosphere.
Visible light is of greatest importance to plants because it is necessary for photosynthesis. It falls between the ranges of 340 nm-700 nm. This part of the spectrum is also called the photosynthetically active radiation or PAR. Factors such as quality of light, intensity of light and the length of the light period (day length) play an important part in an ecosystem.
Quality of Light (Wavelength or Colour):
Plants absorb blue and red light during photosynthesis. In terrestrial ecosystems, the quality of light does not change much. In aquatic ecosystems, the quality of light can be a limiting factor. Both blue and red light are absorbed and as a result they do not penetrate deeply into the water. To compensate for this, some algae have additional pigments, which are able to absorb other colours as well.
Light Intensity (‘Strength’ of Light):
The intensity of light that reaches the earth varies according to the latitude and season of the year. The southern hemisphere receives less than 12 hours of sunlight during the period between 21st March and 23rd of September, but receives more than 12 hours of sunlight during the following six months.
In frogs and lizards, bright light makes the skin colour light while dim light makes the skin colour darker. Human skin responds to bright light by tanning.
Day Length (Length of the Light Period):
Certain plants flower only during certain times of the year. One of the reasons for this is that these plants are able to ‘measure’ the length of the night (dark periods). However, it was thought earlier that it is the day length (light periods) to which plants reacted and this phenomenon was termed photoperiodism. Photoperiodism can be defined as the relative lengths of daylight and darkness that affect the physiology and behaviour of an organism.
Accordingly plants are classified as follows:
a. Short-Day Plants:
These plants flower only if they experience nights, which are longer than a certain critical length. Chrysanthemums (Chrysanthemum sp.), the Poinsettias (Euphorbia pulcherrima) and thorn apple (Datura stramonium) are examples of short- day plants.
b. Long-Day Plants:
These plants flower if they experience nights, which are shorter than a certain critical length, Spinach, wheat, barley and radish are examples of long-day plants.
c. Day-Neutral Plants:
The flowering of day- neutral plants is not influenced by night length. The tomato and the maize plant (Zea mays) are examples of day-neutral plants.
Light requirements of plants differ and as a result distinct layers, or stratification, can be observed in an ecosystem. Plants which grow well in bright sunlight are called heliophytes (Greek helios, Sun) and plants which grow well in shady conditions are known as sciophytes (Greek skia, shade).
Zonation of Light in Aquatic Ecosystems (Fig. 4):
About 10% of light falling on water is reflected back to the atmosphere. The remaining 90% penetrate into the water body. On the basis of penetration of light, the water column of the ocean is divided into three zones supper euphotic zone, a middle disphotic zone and a lower aphotic zone.
The zonation is also categorised as follows:
a. Littoral Zone:
This zone includes the shallow coastal line where light is available up to the bottom. Rooted vegetation occurs along this zone.
b. Limnetic Zone:
This zone includes the water body to a depth to which light can penetrate. This includes the euphotic zone where abundant light penetration occurs and the disphotic zone where light is received but not sufficient for photosynthesis.
c. Profundal Zone:
This zone includes the region where there is no light penetration and no producers are present in this region.
d. Benthic Zone:
The benthic zone includes the bottom of the ocean.
Biological Effects of Light:
a. On Metabolism:
High intensity of light increases metabolic activity in animals by increasing enzyme activity.
b. On Pigmentation:
Light induces photochemical reactions in the formation of colour pigments called melanophores. Animals living in cave, bottom of the ocean do not possess colour.
c. Protective Coloration:
Animals develop colour patterns to conceal themselves from predators to bend with the surroundings. For example, the leaf insect, Phyllium is green in colour.
d. Colour Change in Animals:
The Chamaeleon is able to change its colour according to its background. This happens because of the distribution of the melanophores depending on the light entering the eye.
Light enables organisms to see objects in the environment where it is found. Animals possess specific organs to ‘see’ like the eyespots in protozoa, compound eyes in insects and crustaceans, eyes in vertebrates etc. Animals that live in habitats where there is dim light have large eyes that are powerful as in the owls and loris. In animals that live in habitat where there is no light, the eyes are reduced.
Animals are classified into the following categories according to the influence of light on reproduction:
i. Long-Day Animals:
This group of animals are sexually active when the days are long, e.g. birds.
ii. Short-Day Animals:
This group are sexually active when the days are short, e.g. sheep, deer, goats.
iii. Day-Neutral Animals:
In this group reproduction is influenced by light, e.g. man, cow.
g. Diurnal Migration:
In the oceans, planktons move to the surface in the early morning and evenings and move to the deeper parts of the ocean when there is high intensity. This movement is called diurnal migration.
h. Circadian Rhythm:
The daily rhythm in synchrony with the rotation of the earth is called circadian rhythm. This is endogenous, i.e. initiated by internal factors and is due to a biological clock present in organism. For example, many plants show rhythm of their leaves for sleep. They close or droop during night time and open at daytime. Sleeping and waking in man follow circadian rhythm.
Adaptations of Plants to Changing Light Conditions:
Light requirements of plants differ and as a result distinct layers or stratification can be observed in an ecosystem. Plants which grow well in bright sunlight are called heliophytes (Greek helios, sun) and plants which grow well in shady conditions are known as sciophytes (Greek skia, shade).
Heliophytes have a high rate of respiration and are adapted to high light intensities, while sciophytes have low rate of photosynthesis, respiration, metabolism and growth. The morphological features of heliophytes and sciophytes are summarised in Table 2.
Temperature is an ecological abiotic factor. It is a form of energy and is called the thermal energy. It penetrates into each and every region of the biosphere and affects all forms of life. It influences the various stages of life activities such as growth, metabolism, reproduction, movement, distribution, behaviour, death, etc.
Temperature is usually measured in Fahrenheit or Centigrade. The biosphere obtains its thermal energy from the Sun in the form of solar radiation. It is a variable factor. It varies from place-to-place and time-to-time. It is high in the day and at night it is low. It is high at the sea level and low at high altitudes. It is high at the equator and low in the Polar Regions. It is more in the terrestrial habitat and low in the aquatic habitat. The maximum temperature recorded on land is 85°C as in the desert and the lowest temperature is about – 70°C as in Siberia.
The temperature is high during daytime and low at night. This is called diurnal variation. The temperature on land is high at the sea level, but low at high altitudes. Approximately, an increase in altitude of 150 m results in a decrease in 1°C temperature. On land, maximum temperature is found along the equator. It gradually decreases towards the poles. Temperature varies according to the season. The temperature reaches its maximum during summer, while it is minimal during winter.
Temperature fluctuation in aquatic habitat is less than that of terrestrial habitat.
In lakes and ponds a gradual decrease in temperature from the surface to the bottom is seen. This leads to different layers of water with different temperatures. The arrangement of different layers based on temperature differences is called thermal stratification.
Two types of stratification are observed:
a. Summer stratification and
b. Winter stratification.
a. Summer Stratification:
During summer there are three distinct layers as shown in Fig. 5.
i. Upper Layer or Epilimnion:
The epilimnion is warm and the temperature fluctuates with the temperature of the atmosphere.
ii. Lower Layer or Hypolimnion:
The bottom layer is the hypolimnion. The temperature is about 5-7°C.
iii. Middle Layer or Thermocline or Metalimnion:
The thermocline is characterised by a gradation of temperature from top (at about 21°C) to bottom (at about 7°C). This zone is also called the transition zone.
b. Winter Stratification:
During winter only two layers are seen, an upper layer of ice and a lower layer of water column which is at 4°C. (Fig. 5).
Biological Effects of Temperature:
a. Eurythermal and Stenothermal Organisms:
Organisms that can tolerate wide range of temperature fluctuations are called eurythermal organisms, e.g. man, lizard, amphibians. Those that cannot tolerate wide range of temperature fluctuations are called stenothermal organisms, e.g. corals, snails
b. Poikilothermic and Homeothermic Animals:
Animals in which the body temperature changes according to the fluctuations in the environmental temperature are called poikilothermic or cold-blooded animals or ectotherms. During cold, the body temperature also drops. For example, all animals except birds and mammals.
In birds and mammals, the body temperature remains constant and is not dependent on environment temperature. These animals are called homeotherms or warm- blooded or endotherms. When the environment temperature drops the animal maintains its temperature by metabolic activities.
Hibernation, aestivation, migration are some behavioural adaptations of animals.
Effect of Temperature on Growth and Development:
Temperature affects growth and development of animals. For example, the oyster, Ostraea virginica grows to 1.4 mm when it is reared at 10°C, but when reared at 20°C it grows to 10.3 mm. Similarly, the eggs of the mackerel fish does not develop below 8°C and above 25°C. Low temperature prevents metamorphosis in salamanders and makes the animal neotenous.
Effect of Temperature on Morphology:
The morphological characters of organisms are altered by temperature. Temperature influences the size of animals and the relative proportions of the parts of the body. Three rules have been put forth to understand how the temperature influences various characteristic features.
a. Bergman’s Rule:
The mammals in colder areas are larger is size than in warmer climates. This is called the Bergman’s rule. For example, the penguins found in Antarctica attain a body length of 100- 200cm, whereas the penguins of equatorial Galapagos Islands are about 49cm long.
b. Allen’s Rule:
According to Allen’s rule, extremities of the mammals, like the tail, snout, ears and legs are relatively shorter in colder regions than in warmer regions. In the Arctic rabbit the ears are shorter, while in the Californian rabbit, the ears are longer.
The explanation in both the cases is that endothermic organisms in colder climates should have smaller surface area relative to volume across which they lose heat. Allen’s rule has widespread applicability when compared to Bergman’s rule because of number of factors that affect body size, though it is true at an intra-specific level.
c. Gloger’s Rule:
According to Gloger’s rule the animals in the tropic are darker and heavily pigmented than their counterparts of the colder and dry regions.
Effect of Temperature on Distribution:
Temperature is a limiting factor on the distribution of animals. The distribution of warm-blooded animals is not affected by temperature. But cold-blooded animals are abundant in tropical and temperate regions, and their number rapidly diminishes towards the poles.
Effect of Temperature on Plants:
a. The opening of the flowers of various plants during the day and night is often due to temperature difference between the day and night.
b. The seed of some plants (biennials) normally germinate in the spring or summer. These seeds require a cold treatment of winter. This is called vernalisation. Vernalisation can be induced in seeds artificially. This adaptation ensures that seeds do not germinate during autumn, but only after winter, when the seedlings have better chances to survive.
c. Deciduous trees lose their leaves in winter and enter into a state of dormancy, where the buds are covered for protection against the cold.
d. In the desert due to great temperature variation between day and night organisms exhibit distinct periods of activity, e.g. many cacti flower at night are pollinated by nocturnal insects. Cactus is among the most drought-resistant plants on the planet.
In a cactus:
i. Leaves are modified into spines. These spines protect the plant from animals, shade it from the Sun and also collect moisture. This also reduces transpiration.
ii. Extensive shallow root systems that are spread out just below the surface to allow the plant to absorb water immediately as it rains.
iii. Succulent stems have the ability to store water. This enables the cacti to survive in dry climate and can survive years of drought on the water collected from a single rainfall.
iv. Waxy skin to seal in moisture.
v. Cacti depend on chlorophyll in the outer tissue of stems to conduct photosynthesis for the manufacture of food.
vi. Cacti close their stomata during the day and open them at night to reduce transpiration. These plants exhibit the CAM pathway of photosynthesis.
Many other desert trees and shrubs have also adapted by eliminating leaves – replacing them with thorns, not spines, or by greatly reducing leaf size to eliminate transpiration. Many xerophytes may accumulate proline in the cells of its leaves to maintain osmotic and water potential. Chaperonins, the heat shock proteins provide physiological adaptations to plants to high temperatures. These proteins maintain the structures and avoid denaturation of other proteins.
e. Plants living in cold climates can tolerate frost conditions. When the temperature drops the plant becomes dormant and exhibits slow rate of photosynthesis and respiration. Antifreeze proteins are found in some plants which avoid chilling and frost damage by increasing their sugars and alcohols to lower the freezing point of cell fluids. This causes super cooling of the cell sap for short periods of time without causing freezing.
Structural Adaptations in a Camel;
In hot deserts, temperature is very high. To escape from the heat desert animals have the different adaptations for resistance to heat. This can be understood from the adaptations of a camel: All desert dwellers have adapted to conserve water, food and energy. The camel is one of the best survivors in the desert and it is rightly called the ‘ship of the desert’ because it is adapted very well to the conditions of the desert.
The adaptations in the camel are briefly described below:
a. The camel can store fat in their hump, which is used as energy source whenever food is scarce.
b. Camels have long legs to keep the heat away from the body.
c. Camels have long eye lashes and small ears with lots of hair. They can also close their nostrils. These adaptive features keep them from getting sand in their eyes, ears and nose during sandstorms.
d. The body temperature of the camel ranges from 34°C (93°F) at night up to 41°C (106°F) during the day. Only above this threshold they start to sweat. This allows them to preserve about 5 litres of water a day.
e. A camel can manage for up to 2 weeks without water, and can drink 200 litres at the same time.
f. They have flat padded feet, which are perfect for walking on loose, hot sand.
g. The red blood cells of the camel are oval in shape, unlike those of other animals, which are circular. This facilitates their movement in a dehydrated state.
h. The thick coat of the camel reflects sunlight. It also insulates them from the intense heat that radiates from hot desert sand. The long legs also help by keeping the body further away from the sand. Thick fur and under wool provide warmth during cold desert nights and some insulation against daytime heat.
i. In camels, the body temperature is labile. During day the body temperature rises to 40.6°C, while at night it drops to 33.8°C.
Regular change in temperature at specific intervals of time is called thermoperiodicity.
It is of two types:
a. Diurnal Thermoperiodicity:
The change in temperature in a 24-hour period is called diurnal thermoperiodicity. Animal’s active during the day is called diurnal and those active during the night is called nocturnal.
b. Seasonal Thermoperiodicity:
The variation in temperature in the different seasons of the year is called seasonal thermoperiodicity. It controls important events such as seed germination, flowering, fruiting, leaf shedding, etc. in plants. It also affects growth, development, morphology and coloration in animals.
Water covers 70% of the earth’s surface and is found as vapour in the atmosphere and in the soil as soil water. 97% of the water is found in the oceans and 3% is found as freshwater. Approximately 70% of freshwater is found as ice caps and glaciers, 20% as underground water while the remaining is found in lakes, streams and rivers. Water is essential for life and all organisms depend on it to survive in especially desert areas.
The Water Cycle in Nature:
Water cycles through the biosphere and is constantly exchanged between the physical and the biotic environment. The circulation of water that does not involve living organism is the global water cycle and that which involves living systems is the biological water cycle.
The water or hydrologic cycle is depicted in Fig. 6. Water evaporates from the oceans and bodies of freshwater when the sun’s rays falls on it. Vapourised freshwater rises into the atmosphere, forms clouds, cools and falls as rain over the oceans and the land.
When it rains, some of the water sinks or percolates into the ground and saturates the Earth to a certain level. The top of the saturation level is called the ground water table or simply the water table. Ground water is also sometimes located in a porous layer, called an aquifer, which lies between two sloping layers of impervious rock.
Ground water comes back to the surface naturally as springs or mechanically by pumps or making wells. Water also evaporates from these places to the atmosphere. This completes the global water cycle.
Organisms also use water and become part of the water cycle. Plants absorb water from the soil and return it back to the atmosphere by respiration and transpiration. Animals drink water from water bodies and by eating plants and return water back to the environment by respiration and excretion.
Death and decay of organisms also add water to the physical environment. Water returned to the physical environment forms clouds that come down as rain for being utilised by the organisms. This comprises the biological water cycle.
Adaptations of Plants in Water:
Water constitutes the hydrosphere and includes both fresh and seawater. Aquatic plants are called hydrophytes. These plants possess specialised parenchyma called aerenchyma that possesses air filled spaces in the leaves and stem. This enables the plants to float.
The different types of hydrophytes are summarised in Table 3:
Adaptations of Animals in Aquatic Habitat:
A number of animals live in the aquatic medium, i.e. water. There are animals that are found exclusively in the fresh water, while there are some that are found living in the marine environment; there are some that are capable of living in both fresh and marine water. A few examples of animals that are aquatic are vertebrates like fish, mammals (whales, dolphins, seals, sea lions, etc.), invertebrates like starfish, prawns, lobsters, octopus, oysters, etc.
Adaptations in animals living in water are called aquatic adaptations and organisms living in water are called aquatic organisms. Aquatic organisms found on the surface of the water are called pelagic organisms for which they possess special adaptations. Similarly organisms living in the deep sea, called benthic animals are adapted to live in such conditions.
Adaptations of fish are briefly discussed below:
a. Streamlined Body:
The body of fish is streamlined or boat shaped and therefore it offers little resistance to swimming.
The fins are outgrowths of the body. There are different types of fins such as the pectoral fins, pelvic fins, dorsal fins, anal fins and the caudal fins. These help in locomotion in water. The pectoral, pelvic and dorsal fins act as balancers while the caudal fins act as rudder to change directions.
Fish possess gills that enable exchange of gases between the blood and the surrounding water.
d. Lateral Line Sense Organs:
Lateral line sense organs are canals that extend the entire length of the body. These are filled with mucous and water and contain specialised organs to detect temperature, pressure and mechanical disturbances in water. They can help in echolocation of objects like food and prey.
e. Swim Bladder:
The swim or air bladder in some fish helps in swimming or serves as a hydrostatic organ, or as a sense organ or even as a sound producing organ. The fish can fill or empty the bladder and the fish can float or sink lower in water.
f. Scales and Mucus Glands:
Scales protect the body of the fish. The mucous glands secrete mucous and prevent the diffusion of water through the skin.
Air or atmosphere is the gaseous envelope that surrounds the lithosphere and hydrosphere. The atmosphere is a mixture of gases. Nitrogen makes up four-fifths of it and oxygen makes up a little more than one-fifth. Small quantities of other gases like argon, neon, helium, krypton, xenon, carbon dioxide, hydrogen and ozone are also found.
The most important gases used by plants and animals are oxygen, carbon dioxide and nitrogen.
Oxygen is used by all living organisms during respiration.
b. Carbon Dioxide:
Carbon dioxide is used by green plants during photo-synthesis.
Nitrogen is made available to the plants by certain bacteria and through the action of lightning.
Layers of the Atmosphere:
The atmosphere is made of five or more distinct layers that differ in density, temperature, composition and properties (Fig. 7).
a. Troposphere – 0-10 kms
b. Stratosphere – 10-40 kms
c. Mesosphere – 40-70 kms
d. Thermosphere – 70-400 kms
e. Exosphere – 400 kms and beyond
a. Troposphere (0-10 kms):
The troposphere is the layer with which organisms have intimate contact and it is the seat of weather and climate. It is the densest part of the atmosphere and air pressure drops with increasing altitude. It contains more water vapour and carbon dioxide than any other layer. These two gases affect the heat balance of the Earth.
The temperature at the ground is around 25°C and it drops to about 5°C every km of altitude gained, until it reaches a low of around -60°C at about 10-11 kms. The upper limit of the troposphere is known as tropopause. The branch of the atmospheric science that deals with the characteristics of the troposphere is called ‘micrometeorology’.
b. Stratosphere (10-40 kms):
It is less dense than the troposphere. It contains much the same gases except that there is less water vapour. At about 25 kms is a concentrated layer of ozone. This zone is known as the ozonosphere. This layer absorbs most of the ultraviolet radiation of the Sun. From a low of 60°C at 10 kms, the temperature slowly rises to about the base of the overlying mesosphere.
c. Mesosphere (40-70 kms):
The composition of gases are the same but less dense than the stratosphere. The mesosphere has a layer of ionised or electrified air at 50- 70 kms above the Earth. It is caused by the action of the solar ultraviolet radiation on the air molecules and is charged with electrons. Ozone is also found by the action of UV and X rays on oxygen. The temperature drops to about -90°C at about 80 kms above the surface of the earth.
d. Thermosphere (70-400 kms):
The thermosphere is radically different from the other atmospheric layers. Ozone, carbon dioxide and water are virtually absent. The density is very low, but is dense enough to burn up fast moving meteors. Most of the gas atoms in this layer are electrically charged by the radiation of the Sun.
Three distinct ionised regions are found in this layer, E, F1 and F2 layers. The E layer is found at an altitude of 90-120 kms and is made of nitrogen and oxygen. The F1 has oxygen atoms and in the F2 layer nitrogen ions are predominantly found.
The thermospheric layers are important for communication. They reflect radio waves back to the Earth. There is a wide range of temperature in the thermosphere from a low of about -90°C at 80 kms altitude to several 1000°C at about 500 kms and higher. The motions of ionised gas generate electricity, which in turn causes variations in the Earth’s magnetic field.
e. Exosphere (400 kms and Higher):
This layer extends into space. The gases in this layer are extremely thin. Hydrogen is the chief constituent of this layer.
Differential solar radiation in different regions of the Earth as well as rotation of the Earth causes air to move and form wind. Depending upon the velocity of the wind it is called breeze, gale, storm or hurricane. Dust storms and squall are also modified forms of wind; the former carries dust particles while the latter carries rain or snow.
Winds or air currents arise on a world-wide scale as a result of a complex interaction between hot air expanding and rising (convection) in the mid-latitudes. This has various effects on the rotation of the Earth and results in a centrifugal force which tends to lift the air at the equator. This force is known as the Coriolis force and tends to deflect winds to the left of the southern hemisphere and to the right in the northern hemisphere. Winds carry water vapour, which may condense and fall in the form of rain, snow or hail.
Wind plays a role in pollination and seed dispersal of some plants, as well as the dispersal of some animals, such as insects. Wind erosion can remove and redistribute topsoil, especially where vegetation has been reduced. Warm berg wind results in desiccation, which creates a fire hazard. If plants are exposed to strong prevailing winds are they usually smaller than those in less windy conditions.